[LLVMdev] request for help writing a register allocator

[Török Edwin]

On 2009-10-22 06:02, Susan Horwitz wrote:
> but I don't understand how the code gets generated, so this isn't very 
> helpful.  Is there a way for me to look at the machine instructions before 
> register allocation?
>
>   

You can run llc with -debug which shows debug information from all
codegen phases.
You can also use -debug-only=<name> to debug only a specific codegen pass.

I see output like this, where I think that instead of LINEAR SCAN your
allocator will be run:

********** MACHINEINSTRS **********
entry:
20      MOV32mr <fi#1>, 1, %reg0, 0, %reg0, %EDI<kill>
28      %reg1026<def> = MOV32rm <fi#1>, 1, %reg0, 0, %reg0
36      MOV32mr <fi#0>, 1, %reg0, 0, %reg0, %reg1026<kill>
44      %EAX<def> = MOV32rm <fi#0>, 1, %reg0, 0, %reg0
64      RET %EAX<imp-usekill>
********** LINEAR SCAN **********
********** Function: foo
fixed intervals:
        %reg14,inf = [0,6:0)[6,14:1)[14,22:2)  0@?-(6) 1@? 2@? -> DI
        %reg2,inf = [46,54:1)[54,66:0)  0 at 54-(66) 1@? -> AL
        %reg23,inf = [0,22:0)  0@?-(22) -> EDI
        %reg1,inf = [46,54:1)[54,66:0)  0 at 54-(66) 1@? -> AH
        %reg15,inf = [0,6:0)[6,14:1)[14,22:2)  0@?-(6) 1@? 2@? -> DIL
        %reg3,inf = [46,54:1)[54,66:0)  0 at 54-(66) 1@? -> AX
        %reg19,inf = [46,66:0)  0 at 46-(66) -> EAX

*** CURRENT ***: %reg1026,inf = [30,38:0)  0 at 30-(38)
        processing active intervals:
        processing inactive intervals:
        allocating current interval: EAX
active intervals:
        %reg1026,inf = [30,38:0)  0 at 30-(38) -> EAX
inactive intervals:
        interval %reg1026,inf = [30,38:0)  0 at 30-(38) expired
********** REGISTER MAP **********
[reg1026 -> EAX]


**** Local spiller rewriting function 'foo':
**** Machine Instrs (NOTE! Does not include spills and reloads!) ****
# Machine code for foo():
  <fi#0>: size is 4 bytes, alignment is 4 bytes, at location [SP+8]
  <fi#1>: size is 4 bytes, alignment is 4 bytes, at location [SP+8]
Live Ins: EDI in VR#1025
Live Outs: EAX
....
Best regards
--Edwin