[ARM] Fix MOVCC peephole to not use an incorrect register class

[john.brawn]

Summary

The MOVCC peephole eliminates a MOVCC by making one of its inputs a conditional instruction, but when doing this it should be using both inputs of the MOVCC to decide on the register class to use as otherwise we can get an error when using -verify-machineinstrs.

[dmgreen]

Sounds good to me.

llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp

ARMBaseInstrInfo::optimizeSelect(MachineInstr &MI,
   bool Invert = !DefMI;
   if (!DefMI)
     DefMI = canFoldIntoMOVCC(MI.getOperand(1).getReg(), MRI, this);
   if (!DefMI)
     return nullptr;
 
   // Find new register class to use.
   MachineOperand FalseReg = MI.getOperand(Invert ? 2 : 1);
+  MachineOperand TrueReg = MI.getOperand(Invert ? 1 : 2);
   Register DestReg = MI.getOperand(0).getReg();
-  const TargetRegisterClass *PreviousClass = MRI.getRegClass(FalseReg.getReg());
-  if (!MRI.constrainRegClass(DestReg, PreviousClass))
+  const TargetRegisterClass *FalseClass = MRI.getRegClass(FalseReg.getReg());
+  const TargetRegisterClass *TrueClass = MRI.getRegClass(TrueReg.getReg());
+  if (!MRI.constrainRegClass(DestReg, FalseClass))
+    return nullptr;
+  if (!MRI.constrainRegClass(DestReg, TrueClass))
     return nullptr;
 
   // Create a new predicated version of DefMI.
   // Rfalse is the first use.
   MachineInstrBuilder NewMI =
       BuildMI(*MI.getParent(), MI, MI.getDebugLoc(), DefMI->getDesc(), DestReg);
 
   // Copy all the DefMI operands, excluding its (null) predicate.

llvm/test/CodeGen/ARM/movc-peephole.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -run-pass=peephole-opt %s -o - -verify-machineinstrs | FileCheck %s
+
+# Make sure the MOVCC to conditional instruction peephole doesn't change the
+# register class to one that's invalid.
+
+--- |
+  target triple = "armv7-unknown-unknown"
+  define i32 @test(i32 %x, i32 %y) {
+    ret i32 undef
+  }
+...
+---
+name: test
+tracksRegLiveness: true
+body: |
+  bb.0 (%ir-block.0):
+    liveins: $r0, $r1
+    ; CHECK-LABEL: name: test
+    ; CHECK: liveins: $r0, $r1
+    ; CHECK: [[COPY:%[0-9]+]]:gpr = COPY $r0
+    ; CHECK-NEXT: [[COPY1:%[0-9]+]]:gpr = COPY $r1
+    ; CHECK-NEXT: [[MOVsi:%[0-9]+]]:gpr = MOVsi [[COPY1]], 27, 14 /* CC::al */, $noreg, $noreg
+    ; CHECK-NEXT: [[ORRrsi:%[0-9]+]]:gpr = ORRrsi [[MOVsi]], [[COPY1]], 234, 14 /* CC::al */, $noreg, $noreg
+    ; CHECK-NEXT: [[MOVsi1:%[0-9]+]]:gpr = MOVsi [[COPY1]], 155, 14 /* CC::al */, $noreg, $noreg
+    ; CHECK-NEXT: [[ORRrsi1:%[0-9]+]]:gprnopc = ORRrsi killed [[MOVsi1]], killed [[MOVsi]], 106, 14 /* CC::al */, $noreg, $noreg
+    ; CHECK-NEXT: TSTri [[COPY1]], 1, 14 /* CC::al */, $noreg, implicit-def $cpsr
+    ; CHECK-NEXT: [[UXTH:%[0-9]+]]:gprnopc = UXTH killed [[ORRrsi1]], 0, 0 /* CC::eq */, $cpsr, implicit [[ORRrsi]](tied-def 0)
+    ; CHECK-NEXT: $r0 = COPY killed [[UXTH]]
+    ; CHECK-NEXT: BX_RET 14 /* CC::al */, $noreg, implicit $r0
+    %0:gpr = COPY $r0
+    %1:gpr = COPY $r1
+    %2:gpr = MOVsi %1:gpr, 27, 14, $noreg, $noreg
+    %3:gpr = ORRrsi %2:gpr, %1:gpr, 234, 14, $noreg, $noreg
+    %4:gpr = MOVsi %1:gpr, 155, 14, $noreg, $noreg
+    %5:gprnopc = ORRrsi killed %4:gpr, killed %2:gpr, 106, 14, $noreg, $noreg
+    %6:gprnopc = UXTH killed %5:gprnopc, 0, 14, $noreg
+    TSTri %1:gpr, 1, 14, $noreg, implicit-def $cpsr
+    %7:gpr = MOVCCr %3:gpr, killed %6:gprnopc, 0, $cpsr
+    $r0 = COPY killed %7
+    BX_RET 14, $noreg, implicit $r0