[Bug target/106060] New: Inefficient constant broadcast on x86_64

["goldstein.w.n at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106060

            Bug ID: 106060
           Summary: Inefficient constant broadcast on x86_64
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: goldstein.w.n at gmail dot com
  Target Milestone: ---

```
#include <immintrin.h>

__m256i
shouldnt_have_movabs ()
{
  return _mm256_set1_epi8 (123);
}

__m256i
should_be_cmpeq_abs ()
{
  return _mm256_set1_epi8 (1);
}

__m256i
should_be_cmpeq_add ()
{
  return _mm256_set1_epi8 (-2);
}
```

Compiled with: '-O3 -march=x86-64-v3'

Results in:
```
Disassembly of section .text:

0000000000000000 <shouldnt_have_movabs>:
   0:   48 b8 7b 7b 7b 7b 7b    movabs $0x7b7b7b7b7b7b7b7b,%rax
   7:   7b 7b 7b
   a:   c4 e1 f9 6e c8          vmovq  %rax,%xmm1
   f:   c4 e2 7d 59 c1          vpbroadcastq %xmm1,%ymm0
  14:   c3                      retq
  15:   66 66 2e 0f 1f 84 00    data16 nopw %cs:0x0(%rax,%rax,1)
  1c:   00 00 00 00

0000000000000020 <should_be_cmpeq_abs>:
  20:   48 b8 01 01 01 01 01    movabs $0x101010101010101,%rax
  27:   01 01 01
  2a:   c4 e1 f9 6e c8          vmovq  %rax,%xmm1
  2f:   c4 e2 7d 59 c1          vpbroadcastq %xmm1,%ymm0
  34:   c3                      retq
  35:   66 66 2e 0f 1f 84 00    data16 nopw %cs:0x0(%rax,%rax,1)
  3c:   00 00 00 00

0000000000000040 <should_be_cmpeq_add>:
  40:   48 b8 fe fe fe fe fe    movabs $0xfefefefefefefefe,%rax
  47:   fe fe fe
  4a:   c4 e1 f9 6e c8          vmovq  %rax,%xmm1
  4f:   c4 e2 7d 59 c1          vpbroadcastq %xmm1,%ymm0
  54:   c3                      retq
```

Compiled with: '-O3 -march=x86-64-v4'

Results in:
```
0000000000000000 <shouldnt_have_movabs>:
   0:   48 b8 7b 7b 7b 7b 7b    movabs $0x7b7b7b7b7b7b7b7b,%rax
   7:   7b 7b 7b
   a:   62 f2 fd 28 7c c0       vpbroadcastq %rax,%ymm0
  10:   c3                      retq
  11:   66 66 2e 0f 1f 84 00    data16 nopw %cs:0x0(%rax,%rax,1)
  18:   00 00 00 00
  1c:   0f 1f 40 00             nopl   0x0(%rax)

0000000000000020 <should_be_cmpeq_abs>:
  20:   48 b8 01 01 01 01 01    movabs $0x101010101010101,%rax
  27:   01 01 01
  2a:   62 f2 fd 28 7c c0       vpbroadcastq %rax,%ymm0
  30:   c3                      retq
  31:   66 66 2e 0f 1f 84 00    data16 nopw %cs:0x0(%rax,%rax,1)
  38:   00 00 00 00
  3c:   0f 1f 40 00             nopl   0x0(%rax)

0000000000000040 <should_be_cmpeq_add>:
  40:   48 b8 fe fe fe fe fe    movabs $0xfefefefefefefefe,%rax
  47:   fe fe fe
  4a:   62 f2 fd 28 7c c0       vpbroadcastq %rax,%ymm0
  50:   c3                      retq
```


All functions / targets are inoptimal.

Generating 1/2 can be done without any lane-cross broadcast.

Generating constants like 123 shouldn't first be constant broadcast
into an imm64. That makes it require an 10-byte `movabs` and wastes
spaces.