diff mbox series

[v2,2/3] i686: Do not raise exception traps on fesetexcept (BZ 30989)

Message ID 20231024113716.3911015-3-adhemerval.zanella@linaro.org
State New
Headers show
Series Fix fesetexcept/fesetexceptflag on powerpc and x86 | expand

Commit Message

Adhemerval Zanella Netto Oct. 24, 2023, 11:37 a.m. UTC 
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.
---
 math/test-fesetexcept-traps.c            | 11 +++++++
 sysdeps/i386/fpu/fesetexcept.c           | 41 +++++++++++++++++++++---
 sysdeps/i386/fpu/math-tests-trap-force.h | 29 +++++++++++++++++
 3 files changed, 77 insertions(+), 4 deletions(-)
 create mode 100644 sysdeps/i386/fpu/math-tests-trap-force.h

Comments

Bruno Haible Oct. 24, 2023, 1:43 p.m. UTC | #1 
Looks all good, except for two comments:

Adhemerval Zanella wrote:
> diff --git a/sysdeps/i386/fpu/fesetexcept.c b/sysdeps/i386/fpu/fesetexcept.c
> index 18949e982a..6eeb5ab5b0 100644
> --- a/sysdeps/i386/fpu/fesetexcept.c
> +++ b/sysdeps/i386/fpu/fesetexcept.c
> @@ -17,15 +17,48 @@
>     <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))
> +    {
> +      /* And now similarly for SSE.  */
> +      unsigned int mxcsr;
> +      __asm__ ("stmxcsr %0" : "=m" (*&mxcsr));

The comment "And now similarly for SSE." is odd, since there was no similar code
before it. How about
         /* Set the flags in the SSE unit.  */

> +      /* Set relevant flags.  */
> +      mxcsr |= excepts;
> +
> +      /* Put the new data in effect.  */
> +      __asm__ ("ldmxcsr %0" : : "m" (*&mxcsr));
> +    }
> +  else
> +    {
> +      fenv_t temp;
> +
> +      __asm__ ("fnstenv %0" : "=m" (*&temp));
> +
> +      /* Clear or set relevant flags.  */
> +      temp.__status_word |= temp.__status_word & excepts;

The comment here should be
         /* Set relevant flags.  */
since here, unlike in fesetexceptflag(), we don't need to clear any flags.

Bruno
Adhemerval Zanella Netto Oct. 26, 2023, 6:39 p.m. UTC | #2 
On 24/10/23 10:43, Bruno Haible wrote:
> Looks all good, except for two comments:
> 
> Adhemerval Zanella wrote:
>> diff --git a/sysdeps/i386/fpu/fesetexcept.c b/sysdeps/i386/fpu/fesetexcept.c
>> index 18949e982a..6eeb5ab5b0 100644
>> --- a/sysdeps/i386/fpu/fesetexcept.c
>> +++ b/sysdeps/i386/fpu/fesetexcept.c
>> @@ -17,15 +17,48 @@
>>     <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))
>> +    {
>> +      /* And now similarly for SSE.  */
>> +      unsigned int mxcsr;
>> +      __asm__ ("stmxcsr %0" : "=m" (*&mxcsr));
> 
> The comment "And now similarly for SSE." is odd, since there was no similar code
> before it. How about
>          /* Set the flags in the SSE unit.  */

Ack.

> 
>> +      /* Set relevant flags.  */
>> +      mxcsr |= excepts;
>> +
>> +      /* Put the new data in effect.  */
>> +      __asm__ ("ldmxcsr %0" : : "m" (*&mxcsr));
>> +    }
>> +  else
>> +    {
>> +      fenv_t temp;
>> +
>> +      __asm__ ("fnstenv %0" : "=m" (*&temp));
>> +
>> +      /* Clear or set relevant flags.  */
>> +      temp.__status_word |= temp.__status_word & excepts;
> 
> The comment here should be
>          /* Set relevant flags.  */
> since here, unlike in fesetexceptflag(), we don't need to clear any flags.

Ack.

I changed both locally.
Bruno Haible Oct. 30, 2023, 3:21 p.m. UTC | #3 
Hi Adhemerval,

In the error case, the new code mistakenly masks all floating-point
exceptions (i.e. as if someone had called fedisableexcept (FE_ALL_EXCEPT)).
This is because the FNSTENV instruction is documented as
   "Saves the current FPU operating environment at the memory
    location specified with the destination operand, and then
    masks all floating-point exceptions."

The mistake came from my initial proposed fix of BZ 30990. Sorry about that.

Here's a proposed fix, on top of your patch. I've verified that the sequence
of instructions
    __asm__ ("fnstenv %0" : "=m" (*&temp));
    __asm__ volatile ("fldcw %0" : : "m" (*&temp.__control_word));
does restore the exceptions trapping bits.

Bruno
Adhemerval Zanella Netto Oct. 30, 2023, 4:05 p.m. UTC | #4 
On 30/10/23 12:21, Bruno Haible wrote:
> Hi Adhemerval,
> 
> In the error case, the new code mistakenly masks all floating-point
> exceptions (i.e. as if someone had called fedisableexcept (FE_ALL_EXCEPT)).
> This is because the FNSTENV instruction is documented as
>    "Saves the current FPU operating environment at the memory
>     location specified with the destination operand, and then
>     masks all floating-point exceptions."
> 
> The mistake came from my initial proposed fix of BZ 30990. Sorry about that.
> 
> Here's a proposed fix, on top of your patch. I've verified that the sequence
> of instructions
>     __asm__ ("fnstenv %0" : "=m" (*&temp));
>     __asm__ volatile ("fldcw %0" : : "m" (*&temp.__control_word));
> does restore the exceptions trapping bits.
> 
> Bruno

Right, it would be good to have a regression check for this.  I will update
the patch.
diff mbox series

Patch

diff --git a/math/test-fesetexcept-traps.c b/math/test-fesetexcept-traps.c
index 96f6c4752f..2f63e9ba23 100644
--- a/math/test-fesetexcept-traps.c
+++ b/math/test-fesetexcept-traps.c
@@ -19,6 +19,7 @@ 
 #include <fenv.h>
 #include <stdio.h>
 #include <math-tests.h>
+#include <math-barriers.h>
 
 static int
 do_test (void)
@@ -43,6 +44,16 @@  do_test (void)
      where setting the exception might result in traps the function should
      return a nonzero value.  */
   ret = fesetexcept (FE_ALL_EXCEPT);
+
+  /* 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)
     puts ("fesetexcept (FE_ALL_EXCEPT) succeeded");
   else if (!EXCEPTION_SET_FORCES_TRAP)
diff --git a/sysdeps/i386/fpu/fesetexcept.c b/sysdeps/i386/fpu/fesetexcept.c
index 18949e982a..6eeb5ab5b0 100644
--- a/sysdeps/i386/fpu/fesetexcept.c
+++ b/sysdeps/i386/fpu/fesetexcept.c
@@ -17,15 +17,48 @@ 
    <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))
+    {
+      /* And now similarly for SSE.  */
+      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;
+
+      __asm__ ("fnstenv %0" : "=m" (*&temp));
+
+      /* Clear or set relevant flags.  */
+      temp.__status_word |= 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.  */
+        return -1;
+
+      /* Store the new status word (along with the rest of the environment).  */
+      __asm__ ("fldenv %0" : : "m" (*&temp));
+    }
 
   return 0;
 }
diff --git a/sysdeps/i386/fpu/math-tests-trap-force.h b/sysdeps/i386/fpu/math-tests-trap-force.h
new file mode 100644
index 0000000000..20f4ead98d
--- /dev/null
+++ b/sysdeps/i386/fpu/math-tests-trap-force.h
@@ -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.  */