Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

Linus Torvalds
2009-01-09T03:47:40Z

On Thu, 8 Jan 2009, H. Peter Anvin wrote:
>
> Right. gcc simply doesn't have any way to know how heavyweight an
> asm() statement is

I don't think that's relevant.

First off, gcc _does_ have a perfectly fine notion of how heavy-weight an
"asm" statement is: just count it as a single instruction (and count the
argument setup cost that gcc _can_ estimate).

That would be perfectly fine. If people use inline asms, they tend to use
it for a reason.

However, I doubt that it's the inline asm that was the biggest reason why
gcc decided not to inline - it was probably the constant "switch()"
statement. The inline function actually looks pretty large, if it wasn't
for the fact that we have a constant argument, and that one makes the
switch statement go away.

I suspect gcc has some pre-inlining heuristics that don't take constant
folding and simplifiation into account - if you look at just the raw tree
of the function without taking the optimization into account, it will look
big.

Linus