Patchwork Call math_opt_barrier inside if

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Submitter H.J. Lu
Date Jan. 14, 2016, 11:11 p.m.
Message ID <CAMe9rOpP3OkgYnN3axp++wqRnNxpKegui_nqYx0oFGk6TOKkAQ@mail.gmail.com>
Download mbox | patch
Permalink /patch/10387/
State New
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Comments

H.J. Lu - Jan. 14, 2016, 11:11 p.m.
On Thu, Jan 14, 2016 at 2:38 PM, Joseph Myers <joseph@codesourcery.com> wrote:
> On Thu, 14 Jan 2016, H.J. Lu wrote:
>
>> On Thu, Jan 14, 2016 at 2:13 PM, Joseph Myers <joseph@codesourcery.com> wrote:
>> > On Thu, 14 Jan 2016, H.J. Lu wrote:
>> >
>> >> Since floating-point operation may trigger floating-point exceptions,
>> >> we call math_opt_barrier inside if to prevent code motion.
>> >>
>> >> Tested on x86-64.  OK for trunk?
>> >
>> > Please send a patch updating all implementations for which this issue is
>> > applicable.
>>
>> There are
>>
>> dbl-64/e_sqrt.c:      libc_feholdexcept_setround (&env, FE_TONEAREST);
>> dbl-64/s_fma.c:  libc_feholdexcept_setround (&env, FE_TONEAREST);
>> dbl-64/s_fmaf.c:  libc_feholdexcept_setround (&env, FE_TOWARDZERO);
>>
>> under sysdeps/ieee754.   dbl-64/s_fma.c is the only one with this
>> problem.
>
> I disagree with that analysis.  The question isn't whether there's a call
> to a particular internal interface.  The question is whether there are
> conditionals on arithmetic, such that the arithmetic is exact in the
> conditional case but may be inexact if moved before the conditional.  I
> think at least ldbl-96/s_fmal.c, ldbl-96/s_fma.c and ldbl-128/s_fmal.c
> have the same issue.
>

Here is the updated patch.  OK for trunk?
Joseph Myers - Jan. 15, 2016, 2:11 a.m.
On Thu, 14 Jan 2016, H.J. Lu wrote:

> > I disagree with that analysis.  The question isn't whether there's a call
> > to a particular internal interface.  The question is whether there are
> > conditionals on arithmetic, such that the arithmetic is exact in the
> > conditional case but may be inexact if moved before the conditional.  I
> > think at least ldbl-96/s_fmal.c, ldbl-96/s_fma.c and ldbl-128/s_fmal.c
> > have the same issue.
> >
> 
> Here is the updated patch.  OK for trunk?

OK.

Patch

From 009223c61e461e0feeeadce85d87b0eb3910f38d Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Thu, 14 Jan 2016 12:49:13 -0800
Subject: [PATCH] Call math_opt_barrier inside if

Since floating-point operation may trigger floating-point exceptions,
we call math_opt_barrier inside if to prevent code motion.

	[BZ #19465]
	* sysdeps/ieee754/dbl-64/s_fma.c (__fma): Call math_opt_barrier
	inside if.
	* sysdeps/ieee754/ldbl-128/s_fmal.c (__fmal): Likewise.
	* sysdeps/ieee754/ldbl-96/s_fma.c (__fma): Likewise.
	* sysdeps/ieee754/ldbl-96/s_fmal.c (__fmal): Likewise.
---
 sysdeps/ieee754/dbl-64/s_fma.c    | 5 ++++-
 sysdeps/ieee754/ldbl-128/s_fmal.c | 5 ++++-
 sysdeps/ieee754/ldbl-96/s_fma.c   | 5 ++++-
 sysdeps/ieee754/ldbl-96/s_fmal.c  | 5 ++++-
 4 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/sysdeps/ieee754/dbl-64/s_fma.c b/sysdeps/ieee754/dbl-64/s_fma.c
index bc3677d..a349243 100644
--- a/sysdeps/ieee754/dbl-64/s_fma.c
+++ b/sysdeps/ieee754/dbl-64/s_fma.c
@@ -175,7 +175,10 @@  __fma (double x, double y, double z)
 
   /* Ensure correct sign of exact 0 + 0.  */
   if (__glibc_unlikely ((x == 0 || y == 0) && z == 0))
-    return x * y + z;
+    {
+      x = math_opt_barrier (x);
+      return x * y + z;
+    }
 
   fenv_t env;
   libc_feholdexcept_setround (&env, FE_TONEAREST);
diff --git a/sysdeps/ieee754/ldbl-128/s_fmal.c b/sysdeps/ieee754/ldbl-128/s_fmal.c
index ac30027..728949c 100644
--- a/sysdeps/ieee754/ldbl-128/s_fmal.c
+++ b/sysdeps/ieee754/ldbl-128/s_fmal.c
@@ -179,7 +179,10 @@  __fmal (long double x, long double y, long double z)
 
   /* Ensure correct sign of exact 0 + 0.  */
   if (__glibc_unlikely ((x == 0 || y == 0) && z == 0))
-    return x * y + z;
+    {
+      x = math_opt_barrier (x);
+      return x * y + z;
+    }
 
   fenv_t env;
   feholdexcept (&env);
diff --git a/sysdeps/ieee754/ldbl-96/s_fma.c b/sysdeps/ieee754/ldbl-96/s_fma.c
index 5fbdda7..ab45bcf 100644
--- a/sysdeps/ieee754/ldbl-96/s_fma.c
+++ b/sysdeps/ieee754/ldbl-96/s_fma.c
@@ -41,7 +41,10 @@  __fma (double x, double y, double z)
 
   /* Ensure correct sign of exact 0 + 0.  */
   if (__glibc_unlikely ((x == 0 || y == 0) && z == 0))
-    return x * y + z;
+    {
+      x = math_opt_barrier (x);
+      return x * y + z;
+    }
 
   fenv_t env;
   feholdexcept (&env);
diff --git a/sysdeps/ieee754/ldbl-96/s_fmal.c b/sysdeps/ieee754/ldbl-96/s_fmal.c
index 57774cf..f1467fd 100644
--- a/sysdeps/ieee754/ldbl-96/s_fmal.c
+++ b/sysdeps/ieee754/ldbl-96/s_fmal.c
@@ -177,7 +177,10 @@  __fmal (long double x, long double y, long double z)
 
   /* Ensure correct sign of exact 0 + 0.  */
   if (__glibc_unlikely ((x == 0 || y == 0) && z == 0))
-    return x * y + z;
+    {
+      x = math_opt_barrier (x);
+      return x * y + z;
+    }
 
   fenv_t env;
   feholdexcept (&env);
-- 
2.5.0