Date: Wed, 24 Feb 2010 18:31:50 -0800 From: Patrick Mahan <mahan@mahan.org> To: Juli Mallett <jmallett@freebsd.org> Cc: freebsd-mips@freebsd.org Subject: Re: Writing MIPS assembler instructions in C Message-ID: <201002250236.o1P2a2oX024250@ns.mahan.org> In-Reply-To: <eaa228be1002241807q2613ccecy9773155e68ccda62@mail.gmail.com> References: <17060.1267061906@mahan.org> <eaa228be1002241807q2613ccecy9773155e68ccda62@mail.gmail.com>
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See inline ...
> Hi Patrick,
>
> On Wed, Feb 24, 2010 at 17:38, Patrick Mahan <mahan@mahan.org> wrote:
> > part of some code built on the linux platform and
> > there they are using "%[rt]" which I cannot find
> > a description.
>
> %[foo] can be used to refer to named parameters (input or output,
> maybe even clobbered) of inline assembly instructions rather than
> using %0, %1, etc., which can be hard to read. Consider:
>
> %%%
> static inline bool
> atomic_cmpset64(volatile uint64_t *p, uint64_t o, uint64_t v)
> {
> uint64_t temp;
> int res;
>
> asm volatile (
> "1:\n\t"
> "move %[res], $0\n\t"
> "lld %[temp], %[p]\n\t"
> "bne %[temp], %[o], 2f\n\t"
> "move %[temp], %[v]\n\t"
> "li %[res], 1\n\t"
> "scd %[temp], %[p]\n\t"
> "beqz %[temp], 1b\n\t"
> "2:\n\t"
> : [res] "=&r"(res), [temp] "=&r"(temp), [p] "+m"(*p)
> : [o] "r"(o), [v] "r"(v)
> : "memory"
> );
>
> return (res != 0);
> }
> %%%
>
> The brackets with the input and output parameters specify what names
> to use to refer to them in the assembly listing (here they happen to
> be mostly the same as the variable names, but that's not necessary.)
>
Ah Sooo <*whack!*>......
I did not even twig on that... I had gotten it in my head that %[rt]
was something special to the compiler that caused a certain behavior
to occur.
Now my problem is I still need to force the value pointed to "addr" into
a specific register because there is a jalr to a function else where
that I only have binary access too and it expects it's a value in
that register. Can I coerce this?
Thanks for the education.
Patrick
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