* Linus Torvalds [off-list ref] wrote:

On Fri, 20 Jun 2008, Jeremy Fitzhardinge wrote:

quoted

Blows up on "gcc version 3.4.4 20050314 (prerelease) (Debian 3.4.3-13)":

Yeah, I was a bit worried about that. Gcc sometimes does insane 
things.

We literally just tested that the asm should only _ever_ be generated 
with a constant value, but if some gcc dead-code removal thing doesn't 
work, it will then screw up and try to generate the asm even for a 
non-constant thing.

The fairly trivial fix is probably to just change the "i" to "ir", 
safe in the knowledge that any _sane_ case will never use the "r" 
possibility. I suspect even your insane case will end up then killing 
the bad choice later.

okay - Jeremy, could you try the fix below? (or tip/master, i just 
pushed this out)

(i dont use gcc 3.x myself to build the kernel, had way too many 
miscompilations in randconfig tests in the past.)

	Ingo

-------------->
commit b68b80b8ab39c42707dc126c41e87d46edc97c27
Author: Ingo Molnar [off-list ref]
Date:   Fri Jun 20 21:50:20 2008 +0200

    x86, bitops: make constant-bit set/clear_bit ops faster, gcc workaround
    
    Jeremy Fitzhardinge reported this compiler bug:
    
    Suggestion from Linus: add "r" to the input constraint of the
    set_bit()/clear_bit()'s constant 'nr' branch:
    
    Blows up on "gcc version 3.4.4 20050314 (prerelease) (Debian 3.4.3-13)":
    
     CC      init/main.o
    include2/asm/bitops.h: In function `start_kernel':
    include2/asm/bitops.h:59: warning: asm operand 1 probably doesn't match constraints
    include2/asm/bitops.h:59: warning: asm operand 1 probably doesn't match constraints
    include2/asm/bitops.h:59: warning: asm operand 1 probably doesn't match constraints
    include2/asm/bitops.h:59: error: impossible constraint in `asm'
    include2/asm/bitops.h:59: error: impossible constraint in `asm'
    include2/asm/bitops.h:59: error: impossible constraint in `asm'
    
    Reported-by: Jeremy Fitzhardinge [off-list ref]
    Signed-off-by: Ingo Molnar [off-list ref]

diff --git a/include/asm-x86/bitops.h b/include/asm-x86/bitops.h
index 6c50548..4575de4 100644
--- a/include/asm-x86/bitops.h
+++ b/include/asm-x86/bitops.h

@@ -58,7 +58,7 @@ static inline void set_bit(unsigned int nr, volatile unsigned long *addr)
 	if (IS_IMMEDIATE(nr)) {
 		asm volatile(LOCK_PREFIX "orb %1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "i" (CONST_MASK(nr))
+			: "ir" (CONST_MASK(nr))
 			: "memory");
 	} else {
 		asm volatile(LOCK_PREFIX "bts %1,%0"

@@ -95,7 +95,7 @@ static inline void clear_bit(int nr, volatile unsigned long *addr)
 	if (IS_IMMEDIATE(nr)) {
 		asm volatile(LOCK_PREFIX "andb %1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "i" (~CONST_MASK(nr)));
+			: "ir" (~CONST_MASK(nr)));
 	} else {
 		asm volatile(LOCK_PREFIX "btr %1,%0"
 			: BITOP_ADDR(addr)