Re: [PATCH] Improved SUBREG simplifications in simplify-rtx.cc's simplify_subreg.

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/18/23 04:22, Roger Sayle wrote:
> 
> An x86 backend improvement that I'm working results in combine attempting
> to recognize:
> 
> (set (reg:DI 87 [ xD.2846 ])
>       (ior:DI (subreg:DI (ashift:TI (zero_extend:TI (reg:DI 92))
>                                     (const_int 64 [0x40])) 0)
>               (reg:DI 91)))
> 
> where the lowpart SUBREG has difficulty seeing through the (hi<<64)
> that the lowpart must be zero.  Rather than workaround this in the
> backend, the better fix is to teach simplify-rtx that
> lowpart((hi<<64)|lo) -> lo and highpart((hi<<64)|lo) -> hi, so that
> all backends benefit.  Reducing the number of places where the
> middle-end generates a SUBREG of something other than REG is a
> good thing.
> 
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32}
> with no new failures, except for pr78904-1b.c, for which a backend
> solution has just been proposed.  Ok for mainline?
> 
> 
> 2023-06-18  Roger Sayle  <roger@nextmovesoftware.com>
> 
> gcc/ChangeLog
>          * simplify-rtx.cc (simplify_subreg):  Optimize lowpart SUBREGs
>          of ASHIFT to const0_rtx with sufficiently large shift count.
>          Optimize highpart SUBREGs of ASHIFT as the shift operand when
>          the shift count is the correct offset.  Optimize SUBREGs of
>          multi-word logic operations if the SUBREGs of both operands
>          can be simplified.
OK
Jeff