[Bug target/116129] New: Use SVE INDEX instruction to create constant vectors

[ktkachov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116129

            Bug ID: 116129
           Summary: Use SVE INDEX instruction to create constant vectors
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: aarch64-sve, missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ktkachov at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64

The INDEX (immediates) instruction can be used to create vectors based on a
base value and a step. We should make use of it even if generating fixed-length
vector immediates as it allows base and step values in [-16, 15]. We can use it
when TARGET_SVE is available.

For example:
#include <arm_sve.h>

typedef char v16qi __attribute__((vector_size (16)));

v16qi
foo (void)
{
  return (v16qi){0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
}

svint8_t
foo_sve (void)
{
  return svindex_s8 (0, 1);
}

These two functions should just use:
index   z0.b, #0, #1

Currently function foo will emit an ADR and an LDR from a constant pool