[GlobalISel][AArch64] Select store of zero to WZR/XZR.

These occur very frequently, and are quite trivial to catch.

llvm-svn: 298862

llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp

@@ -775,6 +775,17 @@ bool AArch64InstructionSelector::select(MachineInstr &I) const {
     I.setDesc(TII.get(NewOpc));
 
     I.addOperand(MachineOperand::CreateImm(0));
+
+    // If we're storing a 0, use WZR/XZR.
+    if (auto CVal = getConstantVRegVal(ValReg, MRI)) {
+      if (*CVal == 0 && Opcode == TargetOpcode::G_STORE) {
+        if (I.getOpcode() == AArch64::STRWui)
+          I.getOperand(0).setReg(AArch64::WZR);
+        else if (I.getOpcode() == AArch64::STRXui)
+          I.getOperand(0).setReg(AArch64::XZR);
+      }
+    }
+
     return constrainSelectedInstRegOperands(I, TII, TRI, RBI);
   }
 

llvm/test/CodeGen/AArch64/GlobalISel/select-store.mir

@@ -7,6 +7,10 @@
   define void @store_s32_gpr(i32* %addr) { ret void }
   define void @store_s16_gpr(i16* %addr) { ret void }
   define void @store_s8_gpr(i8* %addr) { ret void }
+
+  define void @store_zero_s64_gpr(i64* %addr) { ret void }
+  define void @store_zero_s32_gpr(i32* %addr) { ret void }
+
   define void @store_s64_fpr(i64* %addr) { ret void }
   define void @store_s32_fpr(i32* %addr) { ret void }
 ...
@@ -119,6 +123,58 @@ body:             |
 
 ...
 
+---
+# CHECK-LABEL: name: store_zero_s64_gpr
+name:            store_zero_s64_gpr
+legalized:       true
+regBankSelected: true
+
+# CHECK:      registers:
+# CHECK-NEXT:  - { id: 0, class: gpr64sp }
+# CHECK-NEXT:  - { id: 1, class: gpr }
+registers:
+  - { id: 0, class: gpr }
+  - { id: 1, class: gpr }
+
+# CHECK:  body:
+# CHECK: %0 = COPY %x0
+# CHECK: STRXui %xzr, %0, 0 :: (store 8 into %ir.addr)
+body:             |
+  bb.0:
+    liveins: %x0, %x1
+
+    %0(p0) = COPY %x0
+    %1(s64) = G_CONSTANT i64 0
+    G_STORE  %1, %0 :: (store 8 into %ir.addr)
+
+...
+
+---
+# CHECK-LABEL: name: store_zero_s32_gpr
+name:            store_zero_s32_gpr
+legalized:       true
+regBankSelected: true
+
+# CHECK:      registers:
+# CHECK-NEXT:  - { id: 0, class: gpr64sp }
+# CHECK-NEXT:  - { id: 1, class: gpr }
+registers:
+  - { id: 0, class: gpr }
+  - { id: 1, class: gpr }
+
+# CHECK:  body:
+# CHECK: %0 = COPY %x0
+# CHECK: STRWui %wzr, %0, 0 :: (store 4 into %ir.addr)
+body:             |
+  bb.0:
+    liveins: %x0
+
+    %0(p0) = COPY %x0
+    %1(s32) = G_CONSTANT i32 0
+    G_STORE  %1, %0 :: (store 4 into %ir.addr)
+
+...
+
 ---
 # CHECK-LABEL: name: store_s64_fpr
 name:            store_s64_fpr