[PATCH] D124118: [Peephole-Opt] For one kind of test-after-add pattern, eliminates test if it's correct to do so.

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

In D124118#3463205 <https://reviews.llvm.org/D124118#3463205>, @davidxl wrote:

> I suspect there is something else going on.
>
> The following case works fine without the patch.
>
> define dso_local noundef i64 @_Z3fooPlll(ptr nocapture noundef %0, i64 noundef %1, i64 noundef %2) local_unnamed_addr #0 {
>
>   %4 = load i64, ptr %0, align 8
>   %5 = and i64 %4, 3
>   %6 = icmp eq i64 %5, 0
>   %7 = select i1 %6, i64 %1, i64 %2
>   store i64 %7, ptr %0, align 8
>   ret i64 %5
>
> }
>
> However a simple change below to replace the operand %2 in the select with %5 (output of and operation), then it stops working.
>
> define dso_local noundef i64 @_Z3fooPlll(ptr nocapture noundef %0, i64 noundef %1, i64 noundef %2) local_unnamed_addr #0 {
>
>   %4 = load i64, ptr %0, align 8
>   %5 = and i64 %4, 3
>   %6 = icmp eq i64 %5, 0
>   %7 = select i1 %6, i64 %1, i64 %5
>   store i64 %7, ptr %0, align 8
>   ret i64 %5
>
> }
>
> Can you investigate the difference?  The behavior difference on ARM is also worth comparing with.

There's a peephole in X86DAGToDAGISel::Select and PostProcessISelDAG that both fail if the AND has users other than the TEST.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D124118/new/

https://reviews.llvm.org/D124118