Re: [x86_64 PATCH] Tweak -Os costs for scalar-to-vector pass.

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Aug 19, 2021 at 6:01 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> Doh!  ENOPATCH.
>
> -----Original Message-----
> From: Roger Sayle <roger@nextmovesoftware.com>
> Sent: 19 August 2021 16:59
> To: 'GCC Patches' <gcc-patches@gcc.gnu.org>
> Subject: [x86_64 PATCH] Tweak -Os costs for scalar-to-vector pass.
>
>
> Back in June I briefly mentioned in one of my gcc-patches posts that a
> change that should have always reduced code size, would mysteriously
> occasionally result in slightly larger code (according to CSiBE):
> https://gcc.gnu.org/pipermail/gcc-patches/2021-June/573233.html
>
> Investigating further, the cause turns out to be that x86_64's
> scalar-to-vector (stv) pass is relying on poor estimates of the size
> costs/benefits.  This patch tweaks the backend's compute_convert_gain method
> to provide slightly more accurate values when compiling with -Os.
> Compilation without -Os is (should be) unaffected.  And for completeness,
> I'll mention that the stv pass is a net win for code size so it's much
> better to improve its heuristics than simply gate the pass on
> !optimize_for_size.
>
> The net effect of this change is to save 1399 bytes on the CSiBE code size
> benchmark when compiling with -Os.
>
> This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
> and "make -k check" with no new failures.
>
> Ok for mainline?

+                   /* xor (2 bytes) vs. xorps (3 bytes).  */
+                   if (src == const0_rtx)
+                     igain -= COSTS_N_BYTES (1);
+                   /* movdi_internal vs. movv2di_internal.  */
+                   /* => mov (5 bytes) vs. movaps (7 bytes).  */
+                   else if (x86_64_immediate_operand (src, SImode))
+                     igain -= COSTS_N_BYTES (2);

doesn't it need two GPR xor for 32bit DImode and two mov?  Thus
the non-SSE cost should be times 'm'?  For const0_rtx we may
eventually re-use the zero reg for the high part so that is eventually
correct.

Also I'm missing a 'else' - in the default case there's no cost/benefit
of using SSE vs. GPR regs?  For SSE it would be a constant pool
load.

I also wonder, since I now see COSTS_N_BYTES for the first time (heh),
whether with -Os we'd need to replace all COSTS_N_INSNS (1)
scaling with COSTS_N_BYTES scaling?  OTOH costs_add_n_insns
uses COSTS_N_INSNS for the size part as well.

That said, it looks like we're eventually mixing apples and oranges
now or even previously?

Thanks,
Richard.

>
>
> 2021-08-19  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         * config/i386/i386-features.c (compute_convert_gain): Provide
>         more accurate values for CONST_INT, when optimizing for size.
>         * config/i386/i386.c (COSTS_N_BYTES): Move definition from here...
>         * config/i386/i386.h (COSTS_N_BYTES): to here.
>
> Roger
> --
>