[PATCH] D22283: X86InstrInfo: No need for liveness analysis in classifyLEAReg()

[Matthias Braun via llvm-commits]

MatzeB created this revision.
MatzeB added reviewers: t.p.northover, ab, qcolombet.
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MatzeB set the repository for this revision to rL LLVM.
Herald added a subscriber: mcrosier.

classifyLEAReg() deals with switching operands from 32bit to 64bit in
order to use a LEA64_32 instruction (for three address code goodness).
It currently performs a liveness analysis to determine the kill/undef
flag for the newly added operand. This should not be necessary:

- If the previous operand had a kill flag, then the 32bit part of the
  register gets killed, this will kill the super register as well.
- If the previous operand had an undef flag then we didn't care what
  value we read, just use the same flag on the new operand.
  (No matter what an operand with an undef flag won't affect liveness)

This makes the code independent of the presence of kill flags because it
avoids a call to MachineBasicBlock::computeRegisterLiveness().

Repository:
  rL LLVM

http://reviews.llvm.org/D22283

Files:
  lib/Target/X86/X86InstrInfo.cpp

Index: lib/Target/X86/X86InstrInfo.cpp
===================================================================
--- lib/Target/X86/X86InstrInfo.cpp
+++ lib/Target/X86/X86InstrInfo.cpp
@@ -2596,24 +2596,8 @@
     ImplicitOp.setImplicit();
 
     NewSrc = getX86SubSuperRegister(Src.getReg(), 64);
-    MachineBasicBlock::LivenessQueryResult LQR =
-        MI.getParent()->computeRegisterLiveness(&getRegisterInfo(), NewSrc, MI);
-
-    switch (LQR) {
-    case MachineBasicBlock::LQR_Unknown:
-      // We can't give sane liveness flags to the instruction, abandon LEA
-      // formation.
-      return false;
-    case MachineBasicBlock::LQR_Live:
-      isKill = MI.killsRegister(SrcReg);
-      isUndef = false;
-      break;
-    default:
-      // The physreg itself is dead, so we have to use it as an <undef>.
-      isKill = false;
-      isUndef = true;
-      break;
-    }
+    isKill = Src.isKill();
+    isUndef = Src.isUndef();
   } else {
     // Virtual register of the wrong class, we have to create a temporary 64-bit
     // vreg to feed into the LEA.


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