[Bug c/35489] New: Inaccurate GCC documentation

[Bug c/35489] New: Inaccurate GCC documentation

[adam at irvine dot com]

I'm not sure whether this is the proper method for reporting errors in
the documentation.  However, I believe there is a serious error in the
GCC documentation that needs to be addressed.

The error is at http://gcc.gnu.org/bugs.html#nonbugs_general.  This
section refers to inherent limitations of floating-point types because
most numbers can be represented only approximately.

The problem is that this "non-bug", which is connected to problem
report #323 and has a hyperlink to it, encompasses many other problem
reports that are not accurately described by this documentation
section.  Some are due to the inherent limitations, but others are
actual bugs in which the compiler is generating incorrect instructions
(such as #12331); others are due to a particular floating-point
processor feature that can be defeated with a flag (but it could be
considered a bug that the compiler is not able to take proper advantage
of this feature); still others could marginally be considered "bugs"
because a compiler could, in theory, generate correct code although
expecting it to do so may not be reasonable.

For honesty's sake, since so many problem reports end up with users
being referred to this section, the documentation should accurately
describe the situation (including the fact that there are known bugs
which may be difficult to fix), rather than giving the false
implication that all such problems are the user's fault for doing
something ignorant.  I believe this to be a serious inaccuracy in the
documentation, and one that I think has been leaving users in an
unhelpful state of confusion.


-- 
           Summary: Inaccurate GCC documentation
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: adam at irvine dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35489

[Bug c/35489] Inaccurate GCC documentation

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2008-03-06 23:58 -------
> but others are
actual bugs in which the compiler is generating incorrect instructions
(such as #12331);

That is not incorrect instructions.  Just different than what you are
expecting.  All can be defected by the compiler option to turn the fp precision
differently.




*** This bug has been marked as a duplicate of 323 ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35489

[Bug c/35489] Inaccurate GCC documentation

[adam at irvine dot com]

------- Comment #2 from adam at irvine dot com  2008-03-07 00:08 -------
(In reply to comment #1)
> > but others are
> actual bugs in which the compiler is generating incorrect instructions
> (such as #12331);
> 
> That is not incorrect instructions.  Just different than what you are
> expecting. 

I suppose that if the chip has an instruction that would allow it to store the
correct value without losing any precision, and the compiler instead chooses to
generate an instruction that causes the computation to lose precision, that
could be considered "different than what I am expecting", yes.

Actually, I like that response.  I might try to use it myself next time one of
our customers reports a problem.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35489

[Bug c/35489] Inaccurate GCC documentation

[manu at gcc dot gnu dot org]

------- Comment #3 from manu at gcc dot gnu dot org  2008-03-10 00:37 -------
@Adam,

If you think that something is wrong in the documentation, please point out
exactly which text should be removed and what should be added. Also, feel free
to submit a documentation patch: http://gcc.gnu.org/contribute.html#docchanges
A patch would certainly help to clarify why you think the current doc is wrong
and what you think should be the correct wording.

As for GCC issues with fp precision, it is an ongoing debate. It is not crystal
clear which optimizations (if any) GCC should avoid for the sake of precision. 

The main sources of info right now are:

http://gcc.gnu.org/wiki/FP_BOF
http://gcc.gnu.org/wiki/Math_Optimization_Flags
http://gcc.gnu.org/wiki/GeertBosch

If after reading those, you think that you can contribute something new or you
still think that some issue should be revisited, then submit an email to the
gcc@gcc.gnu.org list. Please, focus on a particular issue, with examples and
reasoning. As always, patches and testcases would further help your cause.

Thanks.


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35489

[Bug c/35489] Inaccurate GCC documentation

[manu at gcc dot gnu dot org]

------- Comment #4 from manu at gcc dot gnu dot org  2008-03-10 00:52 -------
(In reply to comment #2)
> 
> Actually, I like that response.  I might try to use it myself next time one of
> our customers reports a problem.

I guess that your contracted GCC support developers may give you a reply that
you like even more, so better go ask them.

(That wouldn't be a nice reply, would it? So please understand that your jest
isn't nice to us either. We are happy to receive and fix bug reports, but
politeness certainly helps when asking for something.)


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|0000-00-00 00:00:00         |2008-03-10 00:52:02
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35489