[Codegen] Make sure kill flags are not incorrect from removed machine phi's

We need to clear the kill flags on both SingleValReg and OldReg, to ensure they remain
conservatively correct.

Differential Revision: https://reviews.llvm.org/D58114

llvm-svn: 353847

llvm/lib/CodeGen/OptimizePHIs.cpp

@@ -181,11 +181,12 @@ bool OptimizePHIs::OptimizeBB(MachineBasicBlock &MBB) {
       if (!MRI->constrainRegClass(SingleValReg, MRI->getRegClass(OldReg)))
         continue;
 
-      // for the case SingleValReg taken from copy instr
-      MRI->clearKillFlags(SingleValReg);
-
       MRI->replaceRegWith(OldReg, SingleValReg);
       MI->eraseFromParent();
+
+      // The kill flags on OldReg and SingleValReg may no longer be correct.
+      MRI->clearKillFlags(SingleValReg);
+
       ++NumPHICycles;
       Changed = true;
       continue;

llvm/test/CodeGen/Thumb/opt-phis.mir

@@ -0,0 +1,38 @@
+# RUN: llc -mtriple thumbv6m-none-eabi -run-pass=opt-phis -verify-machineinstrs -o - %s | FileCheck %s
+--- |
+  target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+  target triple = "thumbv6m-arm-none-eabi"
+
+  define i32 @funca() { unreachable }
+
+...
+---
+name:            funca
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    successors: %bb.2(0x40000000), %bb.3(0x40000000)
+
+    %0:tgpr, dead $cpsr = tMOVi8 255, 14, $noreg
+    %1:tgpr, dead $cpsr = tMOVi8 128, 14, $noreg
+    tCMPi8 %0, 0, 14, $noreg, implicit-def $cpsr
+    tBcc %bb.3, 1, $cpsr
+
+  bb.2:
+    successors: %bb.3(0x80000000)
+
+  bb.3:
+    successors: %bb.0(0x80000000)
+
+    %2:tgpr = PHI %0, %bb.2, %0, %bb.0
+    %3:tgpr, $cpsr = tSUBrr %1, killed %2, 14, $noreg
+    %4:tgpr, $cpsr = tADDrr killed %3, killed %0, 14, $noreg
+    tB %bb.0, 14, $noreg
+
+# Make sure the kill flags are not incorrect
+# CHECK-LABEL: name: funca
+# CHECK-NOT: PHI
+# CHECK: tSUBrr %1, %0
+# CHECK: tADDrr killed %3, %0
+
+...