[PATCH] D91511: [JumpThreading] Call eraseBlock when folding a conditional branch

[Kazu Hirata via Phabricator via llvm-commits]

kazu created this revision.
kazu added reviewers: yrouban, MaskRay.
Herald added subscribers: llvm-commits, hiraditya.
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kazu requested review of this revision.

This patch teaches the jump threading pass to call BPI->eraseBlock
when it folds a conditional branch.

Without this patch, BranchProbabilityInfo could end up with stale edge
probabilities for the basic block containing the conditional branch --
one edge probability with less than 1.0 and the other for a removed
edge.

This patch is one of the steps before we can safely re-apply D91017 <https://reviews.llvm.org/D91017>.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D91511

Files:
  llvm/lib/Analysis/BranchProbabilityInfo.cpp
  llvm/lib/Transforms/Scalar/JumpThreading.cpp
  llvm/test/Transforms/JumpThreading/thread-prob-2.ll


Index: llvm/test/Transforms/JumpThreading/thread-prob-2.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/JumpThreading/thread-prob-2.ll
@@ -0,0 +1,22 @@
+; RUN: opt -debug-only=branch-prob -jump-threading -S %s 2>&1 | FileCheck %s
+; REQUIRES: asserts
+
+; Make sure that we clear edge probabilities for bb.cond as we fold
+; the conditional branch in it.
+
+define i32 @foo(i1 %cond) !prof !0 {
+bb.entry:
+  br i1 %cond, label %bb.31, label %bb.cond
+
+bb.cond:
+  br i1 %cond, label %bb.31, label %bb.12
+; CHECK: eraseBlock bb.cond
+
+bb.31:
+  ret i32 31
+
+bb.12:
+  ret i32 12
+}
+
+!0 = !{!"function_entry_count", i64 0}
Index: llvm/lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -1697,6 +1697,8 @@
       BranchInst::Create(OnlyDest, Term);
       Term->eraseFromParent();
       DTU->applyUpdatesPermissive(Updates);
+      if (HasProfileData)
+        BPI->eraseBlock(BB);
 
       // If the condition is now dead due to the removal of the old terminator,
       // erase it.
Index: llvm/lib/Analysis/BranchProbabilityInfo.cpp
===================================================================
--- llvm/lib/Analysis/BranchProbabilityInfo.cpp
+++ llvm/lib/Analysis/BranchProbabilityInfo.cpp
@@ -1183,6 +1183,8 @@
 }
 
 void BranchProbabilityInfo::eraseBlock(const BasicBlock *BB) {
+  LLVM_DEBUG(dbgs() << "eraseBlock " << BB->getName() << "\n");
+
   // Note that we cannot use successors of BB because the terminator of BB may
   // have changed when eraseBlock is called as a BasicBlockCallbackVH callback.
   // Instead we remove prob data for the block by iterating successors by their


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