From f192a27ed3bacdf727f27857adeb6d3762383295 Mon Sep 17 00:00:00 2001
From: Changpeng Fang <Changpeng.Fang@amd.com>
Date: Fri, 9 Oct 2020 16:20:26 -0700
Subject: [PATCH] Sink: Handle instruction sink when a user is dead

Summary:
  The current instruction sink pass uses findNearestCommonDominator of all users to find block to sink the instruction to.
However, a user may be in a dead block, which will result in unexpected behavior.

This patch handles such cases by skipping dead blocks. This patch fixes:
https://bugs.llvm.org/show_bug.cgi?id=47415

Reviewers:
  MaskRay, arsenm

Differential Revision:
  https://reviews.llvm.org/D89166
---
 llvm/lib/Transforms/Scalar/Sink.cpp    |  3 +++
 llvm/test/Transforms/Sink/dead-user.ll | 42 ++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)
 create mode 100644 llvm/test/Transforms/Sink/dead-user.ll

diff --git a/llvm/lib/Transforms/Scalar/Sink.cpp b/llvm/lib/Transforms/Scalar/Sink.cpp
index c430724..89cfbe3 100644
--- a/llvm/lib/Transforms/Scalar/Sink.cpp
+++ b/llvm/lib/Transforms/Scalar/Sink.cpp
@@ -135,6 +135,9 @@ static bool SinkInstruction(Instruction *Inst,
   for (Use &U : Inst->uses()) {
     Instruction *UseInst = cast<Instruction>(U.getUser());
     BasicBlock *UseBlock = UseInst->getParent();
+    // Don't worry about dead users.
+    if (!DT.isReachableFromEntry(UseBlock))
+      continue;
     if (PHINode *PN = dyn_cast<PHINode>(UseInst)) {
       // PHI nodes use the operand in the predecessor block, not the block with
       // the PHI.
diff --git a/llvm/test/Transforms/Sink/dead-user.ll b/llvm/test/Transforms/Sink/dead-user.ll
new file mode 100644
index 0000000..2d21def
--- /dev/null
+++ b/llvm/test/Transforms/Sink/dead-user.ll
@@ -0,0 +1,42 @@
+; Compiler should not be broken with a dead user.
+; RUN: opt -sink -S < %s | FileCheck %s
+
+; CHECK-LABEL: @test(
+; CHECK:       bb.0:
+; CHECK-NEXT:    %conv = sext i16 %p1 to i32
+; CHECK-NEXT:    br i1 undef, label %bb.1, label %bb.3
+
+; CHECK:       bb.1:                                             ; preds = %bb.0
+; CHECK-NEXT:    br label %bb.2
+
+; CHECK:       bb.2:                                             ; preds = %bb.2, %bb.1
+; CHECK-NEXT:    %and.2 = and i32 undef, %conv
+; CHECK-NEXT:    br label %bb.2
+
+; CHECK:       bb.3:                                             ; preds = %bb.3, %bb.0
+; CHECK-NEXT:    %and.3 = and i32 undef, %conv
+; CHECK-NEXT:    br label %bb.3
+
+; CHECK:       dead:                                             ; preds = %dead
+; CHECK-NEXT:    %and.dead = and i32 undef, %conv
+; CHECK-NEXT:    br label %dead
+define void @test(i16 %p1) {
+bb.0:
+  %conv = sext i16 %p1 to i32
+  br i1 undef, label %bb.1, label %bb.3
+
+bb.1:                                             ; preds = %bb.0
+  br label %bb.2
+
+bb.2:                                             ; preds = %bb.2, %bb.1
+  %and.2 = and i32 undef, %conv
+  br label %bb.2
+
+bb.3:                                             ; preds = %bb.3, %bb.0
+  %and.3 = and i32 undef, %conv
+  br label %bb.3
+
+dead:                                             ; preds = %dead
+  %and.dead = and i32 undef, %conv
+  br label %dead
+}
-- 
2.7.4