Commit c59bd568 authored by H. Peter Anvin's avatar H. Peter Anvin

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: default avatarLinus Torvalds <>
Signed-off-by: default avatarH. Peter Anvin <>
Cc: Borislav Petkov <>
LKML-Reference: <>
parent 4677d4a5
......@@ -2,13 +2,15 @@
#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 POPCNT ".byte 0xf3,0x48,0x0f,0xb8,0xc7"
#define POPCNT64 ".byte 0xf3,0x48,0x0f,0xb8,0xc7"
#define REG_IN "D"
#define REG_OUT "a"
/* popcnt %eax, %eax */
#define POPCNT ".byte 0xf3,0x0f,0xb8,0xc0"
#define POPCNT32 ".byte 0xf3,0x0f,0xb8,0xc0"
#define REG_IN "a"
#define REG_OUT "a"
......@@ -23,7 +25,7 @@ static inline unsigned int __arch_hweight32(unsigned int w)
unsigned int res = 0;
asm (ALTERNATIVE("call __sw_hweight32", POPCNT, X86_FEATURE_POPCNT)
asm (ALTERNATIVE("call __sw_hweight32", POPCNT32, X86_FEATURE_POPCNT)
: "="REG_OUT (res)
: REG_IN (w));
......@@ -48,7 +50,7 @@ static inline unsigned long __arch_hweight64(__u64 w)
return __arch_hweight32((u32)w) +
__arch_hweight32((u32)(w >> 32));
asm (ALTERNATIVE("call __sw_hweight64", POPCNT, X86_FEATURE_POPCNT)
asm (ALTERNATIVE("call __sw_hweight64", POPCNT64, X86_FEATURE_POPCNT)
: "="REG_OUT (res)
: REG_IN (w));
#endif /* CONFIG_X86_32 */
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