[SCEVExpander] Do not reuse disjoint or (#80281)

SCEV treats "or disjoint" the same as "add nsw nuw". However, when
expanding, we cannot generally replace an add SCEV node with an "or
disjoint" instruction. Just dropping the poison flag is insufficient in
this case, we would have to actually convert the or into an add.

This is a partial fix for #79861.

(cherry picked from commit 5b8e1a6ebf11b6e93bcc96a0d009febe4bb3d7bc)

2 files changed, 10 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index a1d7f0f..e6f93e7 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -1401,6 +1401,13 @@ canReuseInstruction(ScalarEvolution &SE, const SCEV *S, Instruction *I,
     if (!I)
       return false;
 
+    // Disjoint or instructions are interpreted as adds by SCEV. However, we
+    // can't replace an arbitrary add with disjoint or, even if we drop the
+    // flag. We would need to convert the or into an add.
+    if (auto *PDI = dyn_cast<PossiblyDisjointInst>(I))
+      if (PDI->isDisjoint())
+        return false;
+
     // FIXME: Ignore vscale, even though it technically could be poison. Do this
     // because SCEV currently assumes it can't be poison. Remove this special
     // case once we proper model when vscale can be poison.
diff --git a/llvm/test/Transforms/IndVarSimplify/pr79861.ll b/llvm/test/Transforms/IndVarSimplify/pr79861.ll
index a8e2aa4..7e267d0 100644
--- a/llvm/test/Transforms/IndVarSimplify/pr79861.ll
+++ b/llvm/test/Transforms/IndVarSimplify/pr79861.ll
@@ -75,14 +75,15 @@ define void @expander_or_disjoint(i64 %n) {
 ; CHECK-LABEL: define void @expander_or_disjoint(
 ; CHECK-SAME: i64 [[N:%.*]]) {
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[OR:%.*]] = or i64 [[N]], 1
+; CHECK-NEXT:    [[OR:%.*]] = or disjoint i64 [[N]], 1
+; CHECK-NEXT:    [[TMP0:%.*]] = add i64 [[N]], 1
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[IV_INC:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_INC]] = add i64 [[IV]], 1
 ; CHECK-NEXT:    [[ADD:%.*]] = add i64 [[IV]], [[OR]]
 ; CHECK-NEXT:    call void @use(i64 [[ADD]])
-; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i64 [[IV_INC]], [[OR]]
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i64 [[IV_INC]], [[TMP0]]
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void