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

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