Commit b9b60b31 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

x86: improve bitop code generation with clang 



This uses the new ASM_INPUT_RM macro to avoid the bad code generation
issue that clang has with more generic asm inputs.

This ends up avoiding generating code like this:

 	mov    %r10,(%rsp)
 	tzcnt  (%rsp),%rcx

which now becomes just

 	tzcnt  %r10,%rcx

and in the process ends up also removing a few unnecessary stack frames
when the only use was that pointless "asm uses memory location off stack".

Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 7453b948

arch/x86/include/asm/bitops.h

+5 −5
Original line number Diff line number Diff line
@@ -250,7 +250,7 @@ static __always_inline unsigned long variable__ffs(unsigned long word) 
{

	asm("rep; bsf %1,%0"

		: "=r" (word)

		: "rm" (word));

		: ASM_INPUT_RM (word));

	return word;

}
@@ -297,7 +297,7 @@ static __always_inline unsigned long __fls(unsigned long word) 


	asm("bsr %1,%0"

	    : "=r" (word)

	    : "rm" (word));

	    : ASM_INPUT_RM (word));

	return word;

}
@@ -320,7 +320,7 @@ static __always_inline int variable_ffs(int x) 
	 */

	asm("bsfl %1,%0"

	    : "=r" (r)

	    : "rm" (x), "0" (-1));

	    : ASM_INPUT_RM (x), "0" (-1));

#elif defined(CONFIG_X86_CMOV)

	asm("bsfl %1,%0\n\t"

	    "cmovzl %2,%0"
@@ -377,7 +377,7 @@ static __always_inline int fls(unsigned int x) 
	 */

	asm("bsrl %1,%0"

	    : "=r" (r)

	    : "rm" (x), "0" (-1));

	    : ASM_INPUT_RM (x), "0" (-1));

#elif defined(CONFIG_X86_CMOV)

	asm("bsrl %1,%0\n\t"

	    "cmovzl %2,%0"
@@ -416,7 +416,7 @@ static __always_inline int fls64(__u64 x) 
	 */

	asm("bsrq %1,%q0"

	    : "+r" (bitpos)

	    : "rm" (x));

	    : ASM_INPUT_RM (x));

	return bitpos + 1;

}

#else