[Bug middle-end/34678] Optimization generates incorrect code with -frounding-math option (#pragma STDC FENV_ACCESS not implemented)

[nmm1 at cam dot ac.uk]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34678

--- Comment #25 from Nick Maclaren <nmm1 at cam dot ac.uk> ---
On Jan 10 2014, vincent-gcc at vinc17 dot net wrote:
>
>http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34678
>
>--- Comment #24 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> -
>
>(In reply to Nick Maclaren from comment #23)
>
>> If __STDC_IEC_559__ is unset or does not have the value 1, setting
>> STDC FENV_ACCESS to "on" is undefined behaviour (see 6.10.8.3, 7.6 and
>> Annex F), unless the implementation explicitly chooses to extend the
>> language to support it.
>
>You're wrong. The C standard doesn't say that.

I am sorry, but it is you that is wrong.

>6.10.8.3 says: "__STDC_IEC_559__ The integer constant 1, intended to indica
>te
>conformance to the specifications in annex F (IEC 60559 floating-point
>arithmetic)." and nothing about STDC FENV_ACCESS.

3.4.3 says:
    undefined behavior
    behavior, upon use of a nonportable or erroneous program construct
    or of erroneous data, for which this International Standard imposes
    no requirements

4. Conformance, paragraph 2, says:
    ...  Undefined behavior is otherwise indicated in this International
    Standard by the words "undefined behavior" or by the omission of any
    explicit definition of behavior.  There is no difference in emphasis
    among these three; they all describe "behavior that is undefined".

What "explicit definition of behavior" is there for the case when
STDC FENV_ACCESS is set to "on" but __STDC_IEC_559__ is not set to one?

As there is none, it is undefined behaviour.  gcc can therefore do
whatever it likes.


Regards,
Nick Maclaren.