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

[via llvm-commits]

llvmbot wrote:

<!--IGNORE-->
>
><!--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>


Error: Command failed due to missing milestone.

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