Re: [x86 PATCH] Fine tune STV register conversion costs for -Os.

[Uros Bizjak <ubizjak@gmail.com>]

On Mon, Oct 23, 2023 at 4:47 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> The eagle-eyed may have spotted that my recent testcases for DImode shifts
> on x86_64 included -mno-stv in the dg-options.  This is because the
> Scalar-To-Vector (STV) pass currently transforms these shifts to use
> SSE vector operations, producing larger code even with -Os.  The issue
> is that the compute_convert_gain currently underestimates the size of
> instructions required for interunit moves, which is corrected with the
> patch below.
>
> For the simple test case:
>
> unsigned long long shl1(unsigned long long x) { return x << 1; }
>
> without this patch, GCC -m32 -Os -mavx2 currently generates:
>
> shl1:   push   %ebp              // 1 byte
>         mov    %esp,%ebp         // 2 bytes
>         vmovq  0x8(%ebp),%xmm0   // 5 bytes
>         pop    %ebp              // 1 byte
>         vpaddq %xmm0,%xmm0,%xmm0 // 4 bytes
>         vmovd  %xmm0,%eax        // 4 bytes
>         vpextrd $0x1,%xmm0,%edx  // 6 bytes
>         ret                      // 1 byte  = 24 bytes total
>
> with this patch, we now generate the shorter
>
> shl1:   push   %ebp             // 1 byte
>         mov    %esp,%ebp        // 2 bytes
>         mov    0x8(%ebp),%eax   // 3 bytes
>         mov    0xc(%ebp),%edx   // 3 bytes
>         pop    %ebp             // 1 byte
>         add    %eax,%eax        // 2 bytes
>         adc    %edx,%edx        // 2 bytes
>         ret                     // 1 byte  = 15 bytes total
>
> Benchmarking using CSiBE, shows that this patch saves 1361 bytes
> when compiling with -m32 -Os, and saves 172 bytes when compiling
> with -Os.
>
> 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.  Ok for mainline?
>
>
> 2023-10-23  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         * config/i386/i386-features.cc (compute_convert_gain): Provide
>         more accurate values (sizes) for inter-unit moves with -Os.

LGTM.

Thanks,
Uros.

>
>
> Thanks in advance,
> Roger
> --
>