From 1cf013898bdd44bfd8420a04fa7b84fbb9d6d985 Mon Sep 17 00:00:00 2001
From: Sanjay Patel <spatel@rotateright.com>
Date: Tue, 17 Nov 2020 15:09:06 -0500
Subject: [PATCH] [SLP] avoid unreachable code crash/infloop

Example based on the post-commit comments for D88735.
---
 lib/Transforms/Vectorize/SLPVectorizer.cpp    |  6 ++--
 .../SLPVectorizer/X86/horizontal.ll           | 35 +++++++++++++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/lib/Transforms/Vectorize/SLPVectorizer.cpp b/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 4e4b6a8dd1c..e14fa443fec 100644
--- a/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -7686,10 +7686,12 @@ bool SLPVectorizerPass::vectorizeChainsInBlock(BasicBlock *BB, BoUpSLP &R) {
       // Try to vectorize the incoming values of the PHI, to catch reductions
       // that feed into PHIs.
       for (unsigned I = 0, E = P->getNumIncomingValues(); I != E; I++) {
-        // Skip if the incoming block is the current BB for now.
+        // Skip if the incoming block is the current BB for now. Also, bypass
+        // unreachable IR for efficiency and to avoid crashing.
         // TODO: Collect the skipped incoming values and try to vectorize them
         // after processing BB.
-        if (BB == P->getIncomingBlock(I))
+        if (BB == P->getIncomingBlock(I) ||
+            !DT->isReachableFromEntry(P->getIncomingBlock(I)))
           continue;
 
         Changed |= vectorizeRootInstruction(nullptr, P->getIncomingValue(I),
diff --git a/test/Transforms/SLPVectorizer/X86/horizontal.ll b/test/Transforms/SLPVectorizer/X86/horizontal.ll
index f2f3bd0f02a..5663c88b636 100644
--- a/test/Transforms/SLPVectorizer/X86/horizontal.ll
+++ b/test/Transforms/SLPVectorizer/X86/horizontal.ll
@@ -1731,4 +1731,39 @@ exit:
   ret i32 %sum.1
 }
 
+; Make sure we do not crash or infinite loop on ill-formed IR.
+
+define void @unreachable_block() {
+; CHECK-LABEL: @unreachable_block(
+; CHECK-NEXT:  bb.0:
+; CHECK-NEXT:    br label [[BB_1:%.*]]
+; CHECK:       dead:
+; CHECK-NEXT:    [[T0:%.*]] = add i16 [[T0]], undef
+; CHECK-NEXT:    br label [[BB_1]]
+; CHECK:       bb.1:
+; CHECK-NEXT:    [[T1:%.*]] = phi i16 [ undef, [[BB_0:%.*]] ], [ [[T0]], [[DEAD:%.*]] ]
+; CHECK-NEXT:    ret void
+;
+; STORE-LABEL: @unreachable_block(
+; STORE-NEXT:  bb.0:
+; STORE-NEXT:    br label [[BB_1:%.*]]
+; STORE:       dead:
+; STORE-NEXT:    [[T0:%.*]] = add i16 [[T0]], undef
+; STORE-NEXT:    br label [[BB_1]]
+; STORE:       bb.1:
+; STORE-NEXT:    [[T1:%.*]] = phi i16 [ undef, [[BB_0:%.*]] ], [ [[T0]], [[DEAD:%.*]] ]
+; STORE-NEXT:    ret void
+;
+bb.0:
+  br label %bb.1
+
+dead:
+  %t0 = add i16 %t0, undef ; unreachable IR may depend on itself
+  br label %bb.1
+
+bb.1:
+  %t1 = phi i16 [ undef, %bb.0 ], [ %t0, %dead ]
+  ret void
+}
+
 declare i32 @__gxx_personality_v0(...)