[PATCH] D120651: [SCEV] Only verify BECounts for reachable loops (PR50523)

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: mkazantsev, reames.
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For unreachable loops, any BECount is legal, and since D98706 <https://reviews.llvm.org/D98706> SCEV can make use of this for loops that are unreachable due to constant branches. To avoid false positives, adjust SCEV verification to only check BECounts in reachable loops.

Fixes https://github.com/llvm/llvm-project/issues/50523.


https://reviews.llvm.org/D120651

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Transforms/IndVarSimplify/X86/pr35406.ll


Index: llvm/test/Transforms/IndVarSimplify/X86/pr35406.ll
===================================================================
--- llvm/test/Transforms/IndVarSimplify/X86/pr35406.ll
+++ llvm/test/Transforms/IndVarSimplify/X86/pr35406.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -indvars %s | FileCheck %s
+; RUN: opt -S -passes='loop(indvars),verify<scalar-evolution>' %s | FileCheck %s
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
 target triple = "x86_64-unknown-linux-gnu"
 
Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -13410,6 +13410,25 @@
   SCEVTraversal<FindUsedLoops>(F).visitAll(S);
 }
 
+static void getReachableBlocks(SmallPtrSetImpl<BasicBlock *> &Reachable,
+                               Function &F) {
+  SmallVector<BasicBlock *> Worklist;
+  Worklist.push_back(&F.getEntryBlock());
+  while (!Worklist.empty()) {
+    BasicBlock *BB = Worklist.pop_back_val();
+    if (!Reachable.insert(BB).second)
+      continue;
+
+    const APInt *Cond;
+    BasicBlock *TrueBB, *FalseBB;
+    if (match(BB->getTerminator(),
+              m_Br(m_APInt(Cond), m_BasicBlock(TrueBB), m_BasicBlock(FalseBB))))
+      Worklist.push_back(Cond->isOne() ? TrueBB : FalseBB);
+    else
+      append_range(Worklist, successors(BB));
+  }
+}
+
 void ScalarEvolution::verify() const {
   ScalarEvolution &SE = *const_cast<ScalarEvolution *>(this);
   ScalarEvolution SE2(F, TLI, AC, DT, LI);
@@ -13434,11 +13453,18 @@
   };
 
   SCEVMapper SCM(SE2);
+  SmallPtrSet<BasicBlock *, 16> ReachableBlocks;
+  getReachableBlocks(ReachableBlocks, F);
 
   while (!LoopStack.empty()) {
     auto *L = LoopStack.pop_back_val();
     llvm::append_range(LoopStack, *L);
 
+    // Only verify BECounts in reachable loops. For an unreachable loop,
+    // any BECount is legal.
+    if (!ReachableBlocks.contains(L->getHeader()))
+      continue;
+
     auto *CurBECount = SCM.visit(
         const_cast<ScalarEvolution *>(this)->getBackedgeTakenCount(L));
     auto *NewBECount = SE2.getBackedgeTakenCount(L);


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