Re: [PATCH] 2.4: always_inline for gcc3

[J.A. Magallon]

On 08.27, Alan Cox wrote:
> On Maw, 2003-08-26 at 17:24, J.A. Magallon wrote:
> > gcc3 did not inline big functions, even if they were marked as inline
> > Thread:
> > http://marc.theaimsgroup.com/?t=103632325600005&r=1&w=2
> > Things like memcpy and copy_to/from_user were affected.
> > They were not inlined and you got tons of instances in vmlinux.
> 
> The more interesting question you want to answer first is - was
> gcc right. Repeated code is bad for cache
> 

That would be true for a regular function, too much inlining can be bad.
But __constant_c_and_count_memset is special, you know.
It is written like a big switch:

copy( size )
	switch (size)
		case 1: ...
		case 2: ...
		...

Author and users relay on gcc's optimizer:
- gcc inlines it
- control variable in switch is constant at compile time
- switch is killed and just the suitable case stays.
(\label{1})

If gcc does not inline it, there are a ton of users of copy_to/from
and memset that execute the full switch() (w/o the patch, 150 out-of-line
instances of __constant_c_and_count_memset stay, with the patch
only 16, so it means gcc failed to inline about 140...).
Depending on which users are not inlined, this can be more or less
harmfull in terms of performance, but sure it is not the designed
behaviour.

And of course, I can be totally wrong. Comments welcome.

Ah, all this supposing that gcc still does correctly what I said
in \ref{1} ;)

-- 
J.A. Magallon <jamagallon@xxxxxxx>      \                 Software is like sex:
werewolf.able.es                         \           It's better when it's free
Mandrake Linux release 9.2 (Cooker) for i586
Linux 2.4.22-jam1m (gcc 3.3.1 (Mandrake Linux 9.2 3.3.1-1mdk))
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