Fix __ieee754_logl (-LDBL_MAX) in FE_DOWNWARD mode (bug 17022)
Commit Message
This patch fixes __ieee754_logl (-LDBL_MAX) on x86_64 and x86 not to
subtract 1 from its argument and so cause spurious overflow in
FE_DOWNWARD mode. (For any argument strictly less than -1, it doesn't
matter whether or not 1 is subtracted before computing log1p, as long
as the result doesn't overflow to -Inf.)
Tested x86_64 and x86. (This particular case lacks test coverage,
since the testsuite doesn't cover -lieee, but it will be covered by
tests after the following patch to test pow in all rounding modes,
which was the context in which this bug was found.)
2014-06-04 Joseph Myers <joseph@codesourcery.com>
[BZ #17022]
* sysdeps/i386/fpu/e_logl.S (__ieee754_logl): Do not subtract 1
from arguments -2 or below.
* sysdeps/i386/i686/fpu/e_logl.S (__ieee754_logl): Likewise.
* sysdeps/x86_64/fpu/e_logl.S (__ieee754_logl): Likewise.
Comments
Ping. This patch
<https://sourceware.org/ml/libc-alpha/2014-06/msg00076.html> is pending
review.
Ping^2. This patch
<https://sourceware.org/ml/libc-alpha/2014-06/msg00076.html> is still
pending review.
On Wed, Jun 04, 2014 at 05:35:42PM +0000, Joseph S. Myers wrote:
> This patch fixes __ieee754_logl (-LDBL_MAX) on x86_64 and x86 not to
> subtract 1 from its argument and so cause spurious overflow in
> FE_DOWNWARD mode. (For any argument strictly less than -1, it doesn't
> matter whether or not 1 is subtracted before computing log1p, as long
> as the result doesn't overflow to -Inf.)
>
> Tested x86_64 and x86. (This particular case lacks test coverage,
> since the testsuite doesn't cover -lieee, but it will be covered by
> tests after the following patch to test pow in all rounding modes,
> which was the context in which this bug was found.)
>
> 2014-06-04 Joseph Myers <joseph@codesourcery.com>
>
> [BZ #17022]
> * sysdeps/i386/fpu/e_logl.S (__ieee754_logl): Do not subtract 1
> from arguments -2 or below.
> * sysdeps/i386/i686/fpu/e_logl.S (__ieee754_logl): Likewise.
> * sysdeps/x86_64/fpu/e_logl.S (__ieee754_logl): Likewise.
Looks good to me.
Siddhesh
>
> diff --git a/sysdeps/i386/fpu/e_logl.S b/sysdeps/i386/fpu/e_logl.S
> index edae1d7..828e98a 100644
> --- a/sysdeps/i386/fpu/e_logl.S
> +++ b/sysdeps/i386/fpu/e_logl.S
> @@ -40,8 +40,11 @@ ENTRY(__ieee754_logl)
> fld %st // x : x : log(2)
> sahf
> jc 3f // in case x is NaN or +-Inf
> + movzwl 4+8(%esp), %eax
> + cmpl $0xc000, %eax
> + jae 6f // x <= -2, avoid overflow from -LDBL_MAX - 1.
> 4: fsubl MO(one) // x-1 : x : log(2)
> - fld %st // x-1 : x-1 : x : log(2)
> +6: fld %st // x-1 : x-1 : x : log(2)
> fabs // |x-1| : x-1 : x : log(2)
> fcompl MO(limit) // x-1 : x : log(2)
> fnstsw // x-1 : x : log(2)
> diff --git a/sysdeps/i386/i686/fpu/e_logl.S b/sysdeps/i386/i686/fpu/e_logl.S
> index a0d1107..0ccc8fc 100644
> --- a/sysdeps/i386/i686/fpu/e_logl.S
> +++ b/sysdeps/i386/i686/fpu/e_logl.S
> @@ -39,8 +39,11 @@ ENTRY(__ieee754_logl)
> LOAD_PIC_REG (dx)
> #endif
> fld %st // x : x : log(2)
> + movzwl 4+8(%esp), %eax
> + cmpl $0xc000, %eax
> + jae 5f // x <= -2, avoid overflow from -LDBL_MAX - 1.
> fsubl MO(one) // x-1 : x : log(2)
> - fld %st // x-1 : x-1 : x : log(2)
> +5: fld %st // x-1 : x-1 : x : log(2)
> fabs // |x-1| : x-1 : x : log(2)
> fld MO(limit) // 0.29 : |x-1| : x-1 : x : log(2)
> fcomip %st(1) // |x-1| : x-1 : x : log(2)
> diff --git a/sysdeps/x86_64/fpu/e_logl.S b/sysdeps/x86_64/fpu/e_logl.S
> index 315afc0..047b8db 100644
> --- a/sysdeps/x86_64/fpu/e_logl.S
> +++ b/sysdeps/x86_64/fpu/e_logl.S
> @@ -38,8 +38,11 @@ ENTRY(__ieee754_logl)
> fld %st // x : x : log(2)
> testb $1, %ah
> jnz 3f // in case x is NaN or +-Inf
> + movzwl 8+8(%rsp), %eax
> + cmpl $0xc000, %eax
> + jae 6f // x <= -2, avoid overflow from -LDBL_MAX - 1.
> 4: fsubl MO(one) // x-1 : x : log(2)
> - fld %st // x-1 : x-1 : x : log(2)
> +6: fld %st // x-1 : x-1 : x : log(2)
> fabs // |x-1| : x-1 : x : log(2)
> fcompl MO(limit) // x-1 : x : log(2)
> fnstsw // x-1 : x : log(2)
>
> --
> Joseph S. Myers
> joseph@codesourcery.com
@@ -40,8 +40,11 @@ ENTRY(__ieee754_logl)
fld %st // x : x : log(2)
sahf
jc 3f // in case x is NaN or +-Inf
+ movzwl 4+8(%esp), %eax
+ cmpl $0xc000, %eax
+ jae 6f // x <= -2, avoid overflow from -LDBL_MAX - 1.
4: fsubl MO(one) // x-1 : x : log(2)
- fld %st // x-1 : x-1 : x : log(2)
+6: fld %st // x-1 : x-1 : x : log(2)
fabs // |x-1| : x-1 : x : log(2)
fcompl MO(limit) // x-1 : x : log(2)
fnstsw // x-1 : x : log(2)
@@ -39,8 +39,11 @@ ENTRY(__ieee754_logl)
LOAD_PIC_REG (dx)
#endif
fld %st // x : x : log(2)
+ movzwl 4+8(%esp), %eax
+ cmpl $0xc000, %eax
+ jae 5f // x <= -2, avoid overflow from -LDBL_MAX - 1.
fsubl MO(one) // x-1 : x : log(2)
- fld %st // x-1 : x-1 : x : log(2)
+5: fld %st // x-1 : x-1 : x : log(2)
fabs // |x-1| : x-1 : x : log(2)
fld MO(limit) // 0.29 : |x-1| : x-1 : x : log(2)
fcomip %st(1) // |x-1| : x-1 : x : log(2)
@@ -38,8 +38,11 @@ ENTRY(__ieee754_logl)
fld %st // x : x : log(2)
testb $1, %ah
jnz 3f // in case x is NaN or +-Inf
+ movzwl 8+8(%rsp), %eax
+ cmpl $0xc000, %eax
+ jae 6f // x <= -2, avoid overflow from -LDBL_MAX - 1.
4: fsubl MO(one) // x-1 : x : log(2)
- fld %st // x-1 : x-1 : x : log(2)
+6: fld %st // x-1 : x-1 : x : log(2)
fabs // |x-1| : x-1 : x : log(2)
fcompl MO(limit) // x-1 : x : log(2)
fnstsw // x-1 : x : log(2)