[PATCH] D48930: [InstCombine] Apply foldICmpUsingKnownBits to early bailout case

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

Bailing out on optimizations is a hack, so I'd prefer not to perpetuate that.

I have no context for this patch other than the one test case shown here. I see 2 potential missing folds that would solve that case (and I suspect that we actually have these folds but they are again guarded by min/max bailout hacks):
https://rise4fun.com/Alive/Pe9U

  Name: narrow min/max in instcombine
    %z = zext i16 %x to i32
    %c = icmp ult i32 %z, 255
    %s = select i1 %c, i32 %z, i32 255
    %t = trunc i32 %s to i16
    =>
    %c2 = icmp ult i16 %x, 255
    %t = select i1 %c2, i16 %x, i16 255
    
  Name: fold min/max in instsimplify
    %a = and i16 %x, 255
    %z = zext i16 %a to i32
    %c = icmp ult i32 %z, 255
    %s = select i1 %c, i32 %z, i32 255
    =>
    %s = %z 

Can we add/fix one of those folds instead of trying to solve this in visitICmpInst()?


https://reviews.llvm.org/D48930