[LLVMdev] A question w.r.t fence instruction vs. noalias pointer

[Eli Friedman]

On Wed, Jun 12, 2013 at 6:17 PM, Guo, Xiaoyi <Xiaoyi.Guo at amd.com> wrote:

> Hi,
>
> I have the following test case:
>
> define  void @foo(<2 x float>* noalias nocapture %out, <2 x float>*
> noalias nocapture %data0) nounwind {
> entry:
>   %val1 = load <2 x float>* %data0, align 8
>   store <2 x float> %val1, <2 x float>* %out, align 8
>   fence acq_rel
>   %val2 = load <2 x float>* %data0, align 8
>   store <2 x float> %val2, <2 x float>* %out, align 8
>   ret void
> }
>
> If I run it though GVN with BasicAliasAnalysis, GVN does not remove the
> load after the fence.
>

According to the LLVM atomics rules, the compiler is actually completely
free to either hoist the fence to the top of the function or sink it to the
bottom because your loads/stores aren't atomic.  The fact that LLVM doesn't
actually do this is just because I didn't put much effort into alias
analysis for fences; see
http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/AliasAnalysis.h?revision=182755&view=markup#l431.

-Eli
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