[MachineLICM] Debug intrinsics shouldn't affect hoist decisions

[gberry]

Summary

When checking if an instruction stores to a given frame index, check
that the instruction can write to memory before looking at the memory
operands list to avoid e.g. DBG_VALUE instructions that reference a
frame index preventing a load from that index from being hoisted.

[qcolombet]

LGTM

llvm/trunk/lib/CodeGen/MachineLICM.cpp

while (!Worklist.empty()) {
     }
   }
 
   return Changed;
 }
 
 /// Return true if instruction stores to the specified frame.
 static bool InstructionStoresToFI(const MachineInstr *MI, int FI) {
+  // Check mayStore before memory operands so that e.g. DBG_VALUEs will return
+  // true since they have no memory operands.
+  if (!MI->mayStore())
+     return false;
   // If we lost memory operands, conservatively assume that the instruction
   // writes to all slots.
   if (MI->memoperands_empty())
     return true;
   for (const MachineMemOperand *MemOp : MI->memoperands()) {
     if (!MemOp->isStore() || !MemOp->getPseudoValue())
       continue;
     if (const FixedStackPseudoSourceValue *Value =

llvm/trunk/test/CodeGen/AArch64/mlicm-stack-write-check.mir

+# RUN: llc -mtriple=aarch64 -run-pass machinelicm -verify-machineinstrs -o - %s | FileCheck %s
+---
+name: test
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: gpr64 }
+stack:
+  - { id: 0, size: 8, type: spill-slot }
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: test
+    ; CHECK-LABEL: bb.0:
+    ; CHECK: $x2 = LDRXui %stack.0, 0
+    liveins: $x0, $x1, $x2
+    B %bb.1
+
+  bb.1:
+    ; CHECK-LABEL: bb.1:
+    ; CHECK-NOT: $x2 = LDRXui %stack.0, 0
+    liveins: $x0
+    DBG_VALUE %stack.0, 0
+    $x2 = LDRXui %stack.0, 0 :: (load 8 from %stack.0)
+    $x0 = ADDXrr $x0, $x2
+    $xzr = SUBSXri $x0, 1, 0, implicit-def $nzcv
+    Bcc 11, %bb.1, implicit $nzcv
+    B %bb.2
+
+  bb.2:
+    liveins: $x0
+    %0 = COPY $x0
+    %0 = COPY $x0  ; Force isSSA = false.
+...