Fix __ieee754_logl (-LDBL_MAX) in FE_DOWNWARD mode (bug 17022)
diff mbox

Message ID Pine.LNX.4.64.1406041735120.3719@digraph.polyomino.org.uk
State New
Headers show

Commit Message

Joseph Myers June 4, 2014, 5:35 p.m. UTC
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

Joseph Myers June 11, 2014, 12:18 p.m. UTC | #1
Ping.  This patch 
<https://sourceware.org/ml/libc-alpha/2014-06/msg00076.html> is pending 
review.
Joseph Myers June 18, 2014, 10:39 a.m. UTC | #2
Ping^2.  This patch 
<https://sourceware.org/ml/libc-alpha/2014-06/msg00076.html> is still 
pending review.
Siddhesh Poyarekar June 18, 2014, 12:29 p.m. UTC | #3
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

Patch
diff mbox

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)