i686: Do not raise exception traps on fesetexcept (BZ 30989)

According to ISO C23 (7.6.4.4), fesetexcept is supposed to set
floating-point exception flags without raising a trap (unlike
feraiseexcept, which is supposed to raise a trap if feenableexcept
was called with the appropriate argument).

The flags can be set in the 387 unit or in the SSE unit.  To set
a flag, it is sufficient to do it in the SSE unit, because that is
guaranteed to not trap.  However, on i386 CPUs that have only a
387 unit, set the flags in the 387, as long as this cannot trap.

Checked on i686-linux-gnu.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
This commit is contained in:
Adhemerval Zanella 2023-10-24 08:37:15 -03:00
parent ecb1e7220d
commit 47a9eeb9ba
4 changed files with 99 additions and 25 deletions

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@ -19,6 +19,7 @@
#include <fenv.h>
#include <stdio.h>
#include <math-tests.h>
#include <math-barriers.h>
static int
do_test (void)
@ -41,12 +42,32 @@ do_test (void)
/* Verify fesetexcept does not cause exception traps. For architectures
where setting the exception might result in traps the function should
return a nonzero value. */
return a nonzero value.
Also check if the function does not alter the exception mask. */
ret = fesetexcept (FE_ALL_EXCEPT);
_Static_assert (!(EXCEPTION_SET_FORCES_TRAP && !EXCEPTION_TESTS(float)),
"EXCEPTION_SET_FORCES_TRAP only makes sense if the "
"architecture suports exceptions");
{
int exc_before = fegetexcept ();
ret = fesetexcept (FE_ALL_EXCEPT);
int exc_after = fegetexcept ();
if (exc_before != exc_after)
{
puts ("fesetexcept (FE_ALL_EXCEPT) changed the exceptions mask");
return 1;
}
}
/* Execute some floating-point operations, since on some CPUs exceptions
triggers a trap only at the next floating-point instruction. */
volatile double a = 1.0;
volatile double b = a + a;
math_force_eval (b);
volatile long double al = 1.0L;
volatile long double bl = al + al;
math_force_eval (bl);
if (ret == 0)
{
@ -72,5 +93,4 @@ do_test (void)
return result;
}
#define TEST_FUNCTION do_test ()
#include "../test-skeleton.c"
#include <support/test-driver.c>

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@ -17,15 +17,53 @@
<https://www.gnu.org/licenses/>. */
#include <fenv.h>
#include <ldsodefs.h>
int
fesetexcept (int excepts)
{
fenv_t temp;
/* The flags can be set in the 387 unit or in the SSE unit. To set a flag,
it is sufficient to do it in the SSE unit, because that is guaranteed to
not trap. However, on i386 CPUs that have only a 387 unit, set the flags
in the 387, as long as this cannot trap. */
__asm__ ("fnstenv %0" : "=m" (*&temp));
temp.__status_word |= excepts & FE_ALL_EXCEPT;
__asm__ ("fldenv %0" : : "m" (*&temp));
excepts &= FE_ALL_EXCEPT;
if (CPU_FEATURE_USABLE (SSE))
{
/* Get the control word of the SSE unit. */
unsigned int mxcsr;
__asm__ ("stmxcsr %0" : "=m" (*&mxcsr));
/* Set relevant flags. */
mxcsr |= excepts;
/* Put the new data in effect. */
__asm__ ("ldmxcsr %0" : : "m" (*&mxcsr));
}
else
{
fenv_t temp;
/* Note: fnstenv masks all floating-point exceptions until the fldenv
or fldcw below. */
__asm__ ("fnstenv %0" : "=m" (*&temp));
/* Set relevant flags. */
temp.__status_word |= excepts;
if ((~temp.__control_word) & excepts)
{
/* Setting the exception flags may trigger a trap (at the next
floating-point instruction, but that does not matter).
ISO C23 (7.6.4.4) does not allow it. */
__asm__ volatile ("fldcw %0" : : "m" (*&temp.__control_word));
return -1;
}
/* Store the new status word (along with the rest of the environment). */
__asm__ ("fldenv %0" : : "m" (*&temp));
}
return 0;
}

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@ -0,0 +1,29 @@
/* Configuration for math tests: support for setting exception flags
without causing enabled traps. i686 version.
Copyright (C) 2023 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#ifndef I386_FPU_MATH_TESTS_TRAP_FORCE_H
#define I386_FPU_MATH_TESTS_TRAP_FORCE_H 1
#include <cpu-features.h>
/* Setting exception flags in FPU Status Register results in enabled traps for
those exceptions being taken. */
#define EXCEPTION_SET_FORCES_TRAP !CPU_FEATURE_USABLE (SSE)
#endif /* math-tests-trap-force.h. */

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@ -22,17 +22,8 @@
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
static bool
have_sse2 (void)
{
unsigned int eax, ebx, ecx, edx;
if (!__get_cpuid (1, &eax, &ebx, &ecx, &edx))
return false;
return (edx & bit_SSE2) != 0;
}
#include <cpu-features.h>
#include <support/check.h>
static uint32_t
get_sse_mxcsr (void)
@ -164,13 +155,9 @@ sse_tests (void)
static int
do_test (void)
{
if (!have_sse2 ())
{
puts ("CPU does not support SSE2, cannot test");
return 0;
}
if (!CPU_FEATURE_USABLE (SSE2))
FAIL_UNSUPPORTED ("CPU does not support SSE2");
return sse_tests ();
}
#define TEST_FUNCTION do_test ()
#include <test-skeleton.c>
#include <support/test-driver.c>