[llvm] fee3308 - [SCEV] Fix false-positive recognition of simple recurrences. PR49856
Max Kazantsev via llvm-commits
llvm-commits at lists.llvm.org
Tue Apr 6 23:55:35 PDT 2021
Author: Max Kazantsev
Date: 2021-04-07T13:55:17+07:00
New Revision: fee330824a2b3d7e502a27e1464c418a4663d7a3
URL: https://github.com/llvm/llvm-project/commit/fee330824a2b3d7e502a27e1464c418a4663d7a3
DIFF: https://github.com/llvm/llvm-project/commit/fee330824a2b3d7e502a27e1464c418a4663d7a3.diff
LOG: [SCEV] Fix false-positive recognition of simple recurrences. PR49856
A value from reachable block may come to a Phi node as its input from
unreachable block. This may confuse matchSimpleRecurrence which
has no access to DomTree and can falsely recognize something as a recurrency
because of this effect, as the attached test shows.
Patch `ae7b1e` deals with half of this problem, but it only accounts from
the case when an unreachable instruction comes to Phi as an input.
This patch provides a generalization by checking that no Phi block's
predecessor is unreachable (no matter what the input is).
Differential Revision: https://reviews.llvm.org/D99929
Reviewed By: reames
Added:
llvm/test/Analysis/ScalarEvolution/pr49856.ll
Modified:
llvm/lib/Analysis/ScalarEvolution.cpp
Removed:
################################################################################
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 5a3aa82113da8..a481c23c3d05c 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -5664,17 +5664,18 @@ getRangeForUnknownRecurrence(const SCEVUnknown *U) {
if (!P)
return CR;
+ // Make sure that no Phi input comes from an unreachable block. Otherwise,
+ // even the values that are not available in these blocks may come from them,
+ // and this leads to false-positive recurrence test.
+ for (auto *Pred : predecessors(P->getParent()))
+ if (!DT.isReachableFromEntry(Pred))
+ return CR;
+
BinaryOperator *BO;
Value *Start, *Step;
if (!matchSimpleRecurrence(P, BO, Start, Step))
return CR;
- if (!DT.isReachableFromEntry(P->getParent()) ||
- !DT.isReachableFromEntry(BO->getParent()))
- // If either is in unreachable code, dominance collapses and none of our
- // expected post conditions about loops hold.
- return CR;
-
// If we found a recurrence in reachable code, we must be in a loop. Note
// that BO might be in some subloop of L, and that's completely okay.
auto *L = LI.getLoopFor(P->getParent());
diff --git a/llvm/test/Analysis/ScalarEvolution/pr49856.ll b/llvm/test/Analysis/ScalarEvolution/pr49856.ll
new file mode 100644
index 0000000000000..b9ee43981e78b
--- /dev/null
+++ b/llvm/test/Analysis/ScalarEvolution/pr49856.ll
@@ -0,0 +1,24 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py
+; RUN: opt < %s -analyze -enable-new-pm=0 -scalar-evolution | FileCheck %s
+; RUN: opt < %s -disable-output "-passes=print<scalar-evolution>" 2>&1 | FileCheck %s
+
+define void @test() {
+; CHECK-LABEL: 'test'
+; CHECK-NEXT: Classifying expressions for: @test
+; CHECK-NEXT: %tmp = phi i32 [ 2, %bb ], [ %tmp2, %bb3 ]
+; CHECK-NEXT: --> %tmp U: [1,-2147483648) S: [0,-2147483648)
+; CHECK-NEXT: %tmp2 = add nuw nsw i32 %tmp, 1
+; CHECK-NEXT: --> (1 + %tmp)<nuw> U: [1,-2147483647) S: [1,-2147483647)
+; CHECK-NEXT: Determining loop execution counts for: @test
+;
+bb:
+ br label %bb1
+
+bb1: ; preds = %bb3, %bb
+ %tmp = phi i32 [ 2, %bb ], [ %tmp2, %bb3 ]
+ %tmp2 = add nuw nsw i32 %tmp, 1
+ ret void
+
+bb3: ; No predecessors!
+ br label %bb1
+}
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