[AArch64] Remove superfluous sxtw in peephole opt (#96293)

Across a basic-block we might have in i32 extract from a value that only
operates on upper bits (for example a sxtw). We can replace the COPY
with a new version skipping the sxtw.

Author: davemgreen

llvm/lib/Target/AArch64/AArch64MIPeepholeOpt.cpp

@@ -128,6 +128,7 @@ struct AArch64MIPeepholeOpt : public MachineFunctionPass {
   bool visitINSviGPR(MachineInstr &MI, unsigned Opc);
   bool visitINSvi64lane(MachineInstr &MI);
   bool visitFMOVDr(MachineInstr &MI);
+  bool visitCopy(MachineInstr &MI);
   bool runOnMachineFunction(MachineFunction &MF) override;
 
   StringRef getPassName() const override {
@@ -690,6 +691,34 @@ bool AArch64MIPeepholeOpt::visitFMOVDr(MachineInstr &MI) {
   return true;
 }
 
+// Across a basic-block we might have in i32 extract from a value that only
+// operates on upper bits (for example a sxtw). We can replace the COPY with a
+// new version skipping the sxtw.
+bool AArch64MIPeepholeOpt::visitCopy(MachineInstr &MI) {
+  Register InputReg = MI.getOperand(1).getReg();
+  if (MI.getOperand(1).getSubReg() != AArch64::sub_32 ||
+      !MRI->hasOneNonDBGUse(InputReg))
+    return false;
+
+  MachineInstr *SrcMI = MRI->getUniqueVRegDef(InputReg);
+  MachineInstr *CopyMI = SrcMI;
+  while (SrcMI && SrcMI->isFullCopy() &&
+         MRI->hasOneNonDBGUse(SrcMI->getOperand(1).getReg()))
+    SrcMI = MRI->getUniqueVRegDef(SrcMI->getOperand(1).getReg());
+
+  if (!SrcMI || SrcMI->getOpcode() != AArch64::SBFMXri ||
+      SrcMI->getOperand(2).getImm() != 0 || SrcMI->getOperand(3).getImm() != 31)
+    return false;
+
+  Register SrcReg = SrcMI->getOperand(1).getReg();
+  MRI->constrainRegClass(SrcReg, MRI->getRegClass(InputReg));
+  MI.getOperand(1).setReg(SrcReg);
+  if (CopyMI != SrcMI)
+    CopyMI->eraseFromParent();
+  SrcMI->eraseFromParent();
+  return true;
+}
+
 bool AArch64MIPeepholeOpt::runOnMachineFunction(MachineFunction &MF) {
   if (skipFunction(MF.getFunction()))
     return false;
@@ -771,6 +800,9 @@ bool AArch64MIPeepholeOpt::runOnMachineFunction(MachineFunction &MF) {
       case AArch64::FMOVDr:
         Changed |= visitFMOVDr(MI);
         break;
+      case AArch64::COPY:
+        Changed |= visitCopy(MI);
+        break;
       }
     }
   }

llvm/lib/Target/AArch64/peephole-sxtw.mir

@@ -0,0 +1,46 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -run-pass=aarch64-mi-peephole-opt -o - -mtriple=aarch64-unknown-linux -verify-machineinstrs %s | FileCheck %s
+
+---
+name: removeSxtw
+tracksRegLiveness: true
+body: |
+  bb.0.entry:
+    liveins: $x0
+    ; CHECK-LABEL: name: removeSxtw
+    ; CHECK: liveins: $x0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:gpr64 = COPY $x0
+    ; CHECK-NEXT: [[COPY1:%[0-9]+]]:gpr32sp = COPY [[COPY]].sub_32
+    ; CHECK-NEXT: [[ADDWri:%[0-9]+]]:gpr32sp = ADDWri [[COPY1]], 1, 0
+    ; CHECK-NEXT: $w0 = COPY [[ADDWri]]
+    ; CHECK-NEXT: RET_ReallyLR implicit $w0
+    %0:gpr64 = COPY $x0
+    %1:gpr64 = SBFMXri %0:gpr64, 0, 31
+    %2:gpr32sp = COPY %1.sub_32:gpr64
+    %3:gpr32sp = ADDWri %2:gpr32sp, 1, 0
+    $w0 = COPY %3:gpr32sp
+    RET_ReallyLR implicit $w0
+...
+---
+name: extraCopy
+tracksRegLiveness: true
+body: |
+  bb.0.entry:
+    liveins: $x0
+    ; CHECK-LABEL: name: extraCopy
+    ; CHECK: liveins: $x0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:gpr64 = COPY $x0
+    ; CHECK-NEXT: [[COPY1:%[0-9]+]]:gpr32sp = COPY [[COPY]].sub_32
+    ; CHECK-NEXT: [[ADDWri:%[0-9]+]]:gpr32sp = ADDWri [[COPY1]], 1, 0
+    ; CHECK-NEXT: $w0 = COPY [[ADDWri]]
+    ; CHECK-NEXT: RET_ReallyLR implicit $w0
+    %0:gpr64 = COPY $x0
+    %1:gpr64 = SBFMXri %0:gpr64, 0, 31
+    %2:gpr64all = COPY %1:gpr64
+    %3:gpr32sp = COPY %2.sub_32:gpr64all
+    %4:gpr32sp = ADDWri %3:gpr32sp, 1, 0
+    $w0 = COPY %4:gpr32sp
+    RET_ReallyLR implicit $w0
+...

llvm/test/CodeGen/AArch64/aarch64-mull-masks.ll

@@ -281,8 +281,7 @@ define i64 @smull_ldrsw_shift(ptr %x0, i64 %x1) {
 ; CHECK-LABEL: smull_ldrsw_shift:
 ; CHECK:       // %bb.0: // %entry
 ; CHECK-NEXT:    ldrsw x8, [x0]
-; CHECK-NEXT:    sxtw x9, w1
-; CHECK-NEXT:    smull x0, w8, w9
+; CHECK-NEXT:    smull x0, w8, w1
 ; CHECK-NEXT:    ret
 entry:
   %ext64 = load i32, ptr %x0
@@ -490,8 +489,7 @@ define i64 @smaddl_ldrsw_shift(ptr %x0, i64 %x1, i64 %x2) {
 ; CHECK-LABEL: smaddl_ldrsw_shift:
 ; CHECK:       // %bb.0: // %entry
 ; CHECK-NEXT:    ldrsw x8, [x0]
-; CHECK-NEXT:    sxtw x9, w1
-; CHECK-NEXT:    smaddl x0, w8, w9, x2
+; CHECK-NEXT:    smaddl x0, w8, w1, x2
 ; CHECK-NEXT:    ret
 entry:
   %ext64 = load i32, ptr %x0
@@ -654,8 +652,7 @@ define i64 @smnegl_ldrsw_shift(ptr %x0, i64 %x1) {
 ; CHECK-LABEL: smnegl_ldrsw_shift:
 ; CHECK:       // %bb.0: // %entry
 ; CHECK-NEXT:    ldrsw x8, [x0]
-; CHECK-NEXT:    sxtw x9, w1
-; CHECK-NEXT:    smnegl x0, w8, w9
+; CHECK-NEXT:    smnegl x0, w8, w1
 ; CHECK-NEXT:    ret
 entry:
   %ext64 = load i32, ptr %x0
@@ -818,8 +815,7 @@ define i64 @smsubl_ldrsw_shift(ptr %x0, i64 %x1, i64 %x2) {
 ; CHECK-LABEL: smsubl_ldrsw_shift:
 ; CHECK:       // %bb.0: // %entry
 ; CHECK-NEXT:    ldrsw x8, [x0]
-; CHECK-NEXT:    sxtw x9, w1
-; CHECK-NEXT:    smsubl x0, w8, w9, x2
+; CHECK-NEXT:    smsubl x0, w8, w1, x2
 ; CHECK-NEXT:    ret
 entry:
   %ext64 = load i32, ptr %x0