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SubjectRe: GCC proposal for "@" asm constraint
On Mon, Sep 18, 2000 at 09:37:43PM -0400, John Wehle wrote:
> It's perhaps not optimal, however I'm not sure that it's wrong. In

It's not "wrong" in the sense that something breaks but it's definitely
suboptimal. There's no reason to reload a value that can't change because it's
embedded into the opcodes of the asm and set a static linking time.

> any case if you can supply a small standalone test case (i.e. preprocessed
> source code) I'll take a quick look at things. I take it that you haven't
> tried the current gcc sources?

The first testcase is the current spinlock implementation, the second testcase
adds the "memory" clobber and it generates the _spurious_ reload of `p'. You
should be able to compile with `gcc -O2 -S p-*.i`.

Andrea
# 1 "p.c"
# 1 "/home/andrea/kernel/devel/2.2.18pre9aa1/include/linux/spinlock.h" 1


# 1 "/usr/include/asm/spinlock.h" 1 3



# 134 "/usr/include/asm/spinlock.h" 3






typedef struct {
volatile unsigned int lock;
} spinlock_t;













typedef struct { unsigned long a[100]; } __dummy_lock_t;



# 169 "/usr/include/asm/spinlock.h" 3






































typedef struct {
volatile unsigned int lock;
unsigned long previous;
} rwlock_t;










# 231 "/usr/include/asm/spinlock.h" 3






# 249 "/usr/include/asm/spinlock.h" 3




















# 3 "/home/andrea/kernel/devel/2.2.18pre9aa1/include/linux/spinlock.h" 2


# 1 "p.c" 2


int * p;
spinlock_t lock = (spinlock_t) { 0 } ;

void dummy(int x , int y) {}

main() {
int a, b;
__asm__ __volatile__( "\n1:\t" "lock ; btsl $0,%0\n\t" "jc 2f\n" ".section .text.lock,\"ax\"\n" "2:\t" "testb $1,%0\n\t" "jne 2b\n\t" "jmp 1b\n" ".previous" :"=m" ((*(__dummy_lock_t *)( &lock )) )) ;
a = *p;
__asm__ __volatile__( "lock ; btrl $0,%0" :"=m" ((*(__dummy_lock_t *)( &lock )) )) ;

__asm__ __volatile__( "\n1:\t" "lock ; btsl $0,%0\n\t" "jc 2f\n" ".section .text.lock,\"ax\"\n" "2:\t" "testb $1,%0\n\t" "jne 2b\n\t" "jmp 1b\n" ".previous" :"=m" ((*(__dummy_lock_t *)( &lock )) )) ;
b = *p;
__asm__ __volatile__( "lock ; btrl $0,%0" :"=m" ((*(__dummy_lock_t *)( &lock )) )) ;

dummy(a,b);
}
# 1 "p.c"
# 1 "/home/andrea/kernel/devel/2.2.18pre9aa1/include/linux/spinlock.h" 1


# 1 "/usr/include/asm/spinlock.h" 1 3



# 134 "/usr/include/asm/spinlock.h" 3






typedef struct {
volatile unsigned int lock;
} spinlock_t;













typedef struct { unsigned long a[100]; } __dummy_lock_t;



# 169 "/usr/include/asm/spinlock.h" 3






































typedef struct {
volatile unsigned int lock;
unsigned long previous;
} rwlock_t;










# 231 "/usr/include/asm/spinlock.h" 3






# 249 "/usr/include/asm/spinlock.h" 3




















# 3 "/home/andrea/kernel/devel/2.2.18pre9aa1/include/linux/spinlock.h" 2


# 1 "p.c" 2


int * p;
spinlock_t lock = (spinlock_t) { 0 } ;

void dummy(int x , int y) {}

main() {
int a, b;
__asm__ __volatile__( "\n1:\t" "lock ; btsl $0,%0\n\t" "jc 2f\n" ".section .text.lock,\"ax\"\n" "2:\t" "testb $1,%0\n\t" "jne 2b\n\t" "jmp 1b\n" ".previous" :"=m" ((*(__dummy_lock_t *)( &lock )) ) : : "memory") ;
a = *p;
__asm__ __volatile__( "lock ; btrl $0,%0" :"=m" ((*(__dummy_lock_t *)( &lock )) ) : : "memory") ;

__asm__ __volatile__( "\n1:\t" "lock ; btsl $0,%0\n\t" "jc 2f\n" ".section .text.lock,\"ax\"\n" "2:\t" "testb $1,%0\n\t" "jne 2b\n\t" "jmp 1b\n" ".previous" :"=m" ((*(__dummy_lock_t *)( &lock )) ) : : "memory") ;
b = *p;
__asm__ __volatile__( "lock ; btrl $0,%0" :"=m" ((*(__dummy_lock_t *)( &lock )) ) : : "memory") ;

dummy(a,b);
}
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