s_floorl.s

glibc 库, 不仅可以学习使用库函数,还可以学习函数的具体实现,是提高功力的好资料

/*
 * Written by J.T. Conklin <[e-mail]>.
 * Changes for long double by Ulrich Drepper <[e-mail]>
 * Changes for x86-64 by Andreas Jaeger <[e-mail]>=09
 * Public domain.
 */

#include <machine/asm.h>

ENTRY(__floorl)
	fldt	8(%rsp)

	fstcw	-4(%rsp)		/* store fpu control word */

	/* We use here %edx although only the low 1 bits are defined.
	   But none of the operations should care and they are faster
	   than the 16 bit operations.  */
	movl	$0x400,%edx		/* round towards -oo */
	orl	-4(%rsp),%edx
	andl	$0xf7ff,%edx
	movl	%edx,-8(%rsp)
	fldcw	-8(%rsp)		/* load modified control word */

	frndint				/* round */

	fldcw	-4(%rsp)		/* restore original control word */

	ret
END (__floorl)
weak_alias (__floorl, floorl)