[llvm] bb0842a - [BPI] Incorrect probability reported in case of mulptiple edges.

[Evgeniy Brevnov via llvm-commits]

Author: Evgeniy Brevnov
Date: 2020-04-30T11:41:03+07:00
New Revision: bb0842a3f11e5d5a13d3212b627d9f1521bff553

URL: https://github.com/llvm/llvm-project/commit/bb0842a3f11e5d5a13d3212b627d9f1521bff553
DIFF: https://github.com/llvm/llvm-project/commit/bb0842a3f11e5d5a13d3212b627d9f1521bff553.diff

LOG: [BPI] Incorrect probability reported in case of mulptiple edges.

Summary:
By design 'BranchProbabilityInfo:: getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const' should return sum of probabilities over all edges from Src to Dst. Current implementation is buggy and returns 1/num_of_successors if probabilities are not explicitly set.

Note current implementation of BPI printing has an issue as well and annotates each edge with sum of probabilities over all ages from one basic block to another. That's why 30% probability reported (instead of 10%) in the lit test. This is not urgent issue since only printing is affected.
Note also current implementation assumes that either all or none edges have probabilities set. This is not the only place which uses such assumption. At least we should assert that in verifier. In addition we can think on a more robust API of BPI which would prevent situations.

Reviewers: skatkov, yrouban, taewookoh

Reviewed By: skatkov

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79071

Added: 
    llvm/test/Analysis/BranchProbabilityInfo/switch.ll

Modified: 
    llvm/lib/Analysis/BranchProbabilityInfo.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/BranchProbabilityInfo.cpp b/llvm/lib/Analysis/BranchProbabilityInfo.cpp
index 99f253ca96af..d33af83d586a 100644
--- a/llvm/lib/Analysis/BranchProbabilityInfo.cpp
+++ b/llvm/lib/Analysis/BranchProbabilityInfo.cpp
@@ -936,8 +936,10 @@ BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
                                           const BasicBlock *Dst) const {
   auto Prob = BranchProbability::getZero();
   bool FoundProb = false;
+  uint32_t EdgeCount = 0;
   for (const_succ_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
     if (*I == Dst) {
+      ++EdgeCount;
       auto MapI = Probs.find(std::make_pair(Src, I.getSuccessorIndex()));
       if (MapI != Probs.end()) {
         FoundProb = true;
@@ -945,7 +947,7 @@ BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
       }
     }
   uint32_t succ_num = std::distance(succ_begin(Src), succ_end(Src));
-  return FoundProb ? Prob : BranchProbability(1, succ_num);
+  return FoundProb ? Prob : BranchProbability(EdgeCount, succ_num);
 }
 
 /// Set the edge probability for a given edge specified by PredBlock and an

diff  --git a/llvm/test/Analysis/BranchProbabilityInfo/switch.ll b/llvm/test/Analysis/BranchProbabilityInfo/switch.ll
new file mode 100644
index 000000000000..491fdad94d78
--- /dev/null
+++ b/llvm/test/Analysis/BranchProbabilityInfo/switch.ll
@@ -0,0 +1,53 @@
+; RUN: opt < %s -analyze -branch-prob | FileCheck %s
+; RUN: opt < %s -analyze -lazy-branch-prob | FileCheck %s
+; RUN: opt < %s -passes='print<branch-prob>' -disable-output 2>&1 | FileCheck %s
+
+declare void @g(i32)
+
+; Check correctness of reported probabilities in case of multiple edges between
+; basic blocks. In this case sum of probabilities over all edges should be
+; returned by BranchProbabilityInfo::getEdgeProbability.
+
+define void @test1(i32 %x) {
+;CHECK: edge entry -> return probability is 0x0ccccccd / 0x80000000 = 10.00%
+;CHECK: edge entry -> bb0 probability is 0x26666666 / 0x80000000 = 30.00%
+;CHECK: edge entry -> bb0 probability is 0x26666666 / 0x80000000 = 30.00%
+;CHECK: edge entry -> bb0 probability is 0x26666666 / 0x80000000 = 30.00%
+;CHECK: edge entry -> bb1 probability is 0x26666666 / 0x80000000 = 30.00%
+;CHECK: edge entry -> bb1 probability is 0x26666666 / 0x80000000 = 30.00%
+;CHECK: edge entry -> bb1 probability is 0x26666666 / 0x80000000 = 30.00%
+;CHECK: edge entry -> bb2 probability is 0x26666666 / 0x80000000 = 30.00%
+;CHECK: edge entry -> bb2 probability is 0x26666666 / 0x80000000 = 30.00%
+;CHECK: edge entry -> bb2 probability is 0x26666666 / 0x80000000 = 30.00%
+;CHECK: edge bb0 -> return probability is 0x80000000 / 0x80000000 = 100.00% [HOT edge]
+;CHECK: edge bb1 -> return probability is 0x80000000 / 0x80000000 = 100.00% [HOT edge]
+;CHECK: edge bb2 -> return probability is 0x80000000 / 0x80000000 = 100.00% [HOT edge]
+
+entry:
+  switch i32 %x, label %return [
+    i32 0, label %bb0
+    i32 3, label %bb0
+    i32 6, label %bb0
+    i32 1, label %bb1
+    i32 4, label %bb1
+    i32 7, label %bb1
+    i32 2, label %bb2
+    i32 5, label %bb2
+    i32 8, label %bb2
+  ]
+
+bb0:                                              ; preds = %entry, %entry, %entry
+  tail call void @g(i32 0)
+  br label %return
+
+bb1:                                              ; preds = %entry, %entry, %entry
+  tail call void @g(i32 1)
+  br label %return
+
+bb2:                                              ; preds = %entry, %entry, %entry
+  tail call void @g(i32 2)
+  br label %return
+
+return:                                           ; preds = %bb2, %bb1, %bb0, %entry
+  ret void
+}