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

[Linus Torvalds]

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
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