[Bug c/28795] New: __builtin_isunordered() and __builtin_isnan() should behave consistently

["iano at apple dot com" <gcc-bugzilla at gcc dot gnu dot org>]

I expect this is widespread over the entire GCC family, but at least with
Apple's GCC we have a consistency problem with the meaning of various hacky
math flags in GCC and methods to detect NaN's in GCC:

[ollmia:/tmp] iano% cat main3.c
#include <math.h>
#include <stdio.h>
#include <stdint.h>

int main( void )
{
        union
        {
                int32_t i;
                float   f;
        }u = {-1};

        printf( "__FINITE_MATH_ONLY__ = %d\n", __FINITE_MATH_ONLY__ );
        printf( "__builtin_isunordered(%f,%f) = %d\n", u.f, u.f,
__builtin_isunordered(u.f, u.f) );
        printf( "__builtin_isnan(%f) = %d\n", u.f, __builtin_isnan( u.f) );
        printf( " (%f != %f) = %d\n", u.f, u.f, u.f != u.f );


        return 0;
}
[ollmia:/tmp] iano% gcc main3.c -Wall; ./a.out
__FINITE_MATH_ONLY__ = 0
__builtin_isunordered(nan,nan) = 1
__builtin_isnan(nan) = 1
 (nan != nan) = 1
[ollmia:/tmp] iano% gcc main3.c -Wall -ffinite-math-only; ./a.out
__FINITE_MATH_ONLY__ = 1
__builtin_isunordered(nan,nan) = 1
__builtin_isnan(nan) = 0
 (nan != nan) = 0
[ollmia:/tmp] iano% gcc main3.c -Wall -mno-ieee-fp ; ./a.out
__FINITE_MATH_ONLY__ = 0
__builtin_isunordered(nan,nan) = 1
__builtin_isnan(nan) = 1
 (nan != nan) = 0
[ollmia:/tmp] iano% gcc main3.c -Wall -funsafe-math-optimizations ; ./a.out
__FINITE_MATH_ONLY__ = 0
__builtin_isunordered(nan,nan) = 0
__builtin_isnan(nan) = 0
 (nan != nan) = 0

Here's my plug:  I'm (speaking for) the rare developer who can actually use
these flags responsibly, who has actually verified that NaNs do not occur in my
code. However, to be responsible, I also need to guard my application against
NaNs contained in malicious data sources. So, even though I said that NaNs do
not occur, I still need a way to test for them.  This is very hard to do in a
cross platform way on GCC at the moment.

Most GCC engineers that I've spoken to say that because we've thrown the
standard out the window for speed, GCC should set all these tests to 0.  The
problem is that GCC doesn't seem to have actually done that. What GCC appears
to have done is remove some but not all of them, presumably because it was
convenient for the compiler to do it that way. This doesn't serve the end user.
If there is a philosophy here, either "correct at all costs" or "speed at all
costs", GCC should pick one and stick to it.

Personally, I favor correct at all costs for the __builtins. If the end user
really wants all isnan(x) to return 0 even if x is NaN (which I guarantee you,
he doesn't) he can just define his own test with x != x.  

Since I am personally a math library provider and need my isnan() to work
uniformly all the time, even when the user has a temporary bout with insanity
and turns IEEE-754 conformance off, I favor a __builtin_isnan() that always
works properly. Only then can I pay heed to the GCC advice:

     http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#Other-Builtins
      "GCC provides built-in versions of the ISO C99 floating point comparison
macros that avoid raising exceptions for unordered operands. They have the same
names as the standard macros ( isgreater, isgreaterequal, isless, islessequal,
islessgreater, and isunordered) , with __builtin_ prefixed. We intend for a
library implementor to be able to simply #define each standard macro to its
built-in equivalent."

...with emphasis on the last sentence.  I can not do this until you are
actually C99 compliant *all the time*.  I have to support well written legacy
applications that expect this macro to work *all the time*.


[ollmia:/tmp] iano% gcc -v
Using built-in specs.
Target: i686-apple-darwin8
Configured with: /private/var/tmp/gcc/gcc-5363.obj~28/src/configure
--disable-checking -enable-werror --prefix=/usr --mandir=/share/man
--enable-languages=c,objc,c++,obj-c++
--program-transform-name=/^[cg][^.-]*$/s/$/-4.0/
--with-gxx-include-dir=/include/c++/4.0.0 --with-slibdir=/usr/lib
--build=powerpc-apple-darwin8 --with-arch=nocona --with-tune=generic
--program-prefix= --host=i686-apple-darwin8 --target=i686-apple-darwin8
Thread model: posix
gcc version 4.0.1 (Apple Computer, Inc. build 5363)


-- 
           Summary: __builtin_isunordered() and __builtin_isnan() should
                    behave consistently
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: iano at apple dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28795