[PATCH] D105392: [InstCombine] Add optimization to prevent poison from being propagated.

[Hyeongyu Kim via Phabricator via llvm-commits]

hyeongyukim added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp:3519
+InstCombinerImpl::pushFreezeToPreventPoisonFromPropagating(FreezeInst &OrigFI) {
+  // Try to push freeze through instructions that propagate but don't produce
+  // poison as far as possible.  If operand of freeze follows three conditions
----------------
lebedev.ri wrote:
> Hm, why would multiple `freeze` would be inserted?
> The transform i envisioned is:
> ```
> %a = <whatever> %x
> %a.frozen = freeze %a
> %b = <whatever> %a.frozen
> %c = <whatever> %a
>  =>
> %a = <whatever> %x
> %a.frozen = freeze %a
> %b = <whatever> %a.frozen
> %c = <whatever> %a.frozen
>  =>
> %x.frozen = freeze %x
> %a.frozen = <whatever> %x.frozen
> %b = <whatever> %a.frozen
> %c = <whatever> %a.frozen
> ```
> My question being, why do we not to change other users of `%a` to use `%a.frozen`?
In terms of correctness, it is still fine.
However, `%a`and `%a.frozen` have different semantics, and the optimization performed in `%a` may not be performed in `%a.frozen`.
I thought it might not be good for the frozen values to spread too much, so single-use `%a` is considered only.



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  https://reviews.llvm.org/D105392/new/

https://reviews.llvm.org/D105392