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

[Nikita Popov via llvm-commits]

https://github.com/nikic updated https://github.com/llvm/llvm-project/pull/80281

>From 60cb1bdaa81cd05e5a6701e00bb0bdce6ab2999c Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Thu, 1 Feb 2024 13:02:19 +0100
Subject: [PATCH] [SCEVExpander] Do not reuse disjoint or

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.
---
 llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp | 7 +++++++
 llvm/test/Transforms/IndVarSimplify/pr79861.ll        | 5 +++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index ed55a13072aaa..34597e6b6e83a 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -1398,6 +1398,13 @@ 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 a8e2aa42a365c..7e267d04c94cc 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