[llvm] r252099 - Define portable macros for packed struct definitions:

[David Blaikie via llvm-commits]

On Wed, Nov 4, 2015 at 3:42 PM, Xinliang David Li via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: davidxl
> Date: Wed Nov  4 17:42:56 2015
> New Revision: 252099
>
> URL: http://llvm.org/viewvc/llvm-project?rev=252099&view=rev
> Log:
> Define portable macros for packed struct definitions:
>
>  1. A macro with argument: LLVM_PACKED(StructDefinition)
>  2. A pair of macros defining scope of region with packing:
>     LLVM_PACKED_START
>      struct A { ... };
>      struct B { ... };
>     LLVM_PACKED_END
>

What are we planning to use these for (I imagine there are already some
uses of packed macros/etc in various parts of LLVM that this will be used
to replace? What are they? What new uses do you have in mind?) I'm a little
concerned, because this tend to leads to non-portable code trying to parse
& then use structs with misaligned members, etc... (which, if I understand
correctly, is only a perf penalty on X86, but potentially a correctness
problem on other platforms we support/care about (not to mention the known
bits stuff we do might assume certain bits are zero when they aren't, etc))


>
> Differential Revision: http://reviews.llvm.org/D14337
>
> Modified:
>     llvm/trunk/include/llvm/Support/Compiler.h
>
> Modified: llvm/trunk/include/llvm/Support/Compiler.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/Compiler.h?rev=252099&r1=252098&r2=252099&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/Support/Compiler.h (original)
> +++ llvm/trunk/include/llvm/Support/Compiler.h Wed Nov  4 17:42:56 2015
> @@ -293,6 +293,34 @@
>  # define LLVM_ALIGNAS(x) alignas(x)
>  #endif
>
> +/// \macro LLVM_PACKED
> +/// \brief Used to specify a packed structure.
> +/// LLVM_PACKED(
> +///    struct A {
> +///      int i;
> +///      int j;
> +///      int k;
> +///      long long l;
> +///   });
> +///
> +/// LLVM_PACKED_START
> +/// struct B {
> +///   int i;
> +///   int j;
> +///   int k;
> +///   long long l;
> +/// };
> +/// LLVM_PACKED_END
> +#ifdef _MSC_VER
> +# define LLVM_PACKED(d) __pragma(pack(push, 1)) d __pragma(pack(pop))
> +# define LLVM_PACKED_START __pragma(pack(push, 1))
> +# define LLVM_PACKED_END   __pragma(pack(pop))
> +#else
> +# define LLVM_PACKED(d) d __attribute__((packed))
> +# define LLVM_PACKED_START _Pragma("pack(push, 1)")
> +# define LLVM_PACKED_END   _Pragma("pack(pop)")
> +#endif
> +
>  /// \macro LLVM_PTR_SIZE
>  /// \brief A constant integer equivalent to the value of sizeof(void*).
>  /// Generally used in combination with LLVM_ALIGNAS or when doing
> computation in
>
>
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