[llvm] 707836e - [JumpThreading] Handle zero !prof branch_weights

[Yevgeny Rouban via llvm-commits]

Author: Yevgeny Rouban
Date: 2020-06-12T11:55:15+07:00
New Revision: 707836ed4edb21e7133007f0200c3fd3a04d3409

URL: https://github.com/llvm/llvm-project/commit/707836ed4edb21e7133007f0200c3fd3a04d3409
DIFF: https://github.com/llvm/llvm-project/commit/707836ed4edb21e7133007f0200c3fd3a04d3409.diff

LOG: [JumpThreading] Handle zero !prof branch_weights

Avoid division by zero in updatePredecessorProfileMetadata().

Reviewers: yamauchi
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D81499

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/JumpThreading.cpp

Removed: 
    


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diff  --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index d2d71dde4fd5..9d0500419a7f 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -214,11 +214,16 @@ static void updatePredecessorProfileMetadata(PHINode *PN, BasicBlock *BB) {
   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 @@ static void updatePredecessorProfileMetadata(PHINode *PN, BasicBlock *BB) {
     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)