Re: [PATCH-1, combine] Don't widen shift mode when target has rotate/mask instruction on original mode [PR93738]

[Jeff Law <jeffreyalaw@gmail.com>]

On 7/20/23 18:59, HAO CHEN GUI wrote:
> Hi Jeff,
> 
> 在 2023/7/21 5:27, Jeff Law 写道:
>> Wouldn't it make more sense to just try rotate/mask in the original mode before trying a shift in a widened mode?  I'm not sure why we need a target hook here.
> 
> There is no change to try rotate/mask with the original mode when
> expensive_optimizations is set. The subst widens the shift mode.
But we can add it before the attempt in the wider mode.

> 
>        if (flag_expensive_optimizations)
>          {
>            /* Pass pc_rtx so no substitutions are done, just
>               simplifications.  */
>            if (i1)
>              {
>                subst_low_luid = DF_INSN_LUID (i1);
>                i1src = subst (i1src, pc_rtx, pc_rtx, 0, 0, 0);
>              }
> 
>            subst_low_luid = DF_INSN_LUID (i2);
>            i2src = subst (i2src, pc_rtx, pc_rtx, 0, 0, 0);
>          }
> 
> I don't know if the wider mode is helpful to other targets, so
> I added the target hook.
In this scenario we're often better off relying on rtx_costs (even with 
all its warts) rather than adding yet another target hook.

I'd love to hear from Segher here to see if he's got other ideas.

jeff