[LLVMdev] [LLVMDev] Register Allocation and Kill Flags

[Jeff Kunkel]

I am wondering about register allocation when there is a kill flag on the
MachineOperand. Do I need to remove the kill flag?

This code below is just an example from test\CodeGen\X86\xor.ll


# Machine code for function test3:
Frame Objects:
 fi#-2: size=4, align=4, fixed, at location [SP+8]
 fi#-1: size=4, align=8, fixed, at location [SP+4]
Function Live Outs: %EAX

BB#0: derived from LLVM BB %entry
       %reg16385<def> = MOV32rm <fi#-2>, 1, %reg0, 0, %reg0;
mem:LD4[FixedStack-2] GR32:%reg16385
       %reg16384<def> = MOV32rm <fi#-1>, 1, %reg0, 0, %reg0;
mem:LD4[FixedStack-1] GR32:%reg16384
       %reg16388<def> = MOV32ri 1; GR32:%reg16388
       %reg16392<def> = XOR32ri %reg16385, 4294967294, %EFLAGS<imp-def>;
GR32:%reg16392,16385
       %reg16391<def> = AND32rr *%reg16392<kill>*, %reg16384,
%EFLAGS<imp-def>; GR32:%reg16391,16392,16384
       %reg16389<def> = SHR32ri %reg16391, 1, %EFLAGS<imp-def>;
GR32:%reg16389,16391
       %EAX<def> = COPY %reg16389; GR32:%reg16389
       RET

After my reg allocation I have

# After Register Allocation:
# Machine code for function test3:
Frame Objects:
 fi#-2: size=4, align=4, fixed, at location [SP+8]
 fi#-1: size=4, align=8, fixed, at location [SP+4]
Function Live Outs: %EAX

BB#0: derived from LLVM BB %entry
       %EAX<def> = MOV32rm <fi#-2>, 1, %reg0, 0, %reg0;
mem:LD4[FixedStack-2]
       %ECX<def> = MOV32rm <fi#-1>, 1, %reg0, 0, %reg0;
mem:LD4[FixedStack-1]
       %EDX<def> = MOV32ri 1
       %EAX<def> = XOR32ri %EAX, 4294967294, %EFLAGS<imp-def>
       %EAX<def> = AND32rr *%EAX<kill>*, %ECX, %EFLAGS<imp-def>
       %EAX<def> = SHR32ri %EAX, 1, %EFLAGS<imp-def>
       RET
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