[Bug target/110400] New: Reuse vector register for both scalar and vector value.

[Bug target/110400] New: Reuse vector register for both scalar and vector value.

[crazylht at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110400

            Bug ID: 110400
           Summary: Reuse vector register for both scalar and vector
                    value.
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: crazylht at gmail dot com
  Target Milestone: ---

From PR109812 #c18

Uroš Bizjak 2023-06-21 09:46:43 UTC
One interesting observation:

clang is able to do this:

  0.09 │     │  vmovddup     -0x8(%rdx,%rsi,1),%xmm3              ▒
  ...
  0.11 │     │  vfmadd231sd  %xmm2,%xmm3,%xmm1                    ▒
  ...
  0.74 │     │  vfmadd231pd  %xmm2,%xmm3,%xmm0                    ▒

It figures out that duplicated V2DFmode value in %xmm3 can also be accessed in
the same register as DFmode value.

OTOH, current gcc does:

        vmovsd  (%rsi,%rax,8), %xmm1
        ...
        vmovddup        %xmm1, %xmm4
        ...
        vfmadd231pd     %xmm4, %xmm0, %xmm2
        ...
        vfmadd231sd     %xmm1, %xmm0, %xmm3

The above code needs two registers.

----------------------------------------------------

Similar with below testcase

typedef double v2df __attribute__((vector_size(16)));
v2df c;
double d;
void
foo (double* __restrict a)
{
    c = __extension__(v2df) {*a, *a};
    d = *a;
}

with option: -O2 -mavx2

GCC generates

foo(double*):
        vmovsd  (%rdi), %xmm0
        vmovddup        %xmm0, %xmm1
        vmovsd  %xmm0, d(%rip)
        vmovapd %xmm1, c(%rip)

Clang

foo(double*):                               # @foo(double*)
        vmovddup        (%rdi), %xmm0                   # xmm0 = mem[0,0]
        vmovaps %xmm0, c(%rip)
        vmovlps %xmm0, d(%rip)
        retq

[Bug rtl-optimization/110400] Reuse vector register for both scalar and vector value.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110400

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-06-25
     Ever confirmed|0                           |1
          Component|target                      |rtl-optimization
           Severity|normal                      |enhancement

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. a "CSE" should happen ...