[llvm-dev] copyPhysReg() with undef source operand (llvm-stress)

[Matthias Braun via llvm-dev]

> On May 4, 2017, at 8:44 AM, Jonas Paulsson via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> I would like some advice on how to lower a COPY in copyPhysReg(), where the source operand is undef, because it seems this flag is not available.
> 
> With llvm-stress I got (on SystemZ):
> 
> B#0: derived from LLVM BB %BB
>    Live Ins: %R2D %R4D %R6D
>        %vreg14<def> = IMPLICIT_DEF; GR64Bit:%vreg14
>        %vreg53<def> = IMPLICIT_DEF; GR128Bit:%vreg53
>        %vreg55<def> = LLILL 0; GR64Bit:%vreg55
>        %vreg54<def,tied1> = INSERT_SUBREG %vreg53<tied0>, %vreg55, subreg_h64; GR128Bit:%vreg54,%vreg53 GR64Bit:%vreg55
>        %vreg15<def,tied1> = INSERT_SUBREG %vreg54<tied0>, %vreg14, subreg_l64; GR128Bit:%vreg15,%vreg54 GR64Bit:%vreg14
> 
> -> Process Implicit Definitions
> 
>        %vreg55<def> = LLILL 0; GR64Bit:%vreg55
>        %vreg54<def,tied1> = INSERT_SUBREG %vreg53<undef,tied0>, %vreg55, subreg_h64; GR128Bit:%vreg54,%vreg53 GR64Bit:%vreg55
>        %vreg15<def,tied1> = INSERT_SUBREG %vreg54<tied0>, %vreg14<undef>, subreg_l64; GR128Bit:%vreg15,%vreg54 GR64Bit:%vreg14
> 
> -> Two-Address instruction pass
> 
>        %vreg55<def> = LLILL 0; GR64Bit:%vreg55
>        %vreg54:subreg_h64<def,read-undef> = COPY %vreg55<kill>; GR128Bit:%vreg54 GR64Bit:%vreg55
>        %vreg15<def> = COPY %vreg54<kill>; GR128Bit:%vreg15,%vreg54
>        %vreg15:subreg_l64<def> = COPY %vreg14<undef>; GR128Bit:%vreg15 GR64Bit:%vreg14
> 
> -> reg alloc
> 
>        %R1D<def> = LLILL 0
>        %R12D<def> = COPY %R1D<kill>, %R12Q<imp-def>
>        %R13D<def> = COPY %R0D<undef>, %R12Q<imp-use,kill>, %R12Q<imp-def>
I guess this can be interpreted as an identity copy? (the R0D is undef so you could just as well replace it with R13D here). So maybe you can simply extend the identify copy handling case ExpandPostRA::LowerCopy() instead.

(If that doesn't work then I wouldn't mind if you extend the copyPhysReg() interface with some undef flag)

- Matthiase