[PATCH] D83562: [fix-irreducible] Skip unreachable predecessors.

[Michael Liao via Phabricator via llvm-commits]

hliao created this revision.
hliao added a reviewer: sameerds.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

- Skip unreachable predecessors during header detection in SCC. Those unreachable blocks would be generated in the switch lowering pass in the corner cases or other frontends. Even though they could be removed through the CFG simplification, we should skip them during header detection.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D83562

Files:
  llvm/lib/Transforms/Utils/FixIrreducible.cpp
  llvm/test/Transforms/FixIrreducible/unreachable.ll


Index: llvm/test/Transforms/FixIrreducible/unreachable.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/FixIrreducible/unreachable.ll
@@ -0,0 +1,24 @@
+; RUN: opt %s -fix-irreducible -S -o - | FileCheck %s
+
+; CHECK-LABEL: @unreachable(
+; CHECK: entry:
+; CHECK-NOT: irr.guard:
+define void @unreachable(i32 %n) {
+entry:
+  br label %loop.body
+
+loop.body:
+  br label %inner.block
+
+unreachable.block:
+  br label %inner.block
+
+inner.block:
+  br i1 undef, label %loop.exit, label %loop.latch
+
+loop.latch:
+  br label %loop.body
+
+loop.exit:
+  ret void
+}
Index: llvm/lib/Transforms/Utils/FixIrreducible.cpp
===================================================================
--- llvm/lib/Transforms/Utils/FixIrreducible.cpp
+++ llvm/lib/Transforms/Utils/FixIrreducible.cpp
@@ -281,6 +281,9 @@
     LLVM_DEBUG(dbgs() << "Found headers:");
     for (auto BB : reverse(Blocks)) {
       for (const auto P : predecessors(BB)) {
+        // Skip unreachable predecessors.
+        if (!DT.isReachableFromEntry(P))
+          continue;
         if (!Blocks.count(P)) {
           LLVM_DEBUG(dbgs() << " " << BB->getName());
           Headers.insert(BB);


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