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[LCSSA] Do not remove used PHI nodes in formLCSSAForInstructions 

Summary:
In formLCSSAForInstructions we speculatively add new PHI
nodes, that sometimes ends up without having any uses. It
has been discovered that sometimes an added PHI node can
appear as being unused in one iteration of the Worklist,
although it can end up being used by a PHI node added in
a later iteration. We now check, a second time, that the
PHI node still is unused before we remove it. This avoids
an assert about "Trying to remove a phi with uses." for the
added test case.

Reviewers: davide, mzolotukhin, mattd, dberlin

Reviewed By: mzolotukhin, dberlin

Subscribers: dberlin, mzolotukhin, davide, bjope, uabelho, llvm-commits

Differential Revision: https://reviews.llvm.org/D46422

llvm-svn: 331741
This commit is contained in:
Bjorn Pettersson 2018-05-08 06:59:47 +00:00
parent b0f310d51d
commit 51cebc98f3
2 changed files with 66 additions and 5 deletions
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15
llvm/lib/Transforms/Utils/LCSSA.cpp
View File

@ -226,11 +226,16 @@ bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
insertDebugValuesForPHIs(InstBB, NeedDbgValues);
Changed = true;
}
// Remove PHI nodes that did not have any uses rewritten.
for (PHINode *PN : PHIsToRemove) {
assert (PN->use_empty() && "Trying to remove a phi with uses.");
PN->eraseFromParent();
}
// Remove PHI nodes that did not have any uses rewritten. We need to redo the
// use_empty() check here, because even if the PHI node wasn't used when added
// to PHIsToRemove, later added PHI nodes can be using it. This cleanup is
// not guaranteed to handle trees/cycles of PHI nodes that only are used by
// each other. Such situations has only been noticed when the input IR
// contains unreachable code, and leaving some extra redundant PHI nodes in
// such situations is considered a minor problem.
for (PHINode *PN : PHIsToRemove)
if (PN->use_empty())
PN->eraseFromParent();
return Changed;
}

56
llvm/test/Transforms/LCSSA/remove-phis.ll Normal file
View File

@ -0,0 +1,56 @@
; RUN: opt < %s -lcssa -verify -S -o /dev/null
; This bugpoint reduced test case used to assert when removing unused PHI nodes.
; Just verify that we do not assert/crash.
define void @test() {
entry:
br label %gazank
gazank:
%value = phi i16 [ 0, %entry ], [ undef, %gazonk ]
br i1 undef, label %gazink, label %qqq
gazink:
br i1 undef, label %gazonk, label %infinite.loop.pred
gazonk:
br i1 undef, label %exit1, label %gazank
qqq:
br i1 undef, label %www, label %exit2
www:
br i1 undef, label %qqq, label %foo.pred
foo.pred:
br label %foo
foo:
br i1 undef, label %bar, label %exit1.pred
bar:
br i1 undef, label %foo, label %exit2.pred
unreachable1:
br i1 undef, label %foo, label %exit2.pred
exit1.pred:
br label %exit1
exit1:
ret void
exit2.pred:
br label %exit2
exit2:
ret void
infinite.loop.pred:
br label %infinite.loop
infinite.loop:
%dead = phi i16 [ %value, %infinite.loop.pred ], [ 0, %infinite.loop ]
br label %infinite.loop
}