[PATCH] D29013: Add InstCombine/InstructionSimplify support for Freeze Instruction

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added a comment.

In D29013#1723781 <https://reviews.llvm.org/D29013#1723781>, @reames wrote:

> LGTM.
>
> p.s. Do you have a list of items for this work somewhere?  We clearly need support in a number of passes, is there a master list so that we can split work once the IR def pass lands?


Hi, sorry for the delay. I don't have master list, but possible places where freeze can be considered is as follows:

- The loopunswitch bug can be fixed with freeze. https://reviews.llvm.org/D29015 , https://reviews.llvm.org/D29016 , I'll update the patch today or tomorrow.
- select(c, bop(x,y), bop(x,z)) -> bop(x,(select(c,y,z))  is buggy if c is poison and bop is division because it generates division by poison. This can be resolved by freezing c. It is at InstCombiner::foldSelectOpOp of InstCombineSelect.cpp
- Check whether freeze was regarded as a zero-cost instruction when inlining
- Division hoisting

  divisor = y | 1 // divisor is either non-zero or poison
  while (cond) {
    x = 100 / divisor // Hoisting this x may introduce UB, so y should be freezed in advance.
    use(x)
  }

Similar case is having `if (divisor != 0)` check surrounding the while loop. When divisor is undef (and if branching on undef is nondeterministic jump), hoisting the division is not explained. You can freeze the divisor.

- SelectionDAGBuilder::visitBr() tries to lower br i1 (and (icmp ...), (icmp ...)) into two separated branches, but it is blocked if one of the icmps is freezed. Adding transformations like `freeze (icmp a, const) -> icmp (freeze a), const` is helpful for this.


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https://reviews.llvm.org/D29013