Stop relying on physical register kill flags in isKilled() in the two-address

pass. One of the callers of isKilled() can cope with overapproximation of kills
and the other can't, so I added a flag to indicate this.

In theory this could pessimize code slightly, but in practice most physical
register uses are kills, and most important kills of physical registers are the
only uses of that register prior to register allocation, so we can recognize
them as kills even without kill flags.

This is relevant because LiveIntervals gets rid of all kill flags.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175821 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 10 insertions, 3 deletions

diff --git a/lib/CodeGen/TwoAddressInstructionPass.cpp b/lib/CodeGen/TwoAddressInstructionPass.cpp
index e0dba3f..e312642 100644
--- a/lib/CodeGen/TwoAddressInstructionPass.cpp
+++ b/lib/CodeGen/TwoAddressInstructionPass.cpp
@@ -372,12 +372,19 @@ static bool isPlainlyKilled(MachineInstr *MI, unsigned Reg,
 /// normal heuristics commute the (two-address) add, which lets
 /// coalescing eliminate the extra copy.
 ///
+/// If allowFalsePositives is true then likely kills are treated as kills even
+/// if it can't be proven that they are kills.
 static bool isKilled(MachineInstr &MI, unsigned Reg,
                      const MachineRegisterInfo *MRI,
                      const TargetInstrInfo *TII,
-                     LiveIntervals *LIS) {
+                     LiveIntervals *LIS,
+                     bool allowFalsePositives) {
   MachineInstr *DefMI = &MI;
   for (;;) {
+    // All uses of physical registers are likely to be kills.
+    if (TargetRegisterInfo::isPhysicalRegister(Reg) &&
+        (allowFalsePositives || MRI->hasOneUse(Reg)))
+      return true;
     if (!isPlainlyKilled(DefMI, Reg, LIS))
       return false;
     if (TargetRegisterInfo::isPhysicalRegister(Reg))
@@ -1028,7 +1035,7 @@ tryInstructionTransform(MachineBasicBlock::iterator &mi,
 
   assert(TargetRegisterInfo::isVirtualRegister(regB) &&
          "cannot make instruction into two-address form");
-  bool regBKilled = isKilled(MI, regB, MRI, TII, LIS);
+  bool regBKilled = isKilled(MI, regB, MRI, TII, LIS, true);
 
   if (TargetRegisterInfo::isVirtualRegister(regA))
     scanUses(regA);
@@ -1048,7 +1055,7 @@ tryInstructionTransform(MachineBasicBlock::iterator &mi,
 
     if (regCIdx != ~0U) {
       regC = MI.getOperand(regCIdx).getReg();
-      if (!regBKilled && isKilled(MI, regC, MRI, TII, LIS))
+      if (!regBKilled && isKilled(MI, regC, MRI, TII, LIS, false))
         // If C dies but B does not, swap the B and C operands.
         // This makes the live ranges of A and C joinable.
         TryCommute = true;