[llvm] [LoopPredication] Fix division by zero in case of zero branch weights (PR #66506)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms
            
<details>
<summary>Changes</summary>
Treat the case where all branch weights are zero as if there was no profile.
Fixes #66382

Should be in line with the approach taken by BPI: https://github.com/llvm/llvm-project/blob/74724902ba2fc0dc2734cdb5fae15cb338f1b635/llvm/lib/Analysis/BranchProbabilityInfo.cpp#L430-L434
--
Full diff: https://github.com/llvm/llvm-project/pull/66506.diff

2 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/LoopPredication.cpp (+3) 
- (modified) llvm/test/Transforms/LoopPredication/pr66382.ll (+20-1) 


<pre>
diff --git a/llvm/lib/Transforms/Scalar/LoopPredication.cpp b/llvm/lib/Transforms/Scalar/LoopPredication.cpp
index a58ab093a1f75d3..55079b4a42d2fae 100644
--- a/llvm/lib/Transforms/Scalar/LoopPredication.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopPredication.cpp
@@ -967,6 +967,9 @@ bool LoopPredication::isLoopProfitableToPredicate() {
           Numerator += Weight;
         Denominator += Weight;
       }
+      // If all weights are zero act as if there was no profile data
+      if (Denominator == 0)
+        return BranchProbability::getBranchProbability(1, NumSucc);
       return BranchProbability::getBranchProbability(Numerator, Denominator);
     } else {
       assert(LatchBlock != ExitingBlock &amp;&amp;
diff --git a/llvm/test/Transforms/LoopPredication/pr66382.ll b/llvm/test/Transforms/LoopPredication/pr66382.ll
index 3ac4cac0615f464..f9a14d470453cf0 100644
--- a/llvm/test/Transforms/LoopPredication/pr66382.ll
+++ b/llvm/test/Transforms/LoopPredication/pr66382.ll
@@ -1,4 +1,4 @@
-; XFAIL: *
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
 ; RUN: opt -S -loop-predication-skip-profitability-checks=false -passes=&#x27;require&lt;scalar-evolution&gt;,loop-mssa(loop-predication)&#x27; %s | FileCheck %s
 
 target triple = &quot;x86_64-unknown-linux-gnu&quot;
@@ -6,7 +6,26 @@ target triple = &quot;x86_64-unknown-linux-gnu&quot;
 ; Function Attrs: nocallback nofree nosync willreturn
 declare void @llvm.experimental.guard(i1, ...) #0
 
+; Check that LoopPredication doesn&#x27;t crash on all-zero branch weights
 define void @foo() {
+; CHECK-LABEL: define void @foo() {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[HEADER:%.*]]
+; CHECK:       Header:
+; CHECK-NEXT:    [[J2:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[J_NEXT:%.*]], [[LATCH:%.*]] ]
+; CHECK-NEXT:    call void (i1, ...) @llvm.experimental.guard(i1 false, i32 0) [ &quot;deopt&quot;() ]
+; CHECK-NEXT:    [[J_NEXT]] = add i64 [[J2]], 1
+; CHECK-NEXT:    br i1 false, label [[LATCH]], label [[EXIT:%.*]]
+; CHECK:       Latch:
+; CHECK-NEXT:    [[SPECULATE_TRIP_COUNT:%.*]] = icmp ult i64 [[J2]], 0
+; CHECK-NEXT:    br i1 [[SPECULATE_TRIP_COUNT]], label [[HEADER]], label [[COMMON_RET_LOOPEXIT:%.*]], !prof [[PROF0:![0-9]+]]
+; CHECK:       common.ret.loopexit:
+; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
+; CHECK:       common.ret:
+; CHECK-NEXT:    ret void
+; CHECK:       exit:
+; CHECK-NEXT:    br label [[COMMON_RET]]
+;
 entry:
   br label %Header
 
</pre>
</details>


https://github.com/llvm/llvm-project/pull/66506