[AArch64] Optimize MOV to ORR when load symmetric constants (#51483)

This change looks for cases of symmetric constant loading.
`symmetric constant load` is when the upper 32 bits and lower 32 bits
of a 64-bit register load the same value.

When it finds this, it replaces it with an instruction that loads only
the lower 32 bits of the constant and stores it in the upper and lower
bits simultaneously.

For example:
  renamable $x8 = MOVZXi 49370, 0
  renamable $x8 = MOVKXi $x8, 320, 16
  renamable $x8 = MOVKXi $x8, 49370, 32
  renamable $x8 = MOVKXi $x8, 320, 48
becomes
  renamable $x8 = MOVZXi 49370, 0
  renamable $x8 = MOVKXi $x8, 320, 16
  renamable $x8 = ORRXrs $x8, $x8, 32

Author: ParkHanbum

llvm/lib/Target/AArch64/AArch64ExpandImm.cpp

@@ -509,6 +509,16 @@ static inline void expandMOVImmSimple(uint64_t Imm, unsigned BitSize,
     Imm = ~Imm;
 
   unsigned Opc = (BitSize == 32 ? AArch64::MOVKWi : AArch64::MOVKXi);
+  Shift += 16;
+  Imm16 = (Imm >> Shift) & Mask;
+  if (Imm16 != (isNeg ? Mask : 0))
+    Insn.push_back(
+        {Opc, Imm16, AArch64_AM::getShifterImm(AArch64_AM::LSL, Shift)});
+  if (Imm != 0 && (Imm >> 32) == (Imm & UINT_MAX)) {
+    Insn.push_back({BitSize == 32 ? AArch64::ORRWrs : AArch64::ORRXrs, 0, 32});
+    return;
+  }
+
   while (Shift < LastShift) {
     Shift += 16;
     Imm16 = (Imm >> Shift) & Mask;

llvm/lib/Target/AArch64/AArch64ExpandPseudoInsts.cpp

@@ -168,6 +168,19 @@ bool AArch64ExpandPseudo::expandMOVImm(MachineBasicBlock &MBB,
                 .addImm(I->Op2));
       }
       break;
+    case AArch64::ORRWrs:
+    case AArch64::ORRXrs: {
+      Register DstReg = MI.getOperand(0).getReg();
+      bool DstIsDead = MI.getOperand(0).isDead();
+      MIBS.push_back(
+          BuildMI(MBB, MBBI, MI.getDebugLoc(), TII->get(I->Opcode))
+              .addReg(DstReg, RegState::Define |
+                                  getDeadRegState(DstIsDead && LastItem) |
+                                  RenamableState)
+              .addReg(DstReg)
+              .addReg(DstReg)
+              .addImm(I->Op2));
+    } break;
     case AArch64::ANDXri:
     case AArch64::EORXri:
       if (I->Op1 == 0) {

llvm/test/CodeGen/AArch64/movimm-expand-ldst.mir

@@ -11,8 +11,7 @@ body:             |
     ; CHECK-NEXT: {{  $}}
     ; CHECK-NEXT: renamable $x0 = MOVZXi 49370, 0
     ; CHECK-NEXT: renamable $x0 = MOVKXi $x0, 320, 16
-    ; CHECK-NEXT: renamable $x0 = MOVKXi $x0, 49370, 32
-    ; CHECK-NEXT: renamable $x0 = MOVKXi $x0, 320, 48
+    ; CHECK-NEXT: renamable $x0 = ORRXrs $x0, $x0, 32
     ; CHECK-NEXT: RET undef $lr, implicit $x0
     renamable $x0 = MOVi64imm 90284035103834330
     RET_ReallyLR implicit $x0
@@ -28,8 +27,7 @@ body:             |
     ; CHECK-NEXT: {{  $}}
     ; CHECK-NEXT: renamable $x0 = MOVZXi 320, 0
     ; CHECK-NEXT: renamable $x0 = MOVKXi $x0, 49370, 16
-    ; CHECK-NEXT: renamable $x0 = MOVKXi $x0, 320, 32
-    ; CHECK-NEXT: renamable $x0 = MOVKXi $x0, 49370, 48
+    ; CHECK-NEXT: renamable $x0 = ORRXrs $x0, $x0, 32
     ; CHECK-NEXT: RET undef $lr, implicit $x0
     renamable $x0 = MOVi64imm -4550323095879417536
     RET_ReallyLR implicit $x0