[Bug target/100312] New: __builtin_ia32_maskloadpd256 and friends should be pure

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100312

            Bug ID: 100312
           Summary: __builtin_ia32_maskloadpd256 and friends should be
                    pure
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

The testcase in PR99912 shows that we fail to eliminate dead AVX2 masked loads
since the x86 specific builtins are not marked pure and thus are considered
to clobber their arguments and have other unwanted side-effects.

It looks like the "way" to change this is to remove the builtins from
i386-builtin.def and manually add them to ix86_init_mmx_sse_builtins like
the AVX2 gather builtins (which are properly constructed as pure).