CodeGen: More accurate mayAlias for instructions with multiple MMOs (#166211)

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.

Author: nhaehnle

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;
 }

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