[llvm] [CodeGen] Drop disjoint flag when reassociating (PR #156218)

[Philip Reames via llvm-commits]

https://github.com/preames created https://github.com/llvm/llvm-project/pull/156218

This fixes a latent miscompile.  To understand why the flag can't be preserved, consider the case where a0=0, a1=0, a2=-1, and s3=-1.

>From 6d5c5026669fe74d0586a17946c0f271c4055ece Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Sat, 30 Aug 2025 18:37:10 -0700
Subject: [PATCH] [CodeGen] Drop disjoint flag when reassociating

This fixes a latent miscompile.  To understand why the flag can't be
preserved, consider the case where a0=0, a1=0, a2=-1, and s3=-1.
---
 llvm/lib/CodeGen/TargetInstrInfo.cpp            | 2 ++
 llvm/test/CodeGen/RISCV/machine-combiner-mir.ll | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/CodeGen/TargetInstrInfo.cpp b/llvm/lib/CodeGen/TargetInstrInfo.cpp
index 9e49dddd46ba6..7ba65f0418a6b 100644
--- a/llvm/lib/CodeGen/TargetInstrInfo.cpp
+++ b/llvm/lib/CodeGen/TargetInstrInfo.cpp
@@ -1456,11 +1456,13 @@ void TargetInstrInfo::reassociateOps(
   MIB1->clearFlag(MachineInstr::MIFlag::NoSWrap);
   MIB1->clearFlag(MachineInstr::MIFlag::NoUWrap);
   MIB1->clearFlag(MachineInstr::MIFlag::IsExact);
+  MIB1->clearFlag(MachineInstr::MIFlag::Disjoint);
 
   MIB2->setFlags(IntersectedFlags);
   MIB2->clearFlag(MachineInstr::MIFlag::NoSWrap);
   MIB2->clearFlag(MachineInstr::MIFlag::NoUWrap);
   MIB2->clearFlag(MachineInstr::MIFlag::IsExact);
+  MIB2->clearFlag(MachineInstr::MIFlag::Disjoint);
 
   setSpecialOperandAttr(Root, Prev, *MIB1, *MIB2);
 
diff --git a/llvm/test/CodeGen/RISCV/machine-combiner-mir.ll b/llvm/test/CodeGen/RISCV/machine-combiner-mir.ll
index a74dc57a46c82..79268defa8c7b 100644
--- a/llvm/test/CodeGen/RISCV/machine-combiner-mir.ll
+++ b/llvm/test/CodeGen/RISCV/machine-combiner-mir.ll
@@ -116,8 +116,8 @@ define i64 @test_or_flags(i64 %a0, i64 %a1, i64 %a2, i64 %a3) {
   ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:gpr = COPY $x11
   ; CHECK-NEXT:   [[COPY3:%[0-9]+]]:gpr = COPY $x10
   ; CHECK-NEXT:   [[OR:%[0-9]+]]:gpr = OR [[COPY3]], [[COPY2]]
-  ; CHECK-NEXT:   [[OR1:%[0-9]+]]:gpr = disjoint OR [[COPY1]], [[COPY]]
-  ; CHECK-NEXT:   [[OR2:%[0-9]+]]:gpr = disjoint OR killed [[OR]], killed [[OR1]]
+  ; CHECK-NEXT:   [[OR1:%[0-9]+]]:gpr = OR [[COPY1]], [[COPY]]
+  ; CHECK-NEXT:   [[OR2:%[0-9]+]]:gpr = OR killed [[OR]], killed [[OR1]]
   ; CHECK-NEXT:   $x10 = COPY [[OR2]]
   ; CHECK-NEXT:   PseudoRET implicit $x10
   %t0 = or i64 %a0, %a1