[Bug target/100712] New: The vec_splatid instruction allows the creation of XXSPLTIDP instructions which produces undefined results.

[Bug target/100712] New: The vec_splatid instruction allows the creation of XXSPLTIDP instructions which produces undefined results.

[meissner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100712

            Bug ID: 100712
           Summary: The vec_splatid instruction allows the creation of
                    XXSPLTIDP instructions which produces undefined
                    results.
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: meissner at gcc dot gnu.org
  Target Milestone: ---

The vec_splatid builtin from altivec.h (i.e. __builtin_vec_xxspltid) allows the
user to create an XXSPLTIDP instruction where the value is a single precision
denormal value.  Right now the compiler warns about this usage, but it should
really not allow it.

In addition, the warning is buried in rs6000_emit_xxspltidp_v2df (in rs6000.c).
 A better place might be in rs6000_expand_binop_builtin (in rs6000-calls.)
where other usage warnings are given.

[Bug target/100712] The vec_splatid instruction allows the creation of XXSPLTIDP instructions which produces undefined results.

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100712

--- Comment #1 from Segher Boessenkool <segher at gcc dot gnu.org> ---
As background, the ISA has in the xxspltidp description
  If IMM32 specifies a single-precision denormal value
  (i.e., bits 1:8 equal to 0 and bits 9:31 not equal to 0),
  the result is undefined.

I see no other warnings in rs6000_expand_binop_builtin btw?  There are some
errors there, but those are more basic out-of-range errors.  Something as
specific as this is probably best left where it is now.

Why do you think an error is better than a warning here?  We allow creating UB
everywhere else (but we do warn where possible).