[LLVMdev] strict aliasing and LLVM

[Dan Gohman]

On Oct 29, 2010, at 12:26 AM, Nick Lewycky wrote:

> Xinliang David Li wrote:
>> As simple as
>> 
>> void foo (int n, double *p, int *q)
>> {
>>    for (int i = 0; i < n; i++)
>>      *p += *q;
>> }
>> 
>> clang -O2 -fstrict-aliasing -emit-llvm -o foo.bc -c foo.c
>> llc -enable-tbaa -O2 -filetype=asm -o foo.s foo.bc
> 
> There's a couple things interacting here:
> * clang -fstrict-aliasing -O2 does generate the TBAA info, but it runs the optimizers without enabling the -enable-tbaa flag, so the optimizers never look at it. Oops.
> * clang -fstrict-aliasing -O0 does *not* generate the TBAA info in the resulting .bc file. This is probably intended to speed up -O0 builds even if -fstrict-aliasing is set, but is annoying for debugging what's going on under the hood.
> * If clang -O2 worked by running 'opt' and 'llc' under the hood, we could tell it to pass a flag along to them, but it doesn't. As it stands, you can't turn -enable-tbaa on when running clang.

In case there is any confusion, the -enable-tbaa option is temporary.  TBAA is
a new feature which is still under development.

Dan