syntax for clobber list with inline assembly
hollis at penguinppc.org
Wed Jun 8 11:02:57 EST 2005
On Jun 7, 2005, at 10:00 AM, Segher Boessenkool wrote:
>> I'm writing some inline assembly for ppc. To specify that r3 is
>> used, I know I need to list it in the clobbered list.
>> When doing this, do I specify it as "3" or "r3", or are both valid?
> Both work.
But it should be said that much better style is to let the compiler
choose registers for you, i.e. don't hardcode register numbers at all.
For example (well, it's contrived, but I hope you get the idea):
"ori %0, r3, 0x1\n"
would be better written as
int out, tmp;
"ori %0, %1, 0x1\n"
: "=r"(out), "=r"(tmp)
Be careful about using "r" constraints when dealing with instructions
that treat r0 as a special case, such as addi or lwz (use "b" instead).
Also be careful about gcc arranging outputs and inputs in the same
register; it looks different when you're writing "%0" and "%1" in your
assembly, but it's not.
Inline assembly is very subtle. If you mess up your constraints now,
you might never know until you rebuild with a different compiler
version and things fall apart.
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