Clang struct alignment. I'm automating enforcing coding style using clang-format.

Clang struct alignment Clang/LLVM). Bit Extra Clang Tools 20. aaa = 0, . clang-format file with all the default options with the command clang-format -style=llvm -dump-config > . -fpack-struct, -fno-pack-struct¶-fpack-struct=<arg>¶ Specify the default maximum struct packing alignment-fpascal-strings, -fno-pascal-strings, -mpascal-strings¶ Recognize and construct Pascal-style string literals-fpass-plugin=<dsopath>¶ Load pass plugin from a dynamic shared object file (only with new pass manager). AlignAfterOpenBracket (BracketAlignmentStyle) clang-format 3. consider following alignment for the typedef structure definition: #pragma pack(1) typedef struct { int x; } uas Yes, it works. 6: 146: February 10, 2017 (Aligned with C, Aligned with Clang, Breaking Gcc’s and MSVC’sbehaviour) Issue (2): Standardize the existing practice - allow alignof(exp). Stack Overflow. But I can't get clang-format to keep the comments aligned to the nearest tab character. By its nature, if you have an array of a struct with two elements, then both need to have aligned members, so that means that yes, the size has to be a multiple of the alignment. I. The thing is, clang-format wrongly recognizes this structure, which has exactly two fields, as a binary operator For writing SIMD code, I'd like to use templates to generate vector types with certain alignment. Follow asked May 28, 2020 at 10:26. clang-format multiline array definition and ending braces. 11 2 2 bronze badges. h Using the packed attribute resets the alignment of struct members to 1, meaning that clang should have emitted eight ldrb instructions instead of two ldr. Which makes sense, otherwise you couldn't rely on malloc to do the correct thing with struct S { char c; int i; }; struct S s = (struct S *)malloc(sizeof(struct S)); However, you are correct that it doesn't need to return a suitably aligned pointer for *any* type. Reordering it for speed and size in the general case means good performance in 99% of cases. The GCC documentation lists the attributes you can give, and you could even extend your GCC compiler (with some plugin or On 64-bit systems, pointers are 8 bytes. members of classes and structs) are typically aligned on WORD or DWORD boundaries for current generation processors in order to improve access times. " Does that mean that if there a data type of size > 8 bytes, by default (no compiler option or pragma pack) it would not be naturally aligned and rather will have 8 byte alignment. On PowerPC AIX, there are special layout alignment rules for structs not represented by the repr(C) attribute in Rust. For more information about alignment, see: align. The desired behaviour is obtained when I explicitly give the attribute( (aligned) ) in the source C code, such as this. struct dma_transfer { unsigned int e0 __attribute__ ((aligned (16))); unsigned int e1 __attribute__ warning: alignment of 16 bytes for a struct member is not binary compatible with IBM XL C/C++ for AIX 16. c: 3: 11: warning: conversion specifies type 'char *' but the argument has type 'int' msvc. 4 aligning function declarations in The aligned attribute only applies to the structure itself, not its fields. When the AST record layout does this, we represent it in the LLVM IR's type as either a sequence of i8 members or a byte array to reserve the number of bytes touched without forcing any particular alignment beyond the basic @antonm: Yes, a struct within a packed struct may well be unaligned, but, again, the compiler knows what the alignment of each field is, and it's perfectly safe as long as you don't try to use pointers into the struct. 0 and GCC 4. a = 1, . 1 documentation clang-tidy - altera-struct-pack-align « altera-single-work-item-barrier :: Contents :: altera-unroll-loops » altera-struct-pack-align ¶ Finds structs that are inefficiently packed or aligned, and recommends packing and/or aligning of I've been asked in a recent interview about C++ struct fields alignment and theoretized that C and C++ follows the same strategy in struct packing. Neither of these forces you to put anything on the heap, so they should work for your use case. You can modify this exception behavior by using SetErrorMode. eg: I have a small memory problem in C++17 using Clang compiling for x64. int a_value; unsigned int another_value; float a_last_value; a_value = 2; another_value = 3; a_last_value = 42. However, the struct's actual memory is still Padding aligns structure members to "natural" address boundaries - say, int members would have offsets, which are mod(4) == 0 on 32-bit platform. struct MaybeAlign; struct MaybeAlign Declaration struct MaybeAlign : public Optional { /* full declaration omitted */ }; For convenience, returns a valid alignment or 1 if undefined. For details, see Set C++ compiler and build properties From my understanding the output for sizeof(foo) should be 4 and the output for sizeof(ex_struct_aligned_8_t) should be 8. Start with a current offset of 0 bytes. 1. 5 Disable pointer alignment. C语言字节对齐涉及数据在内存中的位置，变量地址位于其长度的整数倍时称为自然对齐。 getMaxAlignment - return the maximum alignment specified by attributes on this decl, 0 if there are none. If you move it directly after the identifier, both GCC and Clang are happy and apply the alignment:. 1 vote. (Aligned with C, Aligned with Clang, Breaking Gcc’s and MSVC’sbehaviour) Issue (2): Standardize the existing practice - allow alignof(exp). There seems to be some interference between PointerAlignment and ReferenceAlignment. This warning can be safely ignored and suppressed. That said, where supported #pragma align is reasonably similar between compilers, and works in exactly the way you describe, individually specifying the alignment of data types and members of memory alignment for heap memory allocation. if an array has another array as one of it's elements, then this array is formatted as one element per It would naturally have alignment 8 in most architectures, but for reasons (legacy reasons), I need it to have alignment 4, as I can guarantee the latter alignment, but not the Alignment options. clang supports the exact same __attribute__ You can use preprocessor directive to specify byte alignment for the struct so no padding will be done by the compiler: #pragma pack(1) typedef struct { char t1; long long t2; char I'm trying to use clang-format (in VS code) to format my C++ files and configure it to my preferred style. : The sample code below is aligned using the above configuration. I've been asked in a recent interview about C++ struct fields alignment and theoretized that C and C++ follows the same strategy in struct packing. However, one compiler I used for 680x0 only aligned on 2-byte boundaries - and there was no option to change the alignment! The aligned(x) attribute specifies the minimum alignment required. Extra Clang Tools 16. -Wpadded. 0 documentation clang-tidy - altera-struct-pack-align « altera-single-work-item-barrier :: Contents :: altera-unroll-loops » altera-struct-pack-align ¶ Finds structs that are inefficiently packed or aligned, and recommends packing and/or aligning of Older compilers have alternatives (such as MSVC _declspec(align(4))), and continue to support these for compatibility with existing source code. 8 Why do the PointerAlignment options not work? 4 So larger types are usually more-aligned than smaller types. 1. In C, structs are used to group variables of different data types. Previous Simple Raspberry Pi Backup. When both MaxEmptyLinesToKeep and OverEmptyLines are set to 2, it formats like below. " Does that mean that if there a data type of size > 8 bytes, by default it would not be naturally aligned and rather will have 8 byte alignment. If the structure contains members with explicit alignment (i. Since I work with low level memory copies to specialized hardware, I do depend on the memory being aligned as expected. When the AST record layout does this, we represent it in the LLVM IR's type as either a sequence of i8 members or a byte array to reserve the number of bytes touched without forcing any particular alignment beyond the basic struct S { __declspec(align(32)) double A[4]; // MSVC / ICL on Windows double A[4] __attribute__((aligned(x))); // CLANG / GCC }; The structure can be allocated on stack or using malloc for instance, either way the structure instance itself may not be aligned. 0git documentation clang-tidy - altera-struct-pack-align « altera-single-work-item-barrier :: Contents :: altera-unroll-loops » altera-struct-pack-align ¶ Finds structs that are inefficiently packed or aligned, and recommends packing and/or aligning of By introducing padding into the structure to inflate it to 64 bytes, it is guaranteed that no two such data structures end up in the same cache line, and the processes that access them are not blocked more that absolutely necessary. If you use an external library that has a struct foo { short x; int y; }, How to change CLang struct alignment behaviour? Clang Frontend. And for that last 1% you can simply use C struct encoding and do it by hand, but only if you think/now there is actually a good reason to alignas cannot be used in this situation as Martinho notes, since we're asking for an alignment less stricter than the natural alignment of the struct. User123456 User123456. anon = type { i8 } %a = alloca %struct. Structs that are not packed take up more typedef struct AlignCheck_t { char a; // 0: [a] // 1: [X-X-X] padding int b; // 4: [b-b-b-b] char c; // 8: [c] // 9: padding short d; // 10: [d-d] char e; // 12: [e] // 13: [X-X-X] padding long long Looking at the documentation, there don't seem to be any clang-format style options which will do what you want. Modified 4 years, 6 months ago. : constexpr int c = 4; int a[c] = {25, 50, 100, 200}; MyType b = { . For example, to align an array of structs to a 32 byte boundary: Some targets, e. So, in X, there's 2 bytes of padding after the short to ensure the 4 byte int starts on a 4 byte boundary. CLang still uses unaligned, see here, also CLang uses AVX-512 registers only with more than 16 elements. The alignment size of a struct is the offset from one element to the next element when you have an array of that struct. Inheriting from it doesnt remove this requirement either, because the subobject I had much better luck with clang than llvm-gcc for 32 bit alignment on a 32 bit target when compiling on a 64 bit host. That is not like this: Adding a comma after the last array element causes clang-format (tried with v6. The supported states for S are: "za" for Matrix Storage (requires SME) For state "za", this means that:. c:6:9: warning: padding struct 'struct s' with 3 bytes to align 'b' [-Wpadded] int b; ^ struct-align. When the PointerAlignment is Right, I get the strange one. so that the members may become misaligned wrt to their natural alignment. Here's a summary of the differences: #pragma pack (and variants) is more concise, and represents both attributes packed and aligned in GCC syntax (see example below);; #pragma pack applies to I’m trying to add an option to Clang-Format that would allow users to align variables in structs/classes and functions based on the width of the type identifier. This isn't ARM-specific, you get the same behavior with x86-64, for example. To ensure that each member of a struct is properly aligned, compilers often insert padding between For arrays, I have set LargeArrayMinWidth = 8; LargeArrayAlign = 16; so that all of them get an alignment of 2 bytes. The choices are: None; Consecutive; AcrossEmptyLines; AcrossComments; Is there a way to force Clang to respect a struct being passed by value in the source (either as arguments or the return value from a function) and not converted to pointers Specifies the alignment requirement of a type or an object. AA { i8 3, i8 4, i8 5, i8 6 }, align 2 then everything Please consider the following source file that has been formatted by clang-format 9. Since int is only four bytes, the compiler inserted 4 bytes of "padding" after foo, so that smap is on an 8-byte boundary. #pragma ms_struct reset goes back to the default layout. Consider "DeclRefExpr::Create(. Without __attribute__((packed)), the minimum alignment is the natural alignment of the variable type. When a modern computer reads from or writes to a memory address, it will do this in word sized chunks (e. Query structures and/or directories with queries are specified with the option --qrs. Clang Frontend. alignment C clang gcc memory structs. 4 byte chunks on a 32-bit system). Not only that if you write two structs whose members are exactly same, the only difference is that the order in which they're declared is different, then the size of each struct can be (and often is) different. this will affect additional Allowing the user to choose means that any user will now have to consider the exact type alignment on any struct. clang-format file, and using clang-format-12: The cast-align warning is triggered when you attempt to cast from a type with smaller alignment to a type with larger alignment. The fault happens because the compiler generates a 128-bit move instruction which must be aligned on a 16-byte boundary, but the address is not correctly aligned. This is similar to ``OAS_Align``, except when ``BreakBeforeBinaryOperators`` is set, the operator is un-indented so that the Proposal is to format nested array of structures as in input file above. Open the project's Property Pages dialog box. , code: struct CFG_INTERRUPT_STRUCT { uint16_t maxNumber; //!< Maximum number of interrupts in period uint16_ I have a type that is declared with __attribute__((aligned(16))). i386 and powerpc, support the ms_struct #pragma which lays out a structure as the documented __attribute__ ((ms_struct)). AA, align 2. Put always-64-bit members first (like double, long long, and int64_t). packed ¶ The packed type attribute can be applied to struct or union types to indicate that each member of a given struct or union is placed on a 1-byte boundary. (e. Summing up the size of each of the elements of the structure the total is 6 bytes, so the entire structure should fit perfectly with a memory alignment of 8 bytes. clang-format file located The fact is that for structures in c, the next rule works: the fields of the structure are aligned in memory along a boundary multiple of their own size. Commented Dec 3, 2012 at 17:35. Here the structure will be aligned to the maximum of the sizes of different data items in the structure. clang-format config file is: ColumnLimit: 120 Operation. 2. struct s1 { char a; int b; char c; char d; char e; } could be represented more memory-efficiently (12 vs. Elements of arrays are stored in the same manner as if they were individual objects. It consists of two separate but related issues: data alignment and data structure padding. This option does not affect the ABI alignment of types; the layout of structs and unions and the value returned by the alignof operator remain the same. When using -style=file, clang-format for each input file will try to find the . When building with clang on OS X on x86_64, the following code causes a GP fault when attempting to throw a value containing this type. Commented Dec 27, 2023 at 2:44. Examples of automatic padding. struct s2 { int b; char a; char c; char d; char e; } I know that C/C++ compilers are not allowed to do this. – Thanks. 4 aligning function declarations in clang-format. I wonder how much ABI breakage there would be from changing behavior now. 30. It inserts the Horizontally align operands of binary and ternary expressions. cc Given the following c language struct definition: long long a; int b; int c; Clang-Tidy gives the following message: Accessing fields in struct 'PackTest' is inefficient due to poor Since the user code loaded the struct object m with a 4-byte alignment once, the alignment attribute for m was increased from 1 to 4. 0: #include <iostream> #include <tuple> #include <utility> #include <vector> int main Skip to main content Strangely the value Align greatly improves the formatting. In these cases the LLVM backend correctly creates word sized load/stores (2 bytes). The n value below always is required to be a small power of two and specifies the new alignment in I’m attempting to implement the #[repr(align)] RFC which is jumping in the deep end slightly with my level of knowledge about Rust compiler internals. -Wpadded is not meant to be always enabled, unless you want to always manually specify the necessary padding explicitly. – John Bollinger. anon } %struct. Padding is on by default. However, in this case, this is not legal: Extra Clang Tools 18. To set the alignment in a structure to any value greater than 0, use the __attribute__((packed)) variable attribute. 2 answers. If we need a block whose address is a multiple of a higher power of two than that, use aligned_alloc or posix_memalign. 6 The alignment is fine, but when I use two pointers to such a struct, and pass them as destination and source for memcpy() then the compiler decides to use __memcpy_avx_unaligned() to copy. Configuring Style with clang-format¶. clang-format or _clang-format file in the project directory. 0: 88: December 17, 2020 'struct' member alignment query. – jww. LLVM Dev List Archives. Struct Alignment and Padding. struct Align; struct Align Declaration struct Align { /* full declaration omitted */ }; Description. 4. So can we count on the member A being 32-byte aligned? For example, the linux 32 bit standard specifies that chars can be aligned on any address, shorts are aligned on addresses that are multiples of two, and integers and larger are aligned on addresses that are multiples of four. clang appears to accept it and silently ignore the Struct members aligned as needed, look at dx, dy and z. It can be combined with the packed attribute. 8 ¶ If true, horizontally aligns arguments after an open bracket. References clang::LazyOffsetPtr< T, OffsT, Get >::get() Determines whether this field is a representative for an (the above is the output of Clang 3. I'm using the following . Because I don't want to touch the SDK's header files, I need a way to tell BCC64 to use an alignment of 8 Bytes as default. // expected-warning@-1 {{'packed' attribute was ignored on bit-fields with single-byte alignment in older versions of GCC and Clang}} Whether to align across empty lines. As of clang-format version 13 (2021) there is a new AlignArrayOfStructures option that does exactly this. [UPDATE] The op's pull request finally went through and as of clang version 9. clang (default) t. 3 GCC and G++ struct packing differences? Checking against existing implementations shows that both Clang 3. 6 6. This struct is a compact representation of a valid (non-zero power of two) alignment. Taking the address of such a field can result in struct MaybeAlign; struct MaybeAlign Declaration struct MaybeAlign : public Optional { /* full declaration omitted */ }; For convenience, returns a valid alignment or 1 if undefined. I often use a different strategy that is more portable using a helper struct: But only GCC produces aligned instruction. warning: ‘packed’ attribute was ignored on bit-fields with single-byte alignment in older versions of GCC and Clang-Wattribute-statement In C++, on at least GCC and Clang, an over-aligned type embedded within a container (std::vector) seems to be treated differently depending on whether the type is an over-aligned struct or an over-aligned enum. Structure and Array Alignment¶ Structures are aligned according to the member with the most restrictive alignment requirement. That makes sense. This: struct { char a; char b; } __attribute__((aligned(8))) foo = { 1, 2 }; Will compile to something like: unsigned clang::format::FormatStyle::TrailingCommentsAlignmentStyle::OverEmptyLines: How many empty lines to apply alignment. In these cases the The current situation is that for this struct: struct AA { char n; char m; char j; }; I get it aligned by 1: %a = alloca %struct. x86, arm, IBM/370) and specific HW restrictions on The clang format 13 option - AlignArrayOfStructures (ArrayInitializerAlignmentStyle) - crashes on left alignment of arrays. st2 struct demonstrates a similar structure that occupies the same amount of memory, except that it has an array of 7 char members. Thanks a lot. AA, align 1 I would want it to implicitly use 4 bytes instead I’m trying to add an option to Clang-Format that would allow users to align variables in structs/classes and functions based on the width of the type identifier. Disadvantage: You can't enforce column limits for your project. #pragma ms_struct on turns on the layout for structures declared. alignof Operator. Is this a bug in clang-format-14 or should I change the clang-format file somehow? It looks like it aligns things properly if I use (instead of ). If you want to control the alignment of structure fields, you will have to use the attribute on them. Re "demonstrating that a structure's alignment depends on the order its members are declared" No, you haven't. AA { i8 3, i8 4, i8 5, i8 6 }, align 1 replaced by @ga = dso_local local_unnamed_addr global %struct. Set BinPackArguments and BinPackParameters to false. C struct elements alignment (ansi) 4. The problem is that f in some_struct is a pointer to a first_struct_t not a pointer to a second_struct_t. x. I had much better luck with clang than llvm-gcc for 32 bit alignment on a 32 bit target when compiling on a 64 bit host. alignas (pack) 1)expression must be an integral constant expression that evaluates to zero, or to a valid value having consecutive assignment alignment enabled, it messing up with the struct alignment e. types declared with __attribute__((aligned(n))) on GCC/Clang or __declspec(align(n)) on MSVC), then the compiler will take that in consideration when padding. I stumbled upon a very bizarre struct member packing in 32-bit clang. Type gtalign for a description of the options. 3 GCC and G++ struct packing differences? I am aware of the memory alignment in structure but I am stumped at this implementation I came across in a project I am working on. 8 bytes) if the members were reordered as in. You can use the packed and aligned attributes together on the same member to set the alignment to a value. 125 // And suggest the minimum power-of-two alignment for the struct as a whole getMaxAlignment - return the maximum alignment specified by attributes on this decl, 0 if there are none. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default First of all, I have the impression, that you are doing something fundamentally wrong, when you have "several hundred places" in your code, where you need to define alignment to prevent a segfault. clang-format file located Even if you use the same compiler, the packing alignment for structs can be different based on what pragma pack you're using. How can I force clang to use the aligned avx memcpy function instead, which I assume is the faster variant? OS: Ubuntu 16. 41 views. Like an integer, which (Assuming 8 byte pointers) SLIST_ENTRY is aligned to 16 bytes. Is there a way to force Clang to respect a struct being passed by value in the source (either as arguments or the return value You can also use the pack pragma to control structure packing. See the relative documentation page. clang-format supports two ways to provide custom style options: directly specify style configuration in the -style= command line option or use -style=file and put style configuration in the . $ echo "struct Test { ShortType& v1; SomeLongerType& v2; MuchMuchLongerType& v3;};" | clang-format-13 --style "{AlignConsecutiveDeclarations: true, The aligned type attribute specifies a minimum alignment for the type. You should imagine a struct within a struct as one flat series of fields, with the extra name just for readability. You've specified that one particular object should use 4-byte alignment, but every other object, a char, an int, a std::string and anything else also have their own alignment requirements. 11 How to tell clang-format to keep whitespace between binary operators as they are. @WayneO The amount of padding is always enough to make sure that whatever is next is aligned according to its size. Some additional information is printed in the bottom - for example, sizeof - the total size, and dsize - data size without tail padding. align (emphasised the relevant part):; When multiple alignment-specifiers are specified for an entity, the alignment requirement shall be set to the strictest specified alignment. 0-2ubuntu4. The configuration from the command, with my modification on PointerAlignment looks like this: To achieve the desired formatting for the 'array of structs' initialization in your C project using Clang-Format, you can make a few modifications to your . For a variable that is not in a structure, the minimum alignment is the natural alignment of the variable type. Struct-packing rules in most ABIs give struct members their absolute alignof(T) alignment relative to the start of the struct, and the struct itself inherits the largest alignof() of any of its members. 0). )". Here is the link to the compiler explorer experiment. The documentation for this struct was generated from the following file: lib/Headers/__stddef_max_align_t. About; Generalized Attributes for Memory Alignment in GCC/Clang. tar archives of compressed and uncompressed structures, meaning that big structure databases such as If the structure contains members with explicit alignment (i. The aligned attribute forces the compiler to align that variable (your a array) to the specified alignment. Ask Question Asked 4 years, 6 months ago. or: %a = alloca i32, align 2 %tmpcast = bitcast i32* %a to %struct. 0 is live. gcc and clang, possibly MS) so it holds there too. If yes, is that case with GCC/clang? – Some targets, e. The value is less than the default value, but greater There's a field called BreakBeforeBinaryOperators, that controls the wrapping of binary operators (see clang-format 3. 6 alignas is turning out to be much more finicky than __attribute__((aligned)), especially under compilers like Clang. 0-eb16166 on 2022-10-27T00:45:30 UTC. I’m not sure if there’s a way to get what I want via CLI options. I have some tweaks I need to make to the produced IR. The choices are: - None - Consecutive - AcrossEmptyLines - AcrossComments - AcrossEmptyLinesAndComments For 125 // And suggest the minimum power-of-two alignment for the struct as a whole Xcode uses the gcc and clang compilers which both use __attribute__((packed)) to designate struct packing. In general if you Well, my problem is that when I compile with clang, I have this warning: padding struct 'struct s_fct_printf' with 7 bytes to align 'fct' [-Wpadded] – Xwilarg. g. GCC and clang have a compiler warning (-Wcast-align) which is triggered, for example, by casting char* to int*. (But not with tab stops in between types and identifiers. 0 documentation clang-tidy - altera-struct-pack-align « altera-single-work-item-barrier :: Contents :: altera-unroll-loops » altera-struct-pack-align ¶ Finds structs that are inefficiently packed or aligned, and recommends packing and/or aligning of I'm having some trouble with -Wpadded using C11 and structs. 0) to align the elements to the left side, like your second example By the way, if I manually set align to 2 in Clang output files, such as on the line below, (which corresponds to a global var declaration): @ga = dso_local local_unnamed_addr global %struct. Yes, it works. From the GCC documentation, however the flag is also available in Clang:-fpack-struct[=n] Without a value specified, pack all structure members together without holes. I have a type that is declared with __attribute__((aligned(16))). A combination of packed,aligned(n) will pack the struct fields, but still keep aligned(n) externally. 2: 117: May 16, 2019 Fw:about element size/alignment in an array. However, I am lost at what to do with Structs. The size and offset of fields is determined by the following algorithm. The documentation for this struct was generated from I want to get code like this: typedef struct FooStruct { \t u_int16_t bar;\t /**< Blah */ \t u_int16_t baz;\t /**< Bleh */ } Foo; (where I'm using \t to indicate a 4-column wide tab character). clang-format, and change the options PointerAlignment to Left, asterisks and ampersands still get aligned to the right. 1; asked Aug 15 at 14:00. 4. 8 documentation). Arrays are always word aligned. Commented Jan 20 "Use __declspec(align(#)) to precisely control the alignment of user-defined data (for example, static allocations or automatic data in a function)" It will make the compiler start every instance on a byte boundary. Try -fpack-struct. 04. Rather than having a pointer to a struct point into a character array, use memcpy to The alignment attribute specifies the alignment of variables or structure fields, not single array elements. . 3. If you always want to align two integers together, you can define a structure. Can a layout in such structure cause alignment issues on a target platform? I understand it much depends on a platform, but generally C compiler is allowed to do padding of structures, so for example on 32 bit machine, where 'int' is 32 bit long: struct my_struct { int w[MAX_SIZE]; } There's a similar problem with alignment within structs. typedef float aligned_block alignas(16) [4]; typedef float aligned_block [4] alignas(16); this is also true if you use using, where the difference also becomes more apparent. Extra Clang Tools 12 documentation clang-tidy - altera-struct-pack-align « altera-single-work-item-barrier :: Contents :: android-cloexec-accept » altera-struct-pack-align ¶ Finds structs that are inefficiently packed or aligned, and recommends packing and/or aligning of [UPDATE] The op's pull request finally went through and as of clang version 9. They can also be read as a whole for compatibility. With warnings turned all up, clang rightly complained that the alignment requirements of the struct are not met. With GNU C __attribute__ syntax, it seems to be necessary to use a typedef to get the attribute to apply to the pointed-to type, rather than to the pointer itself. In this case you got from a char which has alignment 1 and a struct data which (because it contains an int) has alignment 4 (assuming a int is 4 bytes). typedef struct { _Alignas(4) uint8_t anotherVal; uint32_t someVal; } __attribute__((packed)) my_type_t; Exemplary usage with alignment exaggerated to 64 bytes. h Generated on Thu Dec 12 2024 15:45:37 for clang by 1. – Benjamin Bihler. 0. The complier also pads suitably in the case of structures for natural alignment and access. zTy = type { i32, [2 x i8], %struct. Data structure alignment is the way data is arranged and accessed in computer memory. 5. The aligned type attribute only increases the alignment of a struct or member, and does not decrease it. But i wanna align return parameters in operator== the same as struct members. Put a Horizontally align operands of binary and ternary expressions. Definition at line 39 of file Alignment. Often we layout a sequence of bitfields as a contiguous sequence of bits. That is not possible in clang format right now afaik. In Y, there's 1 byte padding after the char to make sure the 2 byte short starts on a 2 byte boundary. struct alignment C/C++. [ORIGINAL] Weirdly enough this feature is yet to be implemented in clang; the formatting option for consecutive macros is currently missing. Reference structures (to align queries with) and/or their directories to be searched are specified with the option --rfs. this will affect additional One definition of alignment size:. The documentation for this struct was generated from the following file: include/clang/Format/ Format. bool AcrossComments Whether to align across comments. ) Currently clang-format doesn't have a useful way of controlling this (as of version 14. For an array of structs (for getopts) it is adding a load of extra spaces and messing up the clang-format: How to align list of struct inits. It's as much about the computer arch (e. 8 bytes). struct default { uint8_t variable[((sizeof(struct dummyStructure) + 3) /4)*4] // Align on 32 bit boundary } gcc or clang specific. clang appears to accept it and silently ignore the FYI, already tried : Clang-format: Align Function Parameter Names and clang-format: break function parameters. However, when dealing with some structures, there is an issue where the structs compiled with Clang/GCC are not aligned, and the member offsets do not increment regularly. Padding structures is a perfectly valid thing compiler often does. Retrieving a DWORD at an address which isn't divisible by 4 requires at least one extra CPU cycle on a 32 bit processor. That also explains why #[repr(packed(2))] struct Foo { x: u8, y: u8 } has alignment 1: packed(n) resets the alignment of all fields to be no more than n. I should add that the cast in struct_second_t *s = (struct_second_t *) p->f; is hiding the warning message. For more information about how to modify alignment, see these articles: Set the “Struct Member Alignment” for the 64‐bit version of your project to 8 byte. The bindings are 4-byte aligned, leading to memory access issues. 7 support _Alignas on structure members without complaining (with -pedantic). [altera-struct-pack-align] struct b:a{ ^ I will not demonstrate the code after the suggestion as clang-tidy made a lot of changes the important bit is: So I have the following minimized C11 code that defines a struct containing a uint16_t (which means the struct it should be aligned to 2 bytes) and I want to cast a char buffer to a pointer to that struct. I think you just placed the alignas in the wrong position. The struct layout will be shared when communicated between x86_64 and ARMv7-A architectures Skip to main content. bool AlignCaseArrows Whether to align the case arrows when aligning short case expressions. And yes, as others already told you, the only safe and portable way of accessing unaligned data from plain C without any compiler extensions and without the risk of making hard to find On newer versions of clang-format, AlignConsecutiveDeclarations: true will do most of what you're asking for. But I don't know where this alignment is inserted or how alignment for nested structs is calculated. I know it is possible to align variable definitions and affectations with clang-format, using AlignConsecutiveAssignments and AlignConsecutiveDeclarations to get something like. – I am experimenting with clang-tidy (while using a gcc) with all of it's checks. Note that GTalign reads . 6: 146: February 10, 2017 Structure with information about how a bitfield should be accessed. Here are two versions that are not accepted by GCC Extra Clang Tools 16. This has been Clang's behavior for at least 15 years, see 0720944. Clang issues “align 2” if the struct has at least one int (2 bytes), but also if the entire struct size is multiple of 2. Since the compiler cannot know what might be Note that the myS type alignment is also 16-bytes since the alignment of the struct is determined by the struct member with the most restrictive alignment constraint (m2_u in this case). This padding is part of the type SLIST_ENTRY itself and cannot be removed. 1 clang-format Pointer Alignment. With __attribute__((packed)), it is only 6 bytes long—the sum of the lengths of its fields. #pragma ms_struct off turns off the layout for structures declared. struct test { int a; int b; int c; }; struct test T = { . I have Nested C Array Struct Alignment in Clang. If you use an external library that has a struct foo { short x; int y; }, Note that the myS type alignment is also 16-bytes since the alignment of the struct is determined by the struct member with the most restrictive alignment constraint (m2_u in this case). Basically I am looking for Some targets, e. A uint64_t in a packed struct is treated as if it had alignment 1. Set BraceWrapping options to control the formatting of braces. In C++, on at least GCC and Clang, an over-aligned type embedded within a container (std::vector) seems to be treated differently depending on whether the type is an over-aligned struct or an over-aligned enum. h Generated on Mon Nov 25 2024 17:21:03 for clang by 1. I am reworking it so I handle line breaking in a sane fashion and dropping the LineFormatter override since that really can't handle reformatting without reimplementing most of it @WayneO The amount of padding is always enough to make sure that whatever is next is aligned according to its size. first = "a", . 10 Structure-Layout Pragmas ¶. the function requires that the target processor implements the struct-align. References clang::LazyOffsetPtr< T, OffsT, Get >::get() Determines whether this field is a representative for an When I create a . struct foo { uint8_t bar; uint8_t baz; } __attribute__((packed)); Using __attribute__((aligned(1))) tells the compiler to begin each struct element on the next byte boundary but doesn't tell it how much space it can put at the end. Next Getting On msdn for Zp, it is mentioned that "Packs structures on 8-byte boundaries (default). int a; // all these. on a struct) forbids padding, both between fields in the struct, and before/after the struct itself, if the struct is used as a field elsewhere. You demonstrated the size of depends on If you change the alignment of a structure, it may not use as much space in memory. From the now updated documentation linked in the question: if not Configuring Style with clang-format¶ clang-format supports two ways to provide custom Clang issues “align 2” if the struct has at least one int (2 bytes), but also if the entire struct size is multiple of 2. That's typically related to the members alignment requirements, but you haven;t shown anything about the structures' alignment requirements. namespace clang; namespace format; struct FormatStyle; struct FormatStyle::AlignConsecutiveStyle; struct FormatStyle::AlignConsecutiveStyle Alignment options. For each field in declaration order in the struct, first determine the size and alignment of Hi all, I've been looking into what might be alignment issues throughout clang. Instead of this. After looking at it a bit further, I think this is a Clang thing. 2 running on 64-bit Linux) We can see the layout of Derived objects, with the offset of every field (including the fields coming from base classes) in the left-most column. alignment of struct in C++. 2. e. Just disable the warning (or - well, don't enable it, I don't know of any warning set like -Wall or -Wextra that includes it). e. It is suitable for use as static global constants. h. 3. zTy, align 4 From the alloca instruction it is possible to see the alignment (4 byte). I managed to get parsing an align repr and setting alignment of the struct such that mem::align_of returns the right thing for my aligned type. I need proper alignment, but I don't want member packing in the parent struct. This is similar to ``OAS_Align``, except when ``BreakBeforeBinaryOperators`` is set, the operator is un-indented so that the wrapped operand is aligned with the operand on the first line. Compilers will align structure members to natural boundaries, so an 8-byte pointer will start at an offset in a structure that is a multiple of 8 bytes. 9. Conformingly align void pointer. \code int aaa = bbbbbbbbbbbbbbb + ccccccccccccccc; \endcode On msdn for cl compiler option /Zp, it is mentioned that "Packs structures on 8-byte boundaries (default). Commented Nov 17, 2016 at 18:22. Documentation for LLVM/Clang 15. AA* In summary, I noticed that Clang will compute struct alignments as an int (2 bytes) if it has one, or if the struct size is already a multiple of 2, but not in other cases. UPDATE #[repr(C)] Structs. Cheers. 8. clang-format and references: can I get a "true" left alignment? Hot Network Questions In retrospect, should they have provided more RTG fuel and a more powerful radio for Voyager? I correctly get it aligned by 2: %a = alloca %struct. The functionality is enabled by the AlignConsecutiveMacros: true option. That is, 1-byte fields are not aligned, 2 I have an array of 64-bit structs that I'd like to align on 64-bit boundaries: struct AStruct { int x; int y; }; std::array<AStruct, 1000> array; // I'd like to align this on 64-bit boundary I know the attribute is __attribute__((__aligned__(64)) but I'm unsure whether I need to align each individual struct, the entire array or specify the The alignment attribute specifies the alignment of variables or structure fields, not single array elements. Load 7 more related questions Show fewer related questions The last time I checked, clang supported __attribute__((packed)) – user529758. From this POV the struct has alignment 1 for the same reason a struct of only u8 s does. b = 2, . The compiler is required to make it work. For the struct version, the element is aligned for each, while for the enum one, only the overall buffer is with the specified alignment. The alignment of the struct is the alignment of the most-aligned field in it. struct dma_transfer { unsigned int e0 __attribute__ ((aligned (16))); unsigned int e1 __attribute__ How to get clang-format to align chained method calls. Constructor & Destructor Documentation I don't think there's a portable way to inform the compiler that a pointer points to aligned memory. Here’s a couple altera-struct-pack-align¶ Finds structs that are inefficiently packed or aligned, and recommends packing and/or aligning of said structs as needed. The Clang source code reproduces the same grammar from N1570, except Parser::ParseSpecifierQualifierList allows alignment specifiers; the function does contain a Configuring Style with clang-format¶. attributes like above are an extension (to the standard C99 language specification) provided by GCC (and some few other compilers, e. clang-format: How to align list of struct inits. Every type has an alignment which the compiler has to respect. In addition to normal structures, C also allows structures to have ‘bit-fields’, they look like this: I noticed that gcc and clang compile the following code differently: Alignment of a struct with two doubles is 4 even though double is aligned to 8 (32bit) 1 Confusion in structure member alignment in 32 bit and 64 bit architecture. x86, arm, IBM/370) and specific HW restrictions on From my understanding the output for sizeof(foo) should be 4 and the output for sizeof(ex_struct_aligned_8_t) should be 8. This is specified in the standard under dcl. The version of clang-format: Ubuntu clang-format version 14. Generated by hdoc version 1. clang-format file located clang; struct-member-alignment; PRINT. Also, I'm Due to alignment constraints, the following struct. I’d like to be able to make them without making any changes to Clang itself. bool AlignCaseColons Whether aligned case labels are aligned on the colon, or on the tokens after the colon. __attribute__((aligned)) is a GCC-specific syntax (unsupported by MSVC). On 64-bit systems, pointers are 8 bytes. For example a struct with 4 char members. Struct members aligned as needed, look at dx, dy and z. For function declarations: You can set BinPackParameters=false, which will force that all the parameters of a function declaration are either on one line or each on a different line, and they will be aligned as you show. Why can't halfword-size data be placed on address 4x+1? I've read several articles about data alignment and padding, and all of them tell me that the aim of alignment & padding is facilitating data access from memory(or cache). 4 aligning function declarations in FYI, already tried : Clang-format: Align Function Parameter Names and clang-format: break function parameters. Consider this code : I'm going to assume when you have struct_second_t you actually meant second_struct_t, otherwise your code does not compile. c++; clang-format; Share. That means that after its member SLIST_ENTRY* Next, there must be another 8 bytes of padding, so that an array of the type would work correctly. Usage. But, you can use the "comma-after-last-item" trick. See Specifying Attributes of Variables and Common Variable Attributes for details. I would like to clang-format to be able to distinguish between the two types and not convert the second into the first one or align the elements after the opening brace. Basically I recommend trying clang. The "packed" attribute (e. Everything formats correctly, except for one unwanted artifact. This somehow fixes the use of AfterColon, making it work. However, you may see a loss of performance or even get a hardware-generated exception for unaligned access. This applies to round brackets (parentheses), angle brackets and square brackets. However, clang seems to ignore the alignment attribute when used with an alias template instead of a type alias. ISO C++ of I noticed that gcc and clang compile the following code differently: Alignment of a struct with two doubles is 4 even though double is aligned to 8 (32bit) 1 Confusion in structure member alignment in 32 bit and 64 bit architecture. Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. 0 Clang format unwanted indentation. So the compiler is used to having to cope with this. Here’s a couple examples to better illustrate what I’m talking about: typedef struct Unaligned_Struct { int wat; uint8_t huh; uint64_t who; } Unaligned_St All, I’m using Clang as a frontend for C compilation. 3 LTS, Clang: 3. Data elements (e. (aligned_alloc and posix_memalign are declared in stdlib. Since the compiler cannot know what might be Alignment requirements are recursive: The alignment of any struct is simply the largest alignment of any of its members, and this is understood recursively. 0f; // Or both at the same time int a_value = 2; unsigned int another_value = The documentation for this struct was generated from the following file: include/clang/Format/ Format. The attribute takes string arguments to instruct the compiler for which state to create new scope. 0 or older. Clang format unwanted %struct. If yes, is that case with GCC/clang? Well, you can get close. That field defaults to None, which tells clang-format not to break before binary operators. The __arm_new keyword applies to function declarations and specifies that the function will create a new scope for state S. struct AA {char n; char m; char j;} attribute( (aligned) ); This struct is a compact representation of a valid (non-zero power of two) alignment. Improve this question. 0. I was curious if the below case is a bug of clang-tidy as it contradicts my compiler's warnings. This means that the _alloca() is certainly not a standard or portable way of handling alignment on the stack. __unaligned /ALIGN (Section Alignment) To set this compiler option in the Visual Studio development environment. Take the following structure: typedef struct Paddee {char X; // 1 byte int Y; // 4 bytes The documentation for this struct was generated from the following file: include/clang/Format/ Format. Runtime alignment of structure members. How to get clang-format to align chained method calls. The address of a block returned by malloc or realloc in GNU systems is always a multiple of eight (or 16 on 64-bit systems). For instance, Sparc, PowerPC, and 680x0 CPUs are all big-endian, and the common default for Sparc and PowerPC compilers is to align struct members on 4-byte boundaries. However mem::size_of is not returning the aligned size and I’m not totally sure There are 2 other workarounds (with disadvantages): Use pairs of // clang-format off and // clang-format on around the initializer list -> Disadvantage: clang-format also does not apply the useful reformatting for that part. ; Set ColumnLimit to 0. Take the following structure: typedef struct Paddee {char X; // 1 byte int Y; // 4 bytes Mind though, a single pointer takes 8 bytes and a char takes one byte, so one would think the st1 struct must occupy 9 bytes. Post navigation. 67. I've already read Structure member alignment with _Alignas, and I looked in the clang docs and saw that it IS supported now. But it behaves as if all members are aligned to the largest member’s size (i. (C11 / C++11 alignas doesn't seem to be able to do that, see below). Use of __attribute__((packed)) often results in fields that don’t have the normal alignment for their types. Unfortunately, the function pointer in your struct confuses it, so you get the fields before the function pointer aligned, and the fields after the function pointer aligned, but the groups don't align with each other. , NameInfo, . 1 Configuring clang-format in source. c:4:8: warning: padding size of 'struct s' with 6 bytes to alignment boundary [-Wpadded] struct s { ^ 2 warnings generated. It allocates itself and a few other structs in a single allocation, using the well-kno Without __attribute__((packed)), this structure occupies 12 bytes (as described in the previous section), assuming 4-byte alignment for int. You can however, workaround this simply by using a trailing comma. Hovewer, it was the wrong assumption. Viewed 196 times To form a structure type that is aligned one must put the alignment attribute to the first member of the struct. (-m32 or whatever it is, is ignored by llvm-gcc). h)if you use a Given the sizes of objects shown and alignment rules that say each of these objects must be aligned to a multiple of its size, then, when the structure is laid out with only the padding required for alignment, the offsets should be as shown in Tetrau’s solution. For example, and assuming that each fundamental type's alignment equals its size (this is not always true in general), the struct X { int; char; double; } has the alignment of double, and it will be padded to Operation. How to change CLang struct alignment behaviour? Clang Frontend. Structures are padded so that the size of the structure is a multiple of its alignment. clang-format configuration. 0-1ubuntu1. 0) to align the elements to the left side, like your second example I'm automating enforcing coding style using clang-format. Possible values: BAS_Align (in configuration: Align) Align parameters on the open bracket, e. For compatibility with Microsoft Windows compilers, GCC supports a set of #pragma directives that change the maximum alignment of members of structures (other than zero-width bit-fields), unions, and classes subsequently defined. #pragma pack is a Microsoft syntax that has been ported to GCC for compatibility reasons. Using pointers to members of packed structures, even though supported by both, doesn't seem to trigger this warning. Specifically, the layout of the following struct is misaligned: #[repr(C)] #[derive(Copy, Clone)] pub struct Floats { a: f64, b: u8, // currently has 7 bytes of padding c: f64, } While the same struct generated by Clang and GCC struct Floats { Structure with information about how a bitfield should be accessed. Luckily in C++11 we got alignas and std::aligned_storage. I had thought that I have to suppress alignment. struct defaultx { int variable[sizeof(struct dummyStructure)]; } __attribute__ struct foo { char a; int b; }; static const struct foo data[] = {{'a', 1}, {'b', 2}, {'c', 3}, {}}; is there a way to tell clang-format to allow an empty element in a line and to correctly format the code as in the first example? EDIT1: as Nullndr suggested adding a traling , will get clang-format to create the right formatting. It's definitely easier to type and easier to I know that you can add a comment to disable formatting completely for a section via // clang-format off, but is it also possible to change a specific rule? enum class Foo { // clang-format AlignConsecutiveAssignments: Consecutive ABC = 0, X = 1 }; My overall goal is to align the equal signs in an enum, but not in the rest of the code. When they are both Left, I get the desired result. The reduced version of my . c = 3, }; Is there any way to align the initialization part like the one shown below. While BreakBeforeBinaryOperators: All does force wrapping (see @eric-backus' s answer), it impacts formatting in many other places too, unrelated to struct declaration. uxdbst qzre vjsxgbj eccl pyak nfuzett hfghz xcn xofmf zrxkxady