[CodeGenPrepare] avoid crashing from replacing a phi twice

Summary:
This is a fix to bug 41052:
https://bugs.llvm.org/show_bug.cgi?id=41052

While trying to optimize a memory instruction in a dead basic block, we end up registering the same phi for replacement twice. This patch avoids registering more than the first replacement candidate for a phi.

Patch by: JesperAntonsson

Reviewers: skatkov, aprantl

Reviewed By: aprantl

Subscribers: jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59358

llvm-svn: 356260

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index e8498eb375e..44f1709b6e9 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -3282,6 +3282,8 @@ private:
                     PhiNodeSet &PhiNodesToMatch) {
     SmallVector<PHIPair, 8> WorkList;
     Matcher.insert({ PHI, Candidate });
+    SmallSet<PHINode *, 8> MatchedPHIs;
+    MatchedPHIs.insert(PHI);
     WorkList.push_back({ PHI, Candidate });
     SmallSet<PHIPair, 8> Visited;
     while (!WorkList.empty()) {
@@ -3314,8 +3316,10 @@ private:
         if (Matcher.count({ FirstPhi, SecondPhi }))
           continue;
         // So the values are different and does not match. So we need them to
-        // match.
-        Matcher.insert({ FirstPhi, SecondPhi });
+        // match. (But we register no more than one match per PHI node, so that
+        // we won't later try to replace them twice.)
+        if (!MatchedPHIs.insert(FirstPhi).second)
+          Matcher.insert({ FirstPhi, SecondPhi });
         // But me must check it.
         WorkList.push_back({ FirstPhi, SecondPhi });
       }