[PATCH] D81449: [JumpThreading] Handle zero !prof branch_weights

[Yevgeny Rouban via Phabricator via llvm-commits]

yrouban updated this revision to Diff 270032.
yrouban marked an inline comment as done.
yrouban added a comment.

added comment. Moved BP variable to its definition.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D81449/new/

https://reviews.llvm.org/D81449

Files:
  llvm/lib/Transforms/Scalar/JumpThreading.cpp
  llvm/test/Transforms/JumpThreading/threading_prof3.ll


Index: llvm/test/Transforms/JumpThreading/threading_prof3.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/JumpThreading/threading_prof3.ll
@@ -0,0 +1,30 @@
+; RUN: opt -jump-threading -S < %s | FileCheck %s
+; RUN: opt -passes=jump-threading -S < %s | FileCheck %s
+
+; Check that all zero branch weights do not cause a crash.
+define void @zero_branch_weights(i32 %tmp, i32 %tmp3) {
+bb:
+  %tmp1 = icmp eq i32 %tmp, 1
+  br i1 %tmp1, label %bb5, label %bb2
+; CHECK-NOT: br i1 %tmp1,{{.*}} !prof
+
+bb2:                                              
+  %tmp4 = icmp ne i32 %tmp3, 1
+  br label %bb5
+; CHECK: br i1 %tmp4, {{.*}} !prof ![[PROF:[0-9]+]]
+
+bb5:                                             
+  %tmp6 = phi i1 [ false, %bb ], [ %tmp4, %bb2 ]
+  br i1 %tmp6, label %bb8, label %bb7, !prof !{!"branch_weights", i32 0, i32 0}
+
+bb7:                                            
+  br label %bb9
+
+bb8: 
+  br label %bb9
+
+bb9:                                           
+  ret void
+}
+
+;CHECK: ![[PROF]] = !{!"branch_weights", i32 0, i32 0}
Index: llvm/lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -214,11 +214,16 @@
   if (!CondBr)
     return;
 
-  BranchProbability BP;
   uint64_t TrueWeight, FalseWeight;
   if (!CondBr->extractProfMetadata(TrueWeight, FalseWeight))
     return;
 
+  if (TrueWeight + FalseWeight == 0)
+    // Zero branch_weights do not give a hint for getting branch probabilities.
+    // Technically it would result in division by zero denominator, which is
+    // TrueWeight + FalseWeight.
+    return;
+
   // Returns the outgoing edge of the dominating predecessor block
   // that leads to the PhiNode's incoming block:
   auto GetPredOutEdge =
@@ -253,10 +258,11 @@
     if (!CI || !CI->getType()->isIntegerTy(1))
       continue;
 
-    BP = (CI->isOne() ? BranchProbability::getBranchProbability(
-                            TrueWeight, TrueWeight + FalseWeight)
-                      : BranchProbability::getBranchProbability(
-                            FalseWeight, TrueWeight + FalseWeight));
+    BranchProbability BP =
+        (CI->isOne() ? BranchProbability::getBranchProbability(
+                           TrueWeight, TrueWeight + FalseWeight)
+                     : BranchProbability::getBranchProbability(
+                           FalseWeight, TrueWeight + FalseWeight));
 
     auto PredOutEdge = GetPredOutEdge(PN->getIncomingBlock(i), BB);
     if (!PredOutEdge.first)


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