Re: RFC: allowing fwprop to propagate subregs

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Wed, Sep 14, 2011 at 8:24 AM, Richard Sandiford
<rdsandiford@googlemail.com> wrote:
> At the moment, fwprop will propagate constants and registers
> even if no further rtl simplifications are possible:
>
> ?if (REG_P (new_rtx) || CONSTANT_P (new_rtx))
> ? ?flags |= PR_CAN_APPEAR;
>
> What do you think about extending this to subregs? ?The reason for
> asking is that on NEON, vector loads like vld4 are represented as a load
> of a single monolithic register followed by subreg extractions of each
> vector:
>
> ?(set (reg:OI FULL) (...))
> ?(set (reg:V2SI V0) (subreg:V2SI (reg:OI FULL) 0))
> ?(set (reg:V2SI V1) (subreg:V2SI (reg:OI FULL) 16))
> ?(set (reg:V2SI V2) (subreg:V2SI (reg:OI FULL) 32))
> ?(set (reg:V2SI V3) (subreg:V2SI (reg:OI FULL) 48))
>
> Nothing ever propagates these subregs, so the separate moves
> survive until IRA. ?This has three problems:
>
> ?- We generally want the registers allocated to V0...V3 to be the same
> ? ?as FULL, so that the four subreg moves become nops. ?And this often
> ? ?happens in simple examples. ?But if register pressure is relatively
> ? ?high, these moves can sometimes cause IRA to spill in cases where
> ? ?it doesn't if the subregs are used instead of each Vi.
>
> ?- Perhaps related, register pressure becomes harder to estimate.
>
> ?- These moves can interfere with pre-reload scheduling.
>
> In combination with the MODES_TIEABLE_P patch that I posted here:
>
> ? ?http://gcc.gnu.org/ml/gcc-patches/2011-09/msg00626.html
>
> this patch significantly improves the code generated for several libav
> loops. ?Unfortunately, I don't have a setup that can do meaningful
> x86_64 performance measurements, but a diff of the before and after
> output for libav showed many cases where the patch removed moves.
>
> What do you think? ?Alternatives include propagating in lower-subreg,
> or maybe only in the second fwprop pass.
>
> Richard
>
>
> gcc/
> ? ? ? ?* fwprop.c (propagate_rtx): Also set PR_CAN_APPEAR for subregs.
>
> Index: gcc/fwprop.c
> ===================================================================
> --- gcc/fwprop.c ? ? ? ?2011-08-26 09:58:28.829540497 +0100
> +++ gcc/fwprop.c ? ? ? ?2011-08-26 10:14:03.767707504 +0100
> @@ -664,7 +664,7 @@ propagate_rtx (rtx x, enum machine_mode
> ? ? return NULL_RTX;
>
> ? flags = 0;
> - ?if (REG_P (new_rtx) || CONSTANT_P (new_rtx))
> + ?if (REG_P (new_rtx) || CONSTANT_P (new_rtx) || GET_CODE (new_rtx) == SUBREG)
> ? ? flags |= PR_CAN_APPEAR;
> ? if (!for_each_rtx (&new_rtx, varying_mem_p, NULL))
> ? ? flags |= PR_HANDLE_MEM;
>

A SUBREG may not be REG nor CONSTANT. Don't you need
to check REG_P/CONSTANT_P on SUBREG?


-- 
H.J.