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To: cygwin AT cygwin DOT com
Subject: feraiseexcept does not raise traps in Cygwin
Date: Wed, 25 Oct 2023 11:21:47 +0200
Message-ID: <39807966.XM6RcZxFsP@nimes>
MIME-Version: 1.0
From: Bruno Haible via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Bruno Haible <bruno AT clisp DOT org>
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Sender: "Cygwin" <cygwin-bounces+archive-cygwin=delorie DOT com AT cygwin DOT com>
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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>

/* 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;
}

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.)

Bruno


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