linux-kernel - [tip:core/hweight] x86, hweight: Use a 32-bit popcnt for __arch

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Message-ID: <tip-c59bd5688299cddb71183e156e7a3c1409b90df2@git.kernel.org>
Date:	Mon, 17 May 2010 22:57:36 GMT
From:	"tip-bot for H. Peter Anvin" <hpa@...ux.intel.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	hpa@...ux.intel.com, borislav.petkov@....com
Subject: [tip:core/hweight] x86, hweight: Use a 32-bit popcnt for __arch_hweight32()

Commit-ID:  c59bd5688299cddb71183e156e7a3c1409b90df2
Gitweb:     http://git.kernel.org/tip/c59bd5688299cddb71183e156e7a3c1409b90df2
Author:     H. Peter Anvin <hpa@...ux.intel.com>
AuthorDate: Mon, 17 May 2010 15:13:23 -0700
Committer:  H. Peter Anvin <hpa@...ux.intel.com>
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@...ux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@...ux.intel.com>
Cc: Borislav Petkov <borislav.petkov@....com>
LKML-Reference: <alpine.LFD.2.00.1005171443060.4195@...linux-foundation.org>
---
 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"
 #else
 /* 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"
 #endif
@@ -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));
 #else
-	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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