[llvm] fee3308 - [SCEV] Fix false-positive recognition of simple recurrences. PR49856

[Max Kazantsev via llvm-commits]

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
+}