[Bug target/71276] New: frndint generation should not depend on flag_unsafe_math_optimizations

["jsm28 at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71276

            Bug ID: 71276
           Summary: frndint generation should not depend on
                    flag_unsafe_math_optimizations
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---
            Target: i?86-*-* x86_64-*-*

When using x87 floating point, the x86 back end supports generating inline code
sequences using the frndint instruction for the rint / ceil / floor / trunc
built-in functions (for SFmode, DFmode and XFmode).  But those are conditioned
on flag_unsafe_math_optimizations.

There is no need for them to be conditioned on flag_unsafe_math_optimizations. 
For rint, frndint is fully correct.  For the others, the only issue is that it
raises "inexact" for non-integer operands, whereas TS 18661-1 specifies that
these functions should not raise "inexact".  But:

(a) We don't have any options to select TS 18661-1 requirements, and C99 and
C11 leave it unspecified whether "inexact" is raised, so raising it is OK for
currently supported standards.

(b) Even with TS 18661-1 requirements, it would be OK to use this instruction
if !flag_trapping_math.

(c) The documentation of the .md file patterns for ceil / floor / trunc says
nothing about whether "inexact" is raised or not.

So these inlines should be enabled whenever x87 floating point is in use (maybe
subject to code size considerations; you'd need to check how long the sequences
setting / restoring the rounding mode are compared to a call).  And future TS
18661-1 support could disable those for ceil / floor / trunc if flag_ts_18661_1
&& flag_trapping_math.