llvm dev - Jan 2012 - [LLVMdev] Req-sequence, partial defs

Hi,

I'm having an issue with subregisters on my target.

With a pseudo that writes to a 32 bit reg:

        %vreg20<def> = toHi16_low0_pseudo %vreg2; reg32:%vreg20
hi16:%vreg2

expands to

        %vreg2<def> = COPY %a2h; hi16:%vreg2
        %vreg43<def> = mov 0, pred:0, pred:%noreg, %ac0<imp-use>,
%ac1<imp-use>; lo16:%vreg43
        %vreg20<def> = REG_SEQUENCE %vreg2, hi16, %vreg43, lo16;
reg32:%vreg20 hi16:%vreg2 lo16:%vreg43

Becomes

16L             %vreg20:hi16<def,undef> = COPY %a2h<kill>,
%vreg20<imp-def>; reg32:%vreg20
368L            %vreg20:lo16<def> = mov 0, pred:0, pred:%noreg,
%ac0<imp-use>, %ac1<imp-use>; reg32:%vreg20

Becomes

 Live Ins: %a0h %a1_32 %a2h %a3_32
        ... (COPY coalesced as a2h is live in)
        %a2l<def> = mov 0, pred:0, pred:%noreg, %ac0<imp-use>,
%ac1<imp-use>, %a2_32<imp-use>

results in:

** Bad machine code: Using an undefined physical register ***
- function:    lfixedconv8
- basic block:  0x91a1940 (BB#0)
- instruction: %a2l<def> = mov 0, pred:0, pred:%noreg,
%ac0<imp-use>, %ac1<imp-use>, %a2_32<imp-use>
- operand 6:   %a2_32<imp-use>


I cannot quite figure what's wrong here - the a2_32<imp-use> operand
was added by the VirtualRegRewriter, because the partial def was not marked
undef. The reason for this was that the LiveIntervalsAnalysis pass cleaned up
after REG_SEQUENCE lowering by making the first occurence - when the interval
for the reg was empty and thus created - an <def,undef>, but then it does
not do the same for the other subreg, as at that time the interval for the reg
is not empty.

Can anyone say what went wrong?

Thanks,

Jonas

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On Jan 24, 2012, at 7:44 AM, Jonas Paulsson wrote:
>  
> Hi,
>  
> I'm having an issue with subregisters on my target.
>  
> With a pseudo that writes to a 32 bit reg:
>  
>         %vreg20<def> = toHi16_low0_pseudo %vreg2; reg32:%vreg20
hi16:%vreg2
>  
> expands to
>  
>         %vreg2<def> = COPY %a2h; hi16:%vreg2
>         %vreg43<def> = mov 0, pred:0, pred:%noreg,
%ac0<imp-use>, %ac1<imp-use>; lo16:%vreg43
>         %vreg20<def> = REG_SEQUENCE %vreg2, hi16, %vreg43, lo16;
reg32:%vreg20 hi16:%vreg2 lo16:%vreg43
>  
> Becomes
>  
> 16L             %vreg20:hi16<def,undef> = COPY %a2h<kill>,
%vreg20<imp-def>; reg32:%vreg20
> 368L            %vreg20:lo16<def> = mov 0, pred:0, pred:%noreg,
%ac0<imp-use>, %ac1<imp-use>; reg32:%vreg20
>  
> Becomes
>  
> Live Ins: %a0h %a1_32 %a2h %a3_32
>         ... (COPY coalesced as a2h is live in)
>         %a2l<def> = mov 0, pred:0, pred:%noreg, %ac0<imp-use>,
%ac1<imp-use>, %a2_32<imp-use>
>  
> results in:
>  
> ** Bad machine code: Using an undefined physical register ***
> - function:    lfixedconv8
> - basic block:  0x91a1940 (BB#0)
> - instruction: %a2l<def> = mov 0, pred:0, pred:%noreg,
%ac0<imp-use>, %ac1<imp-use>, %a2_32<imp-use>
> - operand 6:   %a2_32<imp-use>
>  
>  
> I cannot quite figure what's wrong here - the a2_32<imp-use>
operand was added by the VirtualRegRewriter, because the partial def was not
marked undef. The reason for this was that the LiveIntervalsAnalysis pass
cleaned up after REG_SEQUENCE lowering by making the first occurence - when the
interval for the reg was empty and thus created - an <def,undef>, but then
it does not do the same for the other subreg, as at that time the interval for
the reg is not empty.
>  
> Can anyone say what went wrong?
The COPY with additional <imp-def> operands shouldn't have been
deleted. It should have been turned into a KILL pseudo-instruction instead,
preserving the operands and the liveness of the super-register.

This bug was fixed by deleting VirtRegRewriter ;-)

/jakob

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