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[libc] Add inline assembly implementation of flsl() using BSR instruction

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Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown 2014-04-24 14:49:08 +01:00
parent 8f0e0e1356
commit dce7107fc0
3 changed files with 33 additions and 15 deletions
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31
src/arch/x86/include/bits/strings.h Normal file
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@ -0,0 +1,31 @@
#ifndef _BITS_STRINGS_H
#define _BITS_STRINGS_H
FILE_LICENCE ( GPL2_OR_LATER );
/**
* Find last (i.e. most significant) set bit
*
* @v value Value
* @ret msb Most significant bit set in value (LSB=1), or zero
*/
static inline __attribute__ (( always_inline )) int __flsl ( long value ) {
long msb_minus_one;
/* If the input value is zero, the BSR instruction returns
* ZF=1 and leaves an undefined value in the output register.
* Perform this check in C rather than asm so that it can be
* omitted in cases where the compiler is able to prove that
* the input is non-zero.
*/
if ( value ) {
__asm__ ( "bsr %1, %0"
: "=r" ( msb_minus_one )
: "rm" ( value ) );
return ( msb_minus_one + 1 );
} else {
return 0;
}
}
#endif /* _BITS_STRINGS_H */

13
src/core/bitops.c
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@ -1,13 +0,0 @@
#include <strings.h>
FILE_LICENCE ( GPL2_OR_LATER );
int __flsl ( long x ) {
unsigned long value = x;
int ls = 0;
for ( ls = 0 ; value ; ls++ ) {
value >>= 1;
}
return ls;
}

4
src/include/strings.h
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@ -5,6 +5,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
#include <limits.h>
#include <string.h>
#include <bits/strings.h>
static inline __attribute__ (( always_inline )) int
__constant_flsl ( unsigned long x ) {
@ -42,8 +43,7 @@ __constant_flsl ( unsigned long x ) {
return r;
}
/* We don't actually have these functions yet */
extern int __flsl ( long x );
int __flsl ( long x );
#define flsl( x ) \
( __builtin_constant_p ( x ) ? __constant_flsl ( x ) : __flsl ( x ) )