[PATCH] D87149: [InstCombine] erase instructions leading up to unreachable

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D87149#2282232 <https://reviews.llvm.org/D87149#2282232>, @lebedev.ri wrote:

> Unreachable BB's were the obvious potential source for non-empty use-count, but i thought instcombine didn't combine instructions in unreachable blocks.
> Maybe that is the bug?

Instcombine tries to remove instructions in unreachable blocks, but we can create circular dependencies that instcombine can't reduce because it doesn't change the CFG. Here's a reduced version of the test (not sure if it's minimal):

  define void @main(i16 %x) {
  entry:
    br label %for.cond
  
  for.cond:
    %p = phi double [ %conv, %g.exit ], [ undef, %entry ]
    br i1 undef, label %for.end, label %for.body
  
  for.body:
    %conv = sitofp i16 %x to double
    unreachable
  
  g.exit:
    br label %for.cond
  
  for.end:
    store double %p, double* undef
    ret void
  }

I guess we could try to eliminate phi operands if an incoming block (g.exit) is unreachable? But simplifycfg should do that already, so I don't know if we want instcombine trying to do that too.


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https://reviews.llvm.org/D87149