[tip:core/hweight] x86, hweight: Use a 32-bit popcnt for __arch_hweight32()

From: tip-bot for H. Peter Anvin
Date: Mon May 17 2010 - 18:58:28 EST

Commit-ID: c59bd5688299cddb71183e156e7a3c1409b90df2
Gitweb: http://git.kernel.org/tip/c59bd5688299cddb71183e156e7a3c1409b90df2
Author: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
AuthorDate: Mon, 17 May 2010 15:13:23 -0700
Committer: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
CommitDate: Mon, 17 May 2010 15:17:16 -0700

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@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
Cc: Borislav Petkov <borislav.petkov@xxxxxxx>
LKML-Reference: <alpine.LFD.2.00.1005171443060.4195@xxxxxxxxxxxxxxxxxxxxxxx>
arch/x86/include/asm/arch_hweight.h | 10 ++++++----
1 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/arch_hweight.h b/arch/x86/include/asm/arch_hweight.h
index d1fc3c2..9686c3d 100644
--- a/arch/x86/include/asm/arch_hweight.h
+++ b/arch/x86/include/asm/arch_hweight.h
@@ -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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