[Bug target/37581] New: IEEE inexact-flag not working on the Alpha

[Bug target/37581] New: IEEE inexact-flag not working on the Alpha

[bagnara at cs dot unipr dot it]

The following shows a problem on the Alpha whereby
the division 2/3 made on floats is flagged as exact.
Here are the details:

$ cat sf.cc
#include <fenv.h>
#include <cstdio>


int main() {
  float x = 2;
  float y = 3;
  feclearexcept(FE_INEXACT);
  x = x / y;
  printf("%d %.1000g\n", fetestexcept(FE_INEXACT) != 0, x);
}
$ g++ -v
Using built-in specs.
Target: alpha-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.2
--program-suffix=-4.2 --enable-clocale=gnu --enable-libstdcxx-debug
--enable-objc-gc --enable-mpfr --disable-libssp --with-long-double-128
--enable-checking=release --build=alpha-linux-gnu --host=alpha-linux-gnu
--target=alpha-linux-gnu
Thread model: posix
gcc version 4.2.4 (Debian 4.2.4-3)
$ g++ -mieee-with-inexact sf.cc
$ ./a.out
0 0.666666686534881591796875
$ cat /proc/cpuinfo
cpu                     : Alpha
cpu model               : EV56
cpu variation           : 7
cpu revision            : 0
cpu serial number       :
system type             : Rawhide
system variation        : Tincup
system revision         : 0
system serial number    : AY74642662
cycle frequency [Hz]    : 399642346 est.
timer frequency [Hz]    : 1200.00
page size [bytes]       : 8192
phys. address bits      : 40
max. addr. space #      : 127
BogoMIPS                : 705.16
kernel unaligned acc    : 0 (pc=0,va=0)
user unaligned acc      : 31 (pc=20000074c18,va=87)
platform string         : AlphaServer 1200 5/400 4MB
cpus detected           : 1
cpus active             : 1
cpu active mask         : 0000000000000001
L1 Icache               : 8K, 1-way, 32b line
L1 Dcache               : 8K, 1-way, 32b line
L2 cache                : 96K, 3-way, 64b line
L3 cache                : 4096K, 1-way, 64b line
$

I attach the generated assembly code, which shows this is not a constant
propagation issue.


-- 
           Summary: IEEE inexact-flag not working on the Alpha
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bagnara at cs dot unipr dot it
  GCC host triplet: alphaev56-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581

[Bug target/37581] IEEE inexact-flag not working on the Alpha

[bagnara at cs dot unipr dot it]

------- Comment #1 from bagnara at cs dot unipr dot it  2008-09-19 07:04 -------
Created an attachment (id=16359)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=16359&action=view)
Assembly code generated with g++ -S -mieee-with-inexact sf.cc


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581

[Bug target/37581] IEEE inexact-flag not working on the Alpha

[ubizjak at gmail dot com]

------- Comment #2 from ubizjak at gmail dot com  2009-01-23 08:47 -------
Works for me with:

--cut here--
#define _GNU_SOURCE
#include <fenv.h>
#include <signal.h>
#include <stdlib.h>

static void foo (int sig)
{
  exit (0);
}

float __attribute__((noinline)) test (float x) { return 2.0f / x; };

int main()
{
  volatile float x;

  signal (SIGFPE, foo);

  feclearexcept (FE_ALL_EXCEPT);
  feenableexcept (FE_INEXACT);

  x = test (3.0f);

  abort ();
}
--cut here--

$ gcc -O2 -lm -mieee-with-inexact sf.c
$ ./a.out
$


-- 

ubizjak at gmail dot com changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581

[Bug target/37581] IEEE inexact-flag not working on the Alpha

[abramobagnara at tin dot it]

------- Comment #3 from abramobagnara at tin dot it  2009-01-24 05:43 -------
The test case you have used it's different from the original that showed the
bug.

Nevertheless it's useful to understand the possible nature of the bug.

It seems that if feenableexcept(FE_INEXACT) is not called
fetestexcept(FE_INEXACT) doesn't work as expected (and as C99 standard
provides).

Please note that according to documentation GNU extension
feenableexcept/fedisablexcept does not enable/disable inexact detection, but
does enable/disable inexact *trapping* via SIGFPE signal.

Here below there is a new test case that shows the wrong behaviour.

$ cat sf2.c 
#define _GNU_SOURCE
#include <fenv.h>
#include <signal.h>
#include <stdio.h>

static void foo (int sig)
{
  printf("inexact\n");
}


float __attribute__((noinline)) test (float x) {
        printf("%f / %f\n", 2.0f, x);
        return 2.0f / x;
}

void t()
{
  volatile float x;
  feclearexcept (FE_ALL_EXCEPT);
  x = test (3.0f);
  printf("fetestexcept(FE_INEXACT) = %d\n", fetestexcept(FE_INEXACT));
  feclearexcept (FE_ALL_EXCEPT);
  x = test (2.0f);
  printf("fetestexcept(FE_INEXACT) = %d\n", fetestexcept(FE_INEXACT));
}

int main() {
  printf("\nWith FE_INEXACT SIGFPE disabled\n");
  t();

  printf("\nWith FE_INEXACT SIGFPE enabled\n");
  signal (SIGFPE, foo);
  feenableexcept (FE_INEXACT);
  t();
}
$ gcc -O2 -lm -mieee-with-inexact sf2.c
$ ./a.out

With FE_INEXACT SIGFPE disabled
2.000000 / 3.000000
fetestexcept(FE_INEXACT) = 0
2.000000 / 2.000000
fetestexcept(FE_INEXACT) = 0

With FE_INEXACT SIGFPE enabled
2.000000 / 3.000000
inexact
fetestexcept(FE_INEXACT) = 2097152
2.000000 / 2.000000
fetestexcept(FE_INEXACT) = 0
$


-- 

abramobagnara at tin dot it changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |abramobagnara at tin dot it


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581

[Bug target/37581] IEEE inexact-flag not working on the Alpha

[bagnara at cs dot unipr dot it]

------- Comment #4 from bagnara at cs dot unipr dot it  2009-01-24 08:08 -------
I don't know why the bug was closed: I can confirm the erroneous behavior is
present also in GCC 4.3.2.


-- 

bagnara at cs dot unipr dot it changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581

[Bug target/37581] IEEE inexact-flag not working on the Alpha

[ubizjak at gmail dot com]

------- Comment #5 from ubizjak at gmail dot com  2009-01-24 10:17 -------
(In reply to comment #4)
> I don't know why the bug was closed: I can confirm the erroneous behavior is
> present also in GCC 4.3.2.

Because it looks to me that this is libc problem with fetestexcept. As far as
the compiler is concerned, GCC correctly decorates trapping instructions with
/sui and puts trap barrier to expected place:

        ...
        jsr $26,($27),__nldbl_printf            !lituse_jsr!5
        ldah $29,0($26)         !gpdisp!6
        divs/sui $f3,$f2,$f0
        trapb
        ...


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581

[Bug target/37581] IEEE inexact-flag not working on the Alpha

[abramobagnara at tin dot it]

------- Comment #6 from abramobagnara at tin dot it  2009-01-24 15:41 -------
(In reply to comment #5)

> Because it looks to me that this is libc problem with fetestexcept. As far as
> the compiler is concerned, GCC correctly decorates trapping instructions with
> /sui and puts trap barrier to expected place:
> 
>         ...
>         jsr $26,($27),__nldbl_printf            !lituse_jsr!5
>         ldah $29,0($26)         !gpdisp!6
>         divs/sui $f3,$f2,$f0
>         trapb
>         ...

I've reported a bug according to the valuable info you provide.

http://sourceware.org/bugzilla/show_bug.cgi?id=9783


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581

[Bug target/37581] IEEE inexact-flag not working on the Alpha

[ubizjak at gmail dot com]

------- Comment #7 from ubizjak at gmail dot com  2009-02-03 11:10 -------
Not a GCC bug.


-- 

ubizjak at gmail dot com changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581