[llvm] Fix SCC enter block collection in BPI (PR #210580)

[via llvm-commits]

llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: SomeFlyingThing

<details>
<summary>Changes</summary>

 Fixes incorrect SCC enter-block collection in BPI, getSccEnterBlocks is documented to collect blocks outside an SCC that have edges into the SCC. It was instead adding the SCC header block itself, which
 prevented estimated cold loop weights from propagating to the actual predecessor entering the irreducible SCC.

---
Full diff: https://github.com/llvm/llvm-project/pull/210580.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/BranchProbabilityInfo.cpp (+1-1) 
- (modified) llvm/test/Analysis/BranchProbabilityInfo/loop.ll (+30) 


``````````diff
diff --git a/llvm/lib/Analysis/BranchProbabilityInfo.cpp b/llvm/lib/Analysis/BranchProbabilityInfo.cpp
index 1b0a8b8e74f4a..feb0b34cd4c48 100644
--- a/llvm/lib/Analysis/BranchProbabilityInfo.cpp
+++ b/llvm/lib/Analysis/BranchProbabilityInfo.cpp
@@ -391,7 +391,7 @@ void BPIConstruction::SccInfo::getSccEnterBlocks(
     if (isSCCHeader(BB, SccNum))
       for (const auto *Pred : predecessors(BB))
         if (getSCCNum(Pred) != SccNum)
-          Enters.push_back(const_cast<BasicBlock *>(BB));
+          Enters.push_back(const_cast<BasicBlock *>(Pred));
   }
 }
 
diff --git a/llvm/test/Analysis/BranchProbabilityInfo/loop.ll b/llvm/test/Analysis/BranchProbabilityInfo/loop.ll
index ffac1cd466641..e6b8429eddaa7 100644
--- a/llvm/test/Analysis/BranchProbabilityInfo/loop.ll
+++ b/llvm/test/Analysis/BranchProbabilityInfo/loop.ll
@@ -722,4 +722,34 @@ exit:
 }
 
 
+; Check that a cold irreducible loop propagates its estimated weight through
+; the blocks entering the SCC.
+define void @test19(i1 %arg, i1 %arg2, i1 %arg3, i1 %arg4) {
+; CHECK: edge %entry -> %dispatch probability is 0x078780e3 / 0x80000000 = 5.88%
+; CHECK: edge %entry -> %exit probability is 0x78787f1d / 0x80000000 = 94.12% [HOT edge]
+
+entry:
+  br i1 %arg, label %dispatch, label %exit
+
+dispatch:
+  br i1 %arg2, label %entry1, label %entry2
+
+entry1:
+  br label %loop1
+
+entry2:
+  br label %loop2
+
+loop1:
+  br i1 %arg3, label %loop2, label %cold
+
+loop2:
+  br i1 %arg4, label %loop1, label %cold
 
+cold:
+  call void @cold()
+  br label %exit
+
+exit:
+  ret void
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/210580