Floating point trouble with x86's extended precision

[John Vickers]

Robert Dewar wrote:
>>The current behavior sometimes results in objects which appear to
>>spontaneously change their values between two read accesses (without
>>any intermediate write access).  I find it hard to believe that such
>>indeterminacy is permitted by any language standard.
> 
> 
> It is certainly permitted by the Fortran 77 standard
> It is certainly permitted by the Ada 83 standard
> It is certainly permitted by the Ada 95 standard (very explicitly)
> 
> As always it is better to go by what is in the standard rather than what
> you believe (or find hard to believe). 
> 
> With regard to the Fortran 77 standard, this is the subject of a well 
> known letter from Kahane, complaining of a bug, but in fact this was
> not a bug according to the standard, though it certainly surprised
> the writer of that letter :-)
> 
> I will let someone else quote language from the C or C++ standard that
> would show that Florians universal quantification at least is correct
> in existential form :-)

;-)

OK.  FWIW, C99 is quite specific about this particular case.

[5.1.2.3#12 of ISO/IEC9899:1999 - extra blank lines
inserted; emphasis is original]

"Implementations employing wide registers have to take care to honor
appropriate semantics.

Values are independent of whether they are represented in a register or in memory.

For example, an implicit /spilling/ of a register is not permitted to alter the value.

Also, an explicit /store/ and /load/ is required to round to the precision of
the storage type.

In particular, casts and assignments are required to perform their specified conversion.

[example floating-point code elided]"


Also, in 6.2.4#2:

"An object exists, has a constant address, and retains its last-stored value
throughout its lifetime."

No special exemption for floating-point types.




So it seems like GCC on i387 is non-conforming (to C99).   And it looks like
the authors might have had i387 in mind when they wrote this.

Whether that non-conformity is considered to be a bug in GCC or not is a different
question.