[PATCH] D84399: [SCEVExpander] Avoid re-using existing casts if it means updating users.

[Florian Hahn via Phabricator via llvm-commits]

fhahn added inline comments.


================
Comment at: llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp:112
+  // If we skipped past MustDominate, use MustDominate as insertion point.
+  if (IP->getParent() == MustDominate->getParent() &&
+      !IP->comesBefore(MustDominate))
----------------
efriedma wrote:
> I'm not sure I understand what's happening here. "I" dominates "MustDominate" because the we're inserting an instruction that uses I. And we're assuming MustDominate itself must be a legal insertion point. So the only way we could skip past MustDominate is if IsInsertedInstruction() is true for MustDominate.
> 
> Is it actually possible for isInsertedInstruction(MustDominate) to be true? If it is, could we check it directly?

> Is it actually possible for isInsertedInstruction(MustDominate) to be true? If it is, could we check it directly?

Unfortunately, at the moment there is a case where we might mark existing instructions as 'inserted' (
https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp#L1247)

This check is indeed a workaround, in case such an instruction is an insert point, so checking directly for it would be more straight forward. I'll update the code, thanks!


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Comment at: llvm/test/Transforms/LoopStrengthReduce/pr27056.ll:39
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[TMP5:%.*]] = inttoptr i64 [[LSR_IV]] to %struct.L*
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq %struct.L* [[UGLYGEP1]], [[TMP]]
----------------
fhahn wrote:
> lebedev.ri wrote:
> > Why were we emitting an unused instruction in the first place?
> That's probably due to the cast re-using logic, which in some cases makes certain values dead by replacing all uses with an equivalent cast.
(which this patch removes)


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D84399/new/

https://reviews.llvm.org/D84399