release/19.x: [ConstraintElim] Bail out on non-dedicated exits when adding exiting conditions (#116627)

[llvmbot]

Backport 52361d0

Requested by: @antoniofrighetto

[llvmbot]

@fhahn What do you think about merging this PR to the release branch?

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: None (llvmbot)

Changes

Backport 52361d0

Requested by: @antoniofrighetto

---

Full diff: https://github.com/llvm/llvm-project/pull/117137.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/ConstraintElimination.cpp (+8-5)
• (modified) llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit.ll (+44)

diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index 37022104d0a9bd..d1c80aa6712433 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -1033,9 +1033,9 @@ void State::addInfoForInductions(BasicBlock &BB) {
       DTN, CmpInst::ICMP_SLT, PN, B,
       ConditionTy(CmpInst::ICMP_SLE, StartValue, B)));
 
-  // Try to add condition from header to the exit blocks. When exiting either
-  // with EQ or NE in the header, we know that the induction value must be u<=
-  // B, as other exits may only exit earlier.
+  // Try to add condition from header to the dedicated exit blocks. When exiting
+  // either with EQ or NE in the header, we know that the induction value must
+  // be u<= B, as other exits may only exit earlier.
   assert(!StepOffset.isNegative() && "induction must be increasing");
   assert((Pred == CmpInst::ICMP_EQ || Pred == CmpInst::ICMP_NE) &&
          "unsupported predicate");
@@ -1043,8 +1043,11 @@ void State::addInfoForInductions(BasicBlock &BB) {
   SmallVector<BasicBlock *> ExitBBs;
   L->getExitBlocks(ExitBBs);
   for (BasicBlock *EB : ExitBBs) {
-    WorkList.emplace_back(FactOrCheck::getConditionFact(
-        DT.getNode(EB), CmpInst::ICMP_ULE, A, B, Precond));
+    // Bail out on non-dedicated exits.
+    if (DT.dominates(&BB, EB)) {
+      WorkList.emplace_back(FactOrCheck::getConditionFact(
+          DT.getNode(EB), CmpInst::ICMP_ULE, A, B, Precond));
+    }
   }
 }
 
diff --git a/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit.ll b/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit.ll
index 15e1d843726278..a04b06e1bf0a52 100644
--- a/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit.ll
+++ b/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit.ll
@@ -763,3 +763,47 @@ exit.2:
   %t.2 = icmp ult i32 %iv, %N
   ret i1 %t.2
 }
+
+define i1 @test_non_dedicated_exit(i16 %n) {
+; CHECK-LABEL: define i1 @test_non_dedicated_exit(
+; CHECK-SAME: i16 [[N:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[COND:%.*]] = icmp slt i16 [[N]], 1
+; CHECK-NEXT:    br i1 [[COND]], label %[[EXIT:.*]], label %[[LOOP_PREHEADER:.*]]
+; CHECK:       [[LOOP_PREHEADER]]:
+; CHECK-NEXT:    [[SUB:%.*]] = add nsw i16 [[N]], -1
+; CHECK-NEXT:    [[EXT:%.*]] = zext nneg i16 [[SUB]] to i32
+; CHECK-NEXT:    br label %[[LOOP:.*]]
+; CHECK:       [[LOOP]]:
+; CHECK-NEXT:    [[INDVAR:%.*]] = phi i32 [ [[INDVAR_INC:%.*]], %[[LOOP_LATCH:.*]] ], [ 0, %[[LOOP_PREHEADER]] ]
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i32 [[INDVAR]], [[EXT]]
+; CHECK-NEXT:    br i1 [[EXITCOND]], label %[[EXIT]], label %[[LOOP_LATCH]]
+; CHECK:       [[LOOP_LATCH]]:
+; CHECK-NEXT:    [[INDVAR_INC]] = add nuw nsw i32 [[INDVAR]], 1
+; CHECK-NEXT:    br label %[[LOOP]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i16 [[N]], 0
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %cond = icmp slt i16 %n, 1
+  br i1 %cond, label %exit, label %loop.preheader
+
+loop.preheader:
+  %sub = add nsw i16 %n, -1
+  %ext = zext nneg i16 %sub to i32
+  br label %loop
+
+loop:
+  %indvar = phi i32 [ %indvar.inc, %loop.latch ], [ 0, %loop.preheader ]
+  %exitcond = icmp eq i32 %indvar, %ext
+  br i1 %exitcond, label %exit, label %loop.latch
+
+loop.latch:
+  %indvar.inc = add nuw nsw i32 %indvar, 1
+  br label %loop
+
+exit:
+  %cmp = icmp sgt i16 %n, 0
+  ret i1 %cmp
+}

[fhahn]

LGTM to cherry pick, thanks!

[github-actions]

@antoniofrighetto (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR.