[LLVMdev] Miscompilation

[Chris Lattner]

On Thu, 13 Dec 2007, Sanghyeon Seo wrote:
> http://llvm.org/bugs/show_bug.cgi?id=1837
> I reduced the above case to this:
> ; test.ll
> define i8* @test() {
> %x1 = malloc i8, i32 16
> %x2 = bitcast i8* %x1 to i8**
> store i8* %x1, i8** %x2
> ret i8* %x1
> }
>
> $ llvm-as test.ll
> $ opt -instcombine test.bc -o testopt.bc
> $ llc test.bc
> $ llc testopt.bc
> $ diff -u test.s testopt.s
> -movl $16, (%esp)
> +movl $8, (%esp)
> call malloc
>
> I'm afraid that I'm not up to fixing this myself. Please help!

Excellent reduction.  It turns out that the reason for this is that the 
LLVM module doesn't have a target data string, so it's assuming (for 
historical reasons) that the target data layout matches SparcV9, which has 
64-bit pointers.  I committed a fix as r45003 of clang.

Be aware that the target layout info is currently stubbed out (see 
TargetInfo::getTargetDescription in clang), so it will always be fixed to 
what darwin/x86 expects (little-endian 32-bit etc).  This should be close 
enough to work fine for linux/i386, but it would be better to improve the 
clang target interface to return the right strings.

-Chris

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