about summary refs log tree commit diff
path: root/sysdeps/i386/fpu/e_powl.S
diff options
context:
space:
mode:
authorJoseph Myers <joseph@codesourcery.com>2012-11-07 13:03:31 +0000
committerJoseph Myers <joseph@codesourcery.com>2012-11-07 13:03:31 +0000
commit60e235ee2ae834bb9f7a884f1b192304b9fdcf33 (patch)
tree4053809680e9b6def9eab8272fc70ac6c6edb16c /sysdeps/i386/fpu/e_powl.S
parent0ab234b7db4991121eb572bf5c4971bfeb2d49a2 (diff)
downloadglibc-60e235ee2ae834bb9f7a884f1b192304b9fdcf33.tar.gz
glibc-60e235ee2ae834bb9f7a884f1b192304b9fdcf33.tar.xz
glibc-60e235ee2ae834bb9f7a884f1b192304b9fdcf33.zip
Fix spurious underflows from pow with results close to 1 (bug 14811).
Diffstat (limited to 'sysdeps/i386/fpu/e_powl.S')
-rw-r--r--sysdeps/i386/fpu/e_powl.S23
1 files changed, 21 insertions, 2 deletions
diff --git a/sysdeps/i386/fpu/e_powl.S b/sysdeps/i386/fpu/e_powl.S
index 933418cf82..ac4842cf63 100644
--- a/sysdeps/i386/fpu/e_powl.S
+++ b/sysdeps/i386/fpu/e_powl.S
@@ -38,6 +38,9 @@ p64:	.byte 0, 0, 0, 0, 0, 0, 0xf0, 0x43
 	.type p78,@object
 p78:	.byte 0, 0, 0, 0, 0, 0, 0xd0, 0x44
 	ASM_SIZE_DIRECTIVE(p78)
+	.type pm79,@object
+pm79:	.byte 0, 0, 0, 0, 0, 0, 0, 0x3b
+	ASM_SIZE_DIRECTIVE(pm79)
 
 	.section .rodata.cst16,"aM",@progbits,16
 
@@ -120,9 +123,25 @@ ENTRY(__ieee754_powl)
 	fucomp	%st(1)		// y : x
 	fnstsw
 	sahf
-	jne	3f
+	je	9f
 
-	/* OK, we have an integer value for y.  */
+	// If y has absolute value at most 0x1p-79, then any finite
+	// nonzero x will result in 1.  Saturate y to those bounds to
+	// avoid underflow in the calculation of y*log2(x).
+	fld	%st		// y : y : x
+	fabs			// |y| : y : x
+	fcompl	MO(pm79)	// y : x
+	fnstsw
+	sahf
+	jnc	3f
+	fstp	%st(0)		// pop y
+	fldl	MO(pm79)	// 0x1p-79 : x
+	testb	$2, %dl
+	jnz	3f		// y > 0
+	fchs			// -0x1p-79 : x
+	jmp	3f
+
+9:	/* OK, we have an integer value for y.  */
 	popl	%eax
 	cfi_adjust_cfa_offset (-4)
 	popl	%edx