linux/arch/x86/include/asm/arch_hweight.h
H. Peter Anvin c59bd56882 x86, hweight: Use a 32-bit popcnt for __arch_hweight32()
Use a 32-bit popcnt instruction for __arch_hweight32(), even on
x86-64.  Even though the input register will *usually* be
zero-extended due to the standard operation of the hardware, it isn't
necessarily so if the input value was the result of truncating a
64-bit operation.

Note: the POPCNT32 variant used on x86-64 has a technically
unnecessary REX prefix to make it five bytes long, the same as a CALL
instruction, therefore avoiding an unnecessary NOP.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Borislav Petkov <borislav.petkov@amd.com>
LKML-Reference: <alpine.LFD.2.00.1005171443060.4195@i5.linux-foundation.org>
2010-05-17 15:17:16 -07:00

61 lines
1.4 KiB
C

#ifndef _ASM_X86_HWEIGHT_H
#define _ASM_X86_HWEIGHT_H
#ifdef CONFIG_64BIT
/* popcnt %edi, %eax -- redundant REX prefix for alignment */
#define POPCNT32 ".byte 0xf3,0x40,0x0f,0xb8,0xc7"
/* popcnt %rdi, %rax */
#define POPCNT64 ".byte 0xf3,0x48,0x0f,0xb8,0xc7"
#define REG_IN "D"
#define REG_OUT "a"
#else
/* popcnt %eax, %eax */
#define POPCNT32 ".byte 0xf3,0x0f,0xb8,0xc0"
#define REG_IN "a"
#define REG_OUT "a"
#endif
/*
* __sw_hweightXX are called from within the alternatives below
* and callee-clobbered registers need to be taken care of. See
* ARCH_HWEIGHT_CFLAGS in <arch/x86/Kconfig> for the respective
* compiler switches.
*/
static inline unsigned int __arch_hweight32(unsigned int w)
{
unsigned int res = 0;
asm (ALTERNATIVE("call __sw_hweight32", POPCNT32, X86_FEATURE_POPCNT)
: "="REG_OUT (res)
: REG_IN (w));
return res;
}
static inline unsigned int __arch_hweight16(unsigned int w)
{
return __arch_hweight32(w & 0xffff);
}
static inline unsigned int __arch_hweight8(unsigned int w)
{
return __arch_hweight32(w & 0xff);
}
static inline unsigned long __arch_hweight64(__u64 w)
{
unsigned long res = 0;
#ifdef CONFIG_X86_32
return __arch_hweight32((u32)w) +
__arch_hweight32((u32)(w >> 32));
#else
asm (ALTERNATIVE("call __sw_hweight64", POPCNT64, X86_FEATURE_POPCNT)
: "="REG_OUT (res)
: REG_IN (w));
#endif /* CONFIG_X86_32 */
return res;
}
#endif