Re: [PATCH GCC]Check tieable TRUNCATE only if inner rtx is register

[Richard Sandiford <richard.sandiford@linaro.org>]

Bin Cheng <Bin.Cheng@arm.com> writes:
> Hi,
> Revision 247881 possibly exposes bug in RTL or ARM backend, while the
> change itself
> may be incomplete too.  Given a TRUNCATE rtx with complicated sub-rtx,
> backend may
> want to know the complicated sub-rtx happens in context of TRUNCATE in
> order to give
> different costs.  This patch adds restriction only checking tieable
> TRUNCATE if its operand
> is register.  This is enough for middle-end, it builds up (truncate:SI
> (reg:DI)) in order to setup
> conversion cost for GIMPLE optimizations.  On the other hand, targets
> like i386/arm don't
> handle register truncation at the moment.
>
> Note, this patch minimize the impact of cost change and workaround
> PR80754 for now.
> So far the effect of change in r247881 is to allow smulsi3_highpart_v6
> pattern to be used
> rather than smullsidi, which means it actually gives RA more free in
> choosing registers.
> If it's because of the cost change that smulsi3_highpart_v6 is now
> preferred, it could be
> suggesting that r247881 is good.  Anyway, it's just my guess before
> looking into rtl passes.
>
> Bootstrap and test ongoing.  Is it OK?
>
> 2017-05-13  Bin Cheng  <bin.cheng@arm.com>
>
> 	* rtlanal.c (rtx_cost): Check tieable TRUNCATE only if inner rtx
> 	is register.
>
> diff --git a/gcc/rtlanal.c b/gcc/rtlanal.c
> index d9f57c3..b211efb 100644
> --- a/gcc/rtlanal.c
> +++ b/gcc/rtlanal.c
> @@ -4165,7 +4165,10 @@ rtx_cost (rtx x, machine_mode mode, enum rtx_code outer_code,
>        break;
>  
>      case TRUNCATE:
> -      if (MODES_TIEABLE_P (mode, GET_MODE (XEXP (x, 0))))
> +      /* If inner rtx isn't register, fall through and call target hook.
> +	 Backend may want to know that sub-rtx is in truncate.  */
> +      if (REG_P (XEXP (x, 0))
> +	  && MODES_TIEABLE_P (mode, GET_MODE (XEXP (x, 0))))
>  	{
>  	  total = 0;
>  	  break;

Even for !REG_P (i.e. even when you still call the hook), 0 seems like
a better default to pass than COSTS_N_INSNS (1).  Would it be worth
guarding just the break with REG_P?

Thanks,
Richard