[LLVMdev] Dead register (was Re: [llvm-commits] [llvm] r145819)

[Jakob Stoklund Olesen]

On Dec 5, 2011, at 12:56 PM, Hal Finkel wrote:

> RegScavenger is complaining about use of an undefined register, CTR8, in
> the BCTR8 instruction, in the following instance (this is from the PPC
> backend):
> 
> BB#38: derived from LLVM BB %for.end50
>    Predecessors according to CFG: BB#36
>        %X3<def> = LD 0, <fi#27>; mem:LD8[FixedStack27]
>        %X4<def> = RLDICR %X3<kill>, 3, 60
>        %X5<def> = LI8 <jt#0>[TF=4]
>        %X5<def> = ADDIS8 %X5<kill>, <jt#0>[TF=8]
>        %X4<def> = LDX %X4<kill>, %X5<kill>; mem:LD8[JumpTable]
>        MTCTR8 %X4<kill>, %CTR8<imp-def,dead>
>        BCTR8 %CTR8<imp-use,kill>, %RM<imp-use>
>    Successors according to CFG: BB#23 BB#15 BB#7 BB#8 BB#9 BB#10 BB#11
> BB#25 BB#12 BB#16 BB#18 BB#13 BB#17
> 
> How could CRT8 be marked implicitly-defined and also dead in the same
> instruction when it is clearly used in the next instruction?

This is the kind of sloppy liveness, I was talking about ;-)

llc -verify-machineinstrs should give you better info.

/jakob