[cfe-dev] Alignment of globals and target datalayout

[Rafael Espíndola]

On 15 April 2014 13:29, Moritz Roth <moritz.roth11 at gmail.com> wrote:
> Hi,
>
> I've run into some interesting behaviour with how globals are aligned - it
> seems like the alignment preferences specified in the target datalayout are
> being ignored. Example:
>
> $ cat test.c
> void f(const char *);
> void g(const char *c) {
>   const char *b = "jgksj";
>   return f(b + 1);
> }
>
> $ clang -target arm-none-linux-gnueabi -mcpu=cortex-m0 test.c -S -emit-llvm
> -o - -O0
> ; ModuleID = 'test.c'
> target datalayout =
> "e-m:e-p:32:32-i1:8:32-i8:8:32-i16:16:32-i64:64-v128:64:128-a:0:32-n32-S64"
> target triple = "thumbv6m-none-linux-gnueabi"
>
> @.str = private unnamed_addr constant [6 x i8] c"jgksj\00", align 1
>
> (...)
>
> Shouldn't @.str be aligned to the preferred alignment of i8, i.e. 4?
>
> If I set MinGlobalAlignment for my target to 32, @.str is aligned to 4 bytes
> (as expected). This leads me to believe that the behaviour is caused by
> ASTContext::getAlignOfGlobalVar, which in turn calls
> ASTContext::getTypeAlign - and from there gets the ABI alignment (which of
> course is 8 bits / 1 in this case).
>
> I imagine a fix would do something like
>
> unsigned ASTContext::getAlignOfGlobalVar(QualType T) const {
>   return std::max(getPreferredTypeAlign(T.getTypePtr()),
> getTargetInfo().getMinGlobalAlign());
> }
>
> However, I'd then also need to change getPreferredTypeAlign so it actually
> considers what the target specifies as the preferred alignment, not just the
> ABI align and type size. Is this the right place to make changes? Or is all
> this intended behaviour?

I think it is safe, but changing getAlignOfGlobalVar to use the
preferred alignment would be a big change for all targets, so it would
need a really large benchmarking run. I think it would also prevent
the linker from merging global strings.

> Any opinions would be greatly appreciated.
>
> Thanks
> Moritz

Cheers,
Rafael