[llvm-branch-commits] [llvm] af970cd - [ScalarEvolution] Handle addrec incoming value in isImpliedViaMerge() (#126236)

[Tom Stellard via llvm-branch-commits]

Author: Nikita Popov
Date: 2025-02-11T14:14:21-08:00
New Revision: af970cd8753c37e7fcf66b6211f2a2d1e261325c

URL: https://github.com/llvm/llvm-project/commit/af970cd8753c37e7fcf66b6211f2a2d1e261325c
DIFF: https://github.com/llvm/llvm-project/commit/af970cd8753c37e7fcf66b6211f2a2d1e261325c.diff

LOG: [ScalarEvolution] Handle addrec incoming value in isImpliedViaMerge() (#126236)

The code already guards against values coming from a previous iteration
using properlyDominates(). However, addrecs are considered to properly
dominate the loop they are defined in.

Handle this special case separately, by checking for expressions that
have computable loop evolution (this should cover cases like a zext of
an addrec as well).

I considered changing the definition of properlyDominates() instead, but
decided against it. The current definition is useful in other context,
e.g. when deciding whether an expression is safe to expand in a given
block.

Fixes https://github.com/llvm/llvm-project/issues/126012.

(cherry picked from commit 7aed53eb1982113e825534f0f66d0a0e46e7a5ed)

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp
    llvm/test/Transforms/IndVarSimplify/pr126012.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 2ce40877b523e..c71202c8dd58e 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -12402,6 +12402,12 @@ bool ScalarEvolution::isImpliedViaMerge(CmpPredicate Pred, const SCEV *LHS,
       // iteration of a loop.
       if (!properlyDominates(L, LBB))
         return false;
+      // Addrecs are considered to properly dominate their loop, so are missed
+      // by the previous check. Discard any values that have computable
+      // evolution in this loop.
+      if (auto *Loop = LI.getLoopFor(LBB))
+        if (hasComputableLoopEvolution(L, Loop))
+          return false;
       if (!ProvedEasily(L, RHS))
         return false;
     }

diff  --git a/llvm/test/Transforms/IndVarSimplify/pr126012.ll b/llvm/test/Transforms/IndVarSimplify/pr126012.ll
index 725ea89b8e651..5189fe020dd3b 100644
--- a/llvm/test/Transforms/IndVarSimplify/pr126012.ll
+++ b/llvm/test/Transforms/IndVarSimplify/pr126012.ll
@@ -1,18 +1,22 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
 ; RUN: opt -S -passes=indvars < %s | FileCheck %s
 
-; FIXME: This is a miscompile.
+; Do not infer that %cmp is true. The %indvar3 input of %indvar2 comes from
+; a previous iteration, so we should not compare it to a value from the current
+; iteration.
 define i32 @test() {
 ; CHECK-LABEL: define i32 @test() {
 ; CHECK-NEXT:  [[ENTRY:.*]]:
 ; CHECK-NEXT:    br label %[[FOR_PREHEADER:.*]]
 ; CHECK:       [[FOR_PREHEADER]]:
 ; CHECK-NEXT:    [[INDVAR1:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[PHI:%.*]], %[[FOR_INC:.*]] ]
-; CHECK-NEXT:    [[INDVAR3:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[INC:%.*]], %[[FOR_INC]] ]
+; CHECK-NEXT:    [[INDVAR2:%.*]] = phi i32 [ 1, %[[ENTRY]] ], [ [[INDVAR3:%.*]], %[[FOR_INC]] ]
+; CHECK-NEXT:    [[INDVAR3]] = phi i32 [ 0, %[[ENTRY]] ], [ [[INC:%.*]], %[[FOR_INC]] ]
 ; CHECK-NEXT:    [[COND1:%.*]] = icmp eq i32 [[INDVAR3]], 0
 ; CHECK-NEXT:    br i1 [[COND1]], label %[[FOR_INC]], label %[[FOR_END:.*]]
 ; CHECK:       [[FOR_END]]:
-; CHECK-NEXT:    [[EXT:%.*]] = zext i1 true to i32
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i32 [[INDVAR2]], 0
+; CHECK-NEXT:    [[EXT:%.*]] = zext i1 [[CMP]] to i32
 ; CHECK-NEXT:    br label %[[FOR_INC]]
 ; CHECK:       [[FOR_INC]]:
 ; CHECK-NEXT:    [[PHI]] = phi i32 [ [[EXT]], %[[FOR_END]] ], [ 0, %[[FOR_PREHEADER]] ]