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+/*
+ * Written by J.T. Conklin <jtc@netbsd.org>.
+ * Adapted for use as log2 by Ulrich Drepper <drepper@cygnus.com>.
+ * Public domain.
+ *
+ * Changed to use fyl2xp1 for values near 1, <drepper@cygnus.com>.
+ * Adapted for x86-64 by Andreas Jaeger <aj@suse.de>.
+ */
+
+#include <machine/asm.h>
+
+	.section .rodata.cst8,"aM",@progbits,8
+
+	.p2align 3
+	.type one,@object
+one:	.double 1.0
+	ASM_SIZE_DIRECTIVE(one)
+	/* It is not important that this constant is precise.  It is only
+	   a value which is known to be on the safe side for using the
+	   fyl2xp1 instruction.  */
+	.type limit,@object
+limit:	.double 0.29
+	ASM_SIZE_DIRECTIVE(limit)
+
+
+#ifdef PIC
+# define MO(op) op##(%rip)
+#else
+# define MO(op) op
+#endif
+
+	.text
+ENTRY(__ieee754_log2l)
+	fldl	MO(one)
+	fldt	8(%rsp)		// x : 1
+	fxam
+	fnstsw
+	fld	%st		// x : x : 1
+	testb	$1, %ah
+	jnz	3f		// in case x is NaN or ħInf
+4:	fsub	%st(2), %st	// x-1 : x : 1
+	fld	%st		// x-1 : x-1 : x : 1
+	fabs			// |x-1| : x-1 : x : 1
+	fcompl	MO(limit)	// x-1 : x : 1
+	fnstsw			// x-1 : x : 1
+	andb	$0x45, %ah
+	jz	2f
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	5f
+	fabs			// log2(1) is +0 in all rounding modes.
+5:	fstp	%st(1)		// x-1 : 1
+	fyl2xp1			// log(x)
+	ret
+
+2:	fstp	%st(0)		// x : 1
+	fyl2x			// log(x)
+	ret
+
+3:	testb	$4, %ah
+	jnz	4b		// in case x is ħInf
+	fstp	%st(1)
+	fstp	%st(1)
+	fadd	%st(0)
+	ret
+END (__ieee754_log2l)
+
+
+ENTRY(__log2l_finite)
+	fldl	MO(one)
+	fldt	8(%rsp)		// x : 1
+	fld	%st		// x : x : 1
+	fsub	%st(2), %st	// x-1 : x : 1
+	fld	%st		// x-1 : x-1 : x : 1
+	fabs			// |x-1| : x-1 : x : 1
+	fcompl	MO(limit)	// x-1 : x : 1
+	fnstsw			// x-1 : x : 1
+	andb	$0x45, %ah
+	jz	2b
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	6f
+	fabs			// log2(1) is +0 in all rounding modes.
+6:	fstp	%st(1)		// x-1 : 1
+	fyl2xp1			// log(x)
+	ret
+END (__log2l_finite)