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SubjectRe: [PATCH][2.6] constant_test_bit doesn't like my gcc
Zwane Mwaikambo <zwane@arm.linux.org.uk> wrote:
>
> cpu_has_foo and friends don't work at all with my gcc 3.2.2-5 and i'm
> branching off into all sorts of tests (which is another story...). i also
> took the liberty of removing the const volatile...

The volatile is rather important.


static inline int constant_test_bit(int nr, const unsigned long *addr)
{
return ((1UL << (nr & 31)) & (((const unsigned long *) addr)[nr >> 5])) != 0;
}

foo(unsigned long *p)
{
while (constant_test_bit(0, p))
;
}


.file "a.c"
.text
.align 2
.p2align 2,,3
.globl foo
.type foo,@function
foo:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %eax
testb $1, (%eax)
.p2align 2,,3
.L2:
jne .L2
leave
ret
.Lfe1:
.size foo,.Lfe1-foo
.ident "GCC: (GNU) 3.2 20020903 (Red Hat Linux 8.0 3.2-7)"


See, the little busywait loop doesn't reevaluate the test each time around:
it locks up instead.

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