[llvm] CodeGen: More accurate mayAlias for instructions with multiple MMOs (PR #166211)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle updated https://github.com/llvm/llvm-project/pull/166211

>From c3abece5bc6dec21ed18e6d6272000c59c88127b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicolai=20H=C3=A4hnle?= <nicolai.haehnle at amd.com>
Date: Fri, 3 Oct 2025 18:20:22 -0700
Subject: [PATCH] CodeGen: More accurate mayAlias for instructions with
 multiple MMOs

There can only be meaningful aliasing between the memory accesses of
different instructions if at least one of the accesses modifies memory.

This check is applied at the instruction-level earlier in the method.
This change merely extends the check on a per-MMO basis.

This affects a SystemZ test because PFD instructions are both mayLoad
and mayStore but may carry a load-only MMO which is now no longer
treated as aliasing loads. The PFD instructions are from llvm.prefetch
generated by loop-data-prefetch.

commit-id:667859fc
---
 llvm/lib/CodeGen/MachineInstr.cpp             | 8 ++++++--
 llvm/test/CodeGen/SystemZ/vec-load-element.ll | 4 ++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/CodeGen/MachineInstr.cpp b/llvm/lib/CodeGen/MachineInstr.cpp
index 8ad9245a47684..37e5c517d24d8 100644
--- a/llvm/lib/CodeGen/MachineInstr.cpp
+++ b/llvm/lib/CodeGen/MachineInstr.cpp
@@ -1547,10 +1547,14 @@ bool MachineInstr::mayAlias(BatchAAResults *AA, const MachineInstr &Other,
 
   // Check each pair of memory operands from both instructions, which can't
   // alias only if all pairs won't alias.
-  for (auto *MMOa : memoperands())
-    for (auto *MMOb : Other.memoperands())
+  for (auto *MMOa : memoperands()) {
+    for (auto *MMOb : Other.memoperands()) {
+      if (!MMOa->isStore() && !MMOb->isStore())
+        continue;
       if (MemOperandsHaveAlias(MFI, AA, UseTBAA, MMOa, MMOb))
         return true;
+    }
+  }
 
   return false;
 }
diff --git a/llvm/test/CodeGen/SystemZ/vec-load-element.ll b/llvm/test/CodeGen/SystemZ/vec-load-element.ll
index 2baaed19546df..9bef279d7c0fa 100644
--- a/llvm/test/CodeGen/SystemZ/vec-load-element.ll
+++ b/llvm/test/CodeGen/SystemZ/vec-load-element.ll
@@ -5,8 +5,8 @@
 ; CHECK-LABEL: .LBB0_1:
 ; CHECK-NOT: l %r
 ; CHECK-NOT: vlvgf
-; CHECK: pfd
-; CHECK: vlef
+; CHECK-DAG: pfd
+; CHECK-DAG: vlef
 
 %type0 = type { i32, [400 x i8], i32, i32, i32, i32, i32, i32, i32, i32, i32, i32 }
 @Mem = external global [150 x %type0], align 4