[llvm] [X86, SimplifyCFG] Support hoisting load/store with conditional faulting (Part II) (PR #108812)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> > > For cases like this why aren't we folding to:
> > > ```
> > > void src(int a, int *c, int *d) {
> > >   if (a)
> > >    *c = a;
> > >   else
> > >    *d = a;
> > > }
> > > 
> > > void tgt(int a, int *c, int *d) {
> > >   int *p = a ? c : d;
> > >   *p = a;
> > > }
> > > ```  
> > >   
> > > or the even more general:
> > > ```
> > > void src(int s, int a, int b, int *c, int *d) {
> > >   if (s)
> > >    *c = a;
> > >   else
> > >    *d = b;
> > > }
> > > 
> > > void tgt(int s, int a, int b, int *c, int *d) {
> > >   int *p = s ? c : d;
> > >   int v = s ? a : b;
> > >   *p = v;
> > > }
> > > ```
> > 
> > 
> > That's a good question. It doesn't rely on hardware feature, so can be a general branch to select transformation. I'm also wondering why didn't we do it before, especially the first one. Is there any concern on possible sideeffect of memory operand?
> 
> From this perspective, should we implement this optimization w/o any HW feature and do further optimization w/ CFCMOV at backend?

Actually, we have already had this optimization done by SimplifyCFG, we just need to increase NumPHIInsts from 1 to 2:
```
$ cat tmp.c
void foo(int a, int *c, int *d) {
  if (a)
   *c = a;
  else
   *d = a;
}

void bar(int s, int a, int b, int *c, int *d) {
  if (s)
   *c = a;
  else
   *d = b;
}

$ clang -S tmp.c -O2 -o - | grep -E ':|^\s[a-z]'
foo:                                    # @foo
# %bb.0:                                # %entry
        testl   %edi, %edi
        cmoveq  %rdx, %rsi
        movl    %edi, (%rsi)
        retq
.Lfunc_end0:
bar:                                    # @bar
# %bb.0:                                # %entry
        testl   %edi, %edi
        cmoveq  %r8, %rcx
        cmovel  %edx, %esi
        movl    %esi, (%rcx)
        retq
```

https://github.com/llvm/llvm-project/pull/108812