On 2023-10-25 03:21, Bruno Haible via Cygwin wrote:
Hi,
Seen on
- Cygwin 3.4.6 or 2.9.0 on x86_64.
- Cygwin 2.9.0 on i386.
According to ISO C 23 § 7.6.4.3
"The feraiseexcept function attempts to raise the supported floating-point
exceptions represented by its argument. 266)
Footnote 266) The effect is intended to be similar to that of floating-point
exceptions raised by arithmetic operations. Hence, implementation extensions
associated with raising a floating-point exception (for example, enabled
traps or IEC 60559 alternate exception handling) should be honored."
This does not work. How to reproduce:
=================================== foo.c ===================================
#define _GNU_SOURCE 1
#include <fenv.h>
#include <assert.h>
int
main ()
{
/* Clear FE_INVALID exceptions from past operations. */
if (feclearexcept (FE_INVALID))
return 1;
/* An FE_INVALID exception shall trigger a SIGFPE signal, which by default
terminates the program. */
if (feenableexcept (FE_INVALID) == -1)
return 2;
if (feraiseexcept (FE_INVALID))
return 3;
return 0;
}
=============================================================================
For x86_64:
$ x86_64-pc-cygwin-gcc -Wall foo.c
$ ./a.exe; echo $?
For i386:
$ i686-pc-cygwin-gcc -Wall foo.c
$ ./a.exe; echo $?
Expected result (like seen e.g. on Linux/glibc):
Floating-point exception (core dumped)
136
Actual result:
0
The workaround for x86_64 is to redefine feeraiseexcept in the same way
as glibc does. This modified test program includes the workaround:
=================================== foo.c ===================================
#define _GNU_SOURCE 1
#include <fenv.h>
#include <assert.h>
#ifdef REDEFFE
/* The floating-point environment of the 387 unit. */
typedef struct
{
/* 7 32-bit words: */
unsigned short __control_word; /* fctrl register */
unsigned short __reserved1;
unsigned short __status_word; /* fstat register */
unsigned short __reserved2;
unsigned int more[5];
}
x86_387_fenv_t;
int
feraiseexcept (int exceptions)
{
exceptions &= FE_ALL_EXCEPT;
if ((exceptions & ~(FE_INVALID | FE_DIVBYZERO)) == 0 && 0)
{
/* First: invalid exception. */
if (exceptions & FE_INVALID)
{
double volatile a;
double volatile b;
a = 0; b = a / a;
(void) b;
}
/* Next: division by zero. */
if (exceptions & FE_DIVBYZERO)
{
double volatile a, b;
double volatile c;
a = 1; b = 0; c = a / b;
(void) c;
}
}
else
{
/* The general case. */
/* Set the bits in the 387 unit. */
x86_387_fenv_t env;
unsigned short orig_status_word;
__asm__ __volatile__ ("fnstenv %0" : "=m" (*&env));
orig_status_word = env.__status_word;
env.__status_word |= exceptions;
if (env.__status_word != orig_status_word)
{
__asm__ __volatile__ ("fldenv %0" : : "m" (*&env));
/* A trap (if enabled) is triggered only at the next floating-point
instruction. Force it to occur here. */
__asm__ __volatile__ ("fwait");
}
}
return 0;
}
#endif /* REDEFFE */
int
main ()
{
/* Clear FE_INVALID exceptions from past operations. */
if (feclearexcept (FE_INVALID))
return 1;
/* An FE_INVALID exception shall trigger a SIGFPE signal, which by default
terminates the program. */
if (feenableexcept (FE_INVALID) == -1)
return 2;
if (feraiseexcept (FE_INVALID))
return 3;
return 0;
}
=============================================================================
(This workaround *should* also work on i386, but it doesn't. I don't know
why.)
Thanks for the report Bruno,
Confirmed on Cygwin current stable with #ifdef added for ease of rebuild:
$ uname -srvmo
CYGWIN_NT-10.0-19045 3.4.9-1.x86_64 2023-09-06 11:19 UTC x86_64 Cygwin
$ gcc -UREDEFFE -o feraiseexcept{,.c} && ./feraiseexcept; echo $?
0
$ gcc -DREDEFFE -o feraiseexcept{,.c} && ./feraiseexcept; echo $?
Floating point exception (core dumped)
136
Cygwin no longer supports x86/i686 32 bit.
Anyone have a Cygwin 3.5.0 test release environment available, just in case it
might be different in some way, and could confirm?
The code is in newlib, so redirecting there:
https://sourceware.org/cgit/newlib-cygwin/tree/newlib/libm/machine/shared_x86/fenv.c#n253
That only has similar common code as in glibc, not the specific exceptions.
Please, could anyone test on a non-Cygwin newlib x86/_64 build?
The BSDs store, add exceptions, then load first the X87 then SSE MXCSR
environments:
https://cgit.freebsd.org/src/tree/lib/msun/amd64/fenv.c#n32
https://github.com/NetBSD/src/blob/trunk/lib/libm/arch/x86_64/fenv.c#L195
https://github.com/openbsd/src/blob/master/lib/libm/arch/amd64/fenv.c#L116
which is handled in newlib fenv.c functions by fgetenv/fsetenv *except* in
feraiseexcept!
--
Take care. Thanks, Brian Inglis Calgary, Alberta, Canada
La perfection est atteinte Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer but when there is no more to cut
-- Antoine de Saint-Exupéry
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