[clang] [clang] Implement gcc_struct attribute on Itanium targets (PR #71148)

[Erich Keane via cfe-commits]

Martin =?utf-8?q?Storsjö?= <martin at martin.st>,
Martin =?utf-8?q?Storsjö?= <martin at martin.st>,
Martin =?utf-8?q?Storsjö?= <martin at martin.st>,
Martin =?utf-8?q?Storsjö?= <martin at martin.st>,
Martin =?utf-8?q?Storsjö?= <martin at martin.st>
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In-Reply-To: <llvm.org/llvm/llvm-project/pull/71148 at github.com>


erichkeane wrote:

> It turns out that this change did break the `Sema/struct-packed-align.c` testcase when built in mingw mode (see https://github.com/mstorsjo/llvm-mingw/actions/runs/20208875331/job/58011647890); this can be observed in any build if the testcase is amended like this:
> 
> ```diff
> diff --git a/clang/test/Sema/struct-packed-align.c b/clang/test/Sema/struct-packed-align.c
> index d6d0724da49f..c996a58c11e4 100644
> --- a/clang/test/Sema/struct-packed-align.c
> +++ b/clang/test/Sema/struct-packed-align.c
> @@ -1,5 +1,6 @@
>  // RUN: %clang_cc1 %s -fsyntax-only -verify
>  // RUN: %clang_cc1 %s -fsyntax-only -triple=x86_64-windows-coff -verify
> +// RUN: %clang_cc1 %s -fsyntax-only -triple=x86_64-windows-gnu -verify
>  // RUN: %clang_cc1 %s -fsyntax-only -triple=x86_64-scei-ps4 -verify
>  // RUN: %clang_cc1 %s -fsyntax-only -triple=x86_64-sie-ps5 -verify
>  
> ```
> 
> With this change, we're getting the following new failures:
> 
> ```
> # | error: 'expected-error' diagnostics seen but not expected:
> # |   File /home/martin/code/llvm-project/clang/test/Sema/struct-packed-align.c Line 140: 'n1' declared as an array with a negative size
> # |   File /home/martin/code/llvm-project/clang/test/Sema/struct-packed-align.c Line 141: 'n2' declared as an array with a negative size
> # |   File /home/martin/code/llvm-project/clang/test/Sema/struct-packed-align.c Line 175: 'o1' declared as an array with a negative size
> # | 3 errors generated.
> ```
> 
> Among other, this part of the testcase now behaves differently:
> 
> ```c
> typedef long long  __attribute__((aligned(1))) nt;
> 
> struct nS {
>   char buf_nr;
>   nt start_lba;
> };
> 
> #if defined(_WIN32) && !defined(__declspec) // _MSC_VER is unavailable in cc1.
> // Alignment doesn't affect packing in MS mode.
> extern int n1[sizeof(struct nS) == 16 ? 1 : -1];
> extern int n2[__alignof(struct nS) == 8 ? 1 : -1];
> #else
> extern int n1[sizeof(struct nS) == 9 ? 1 : -1];
> extern int n2[__alignof(struct nS) == 1 ? 1 : -1];
> #endif
> ```

That seems reasonably revert worrthy until we can re-apply this.  Would you like to create the revert patch? 

https://github.com/llvm/llvm-project/pull/71148