[Bug target/101296] Addition of x86 addsub SLP patterned slowed down 433.milc by 12% on znver2 with -Ofast -flto

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101296

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot 
gnu.org
   Last reconfirmed|                            |2021-07-02
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I will have a look next week.  A quick look shows FMAs being used and addsub
can break FMA detection until we get general optab support for fmaddsub
and friends.  So it might be { fma, fms } + blend compared to addsub + mul
where the former maybe has lower latency though Agner says FMA (5c) + blend
(1c)
vs ADDSUB (3c) + MUL (3c).  As said, I have to look into this in more detail.

double a[4], b[4], c[4];

void foo ()
{
  c[0] = a[0] - b[0] * c[0];
  c[1] = a[1] + b[1] * c[1];
  c[2] = a[2] - b[2] * c[2];
  c[3] = a[3] + b[3] * c[3];
}

        vmovapd a(%rip), %ymm2
        vmovapd b(%rip), %ymm1
        vmovapd b(%rip), %ymm0
        vfmadd132pd     c(%rip), %ymm2, %ymm1
        vfnmadd132pd    c(%rip), %ymm2, %ymm0
        vshufpd $10, %ymm1, %ymm0, %ymm0
        vmovapd %ymm0, c(%rip)

vs.

        vmovapd b(%rip), %ymm1
        vmovapd a(%rip), %ymm2
        vmulpd  c(%rip), %ymm1, %ymm0
        vaddsubpd       %ymm0, %ymm2, %ymm0
        vmovapd %ymm0, c(%rip)