-O3.... again

[Timothy Miller <miller@techsource.com>]

I know this has been beat to death, but I was wondering about something, 
and google isn't being forthcoming.

I understand that the biggest problem with -O3 is the automatic function 
inlining.  It tends to make things worse, due to cache misses.

Well, the default maximum for automatic inlining for GCC (--param 
max-inline-insns-auto=n) is 300 pseudo instructions, which sounds 
awfully high to me (although I don't know what a pseudo instruction 
quite corresponds to).

Has anyone tinkered with different values for -finline-limit, or 
specifically max-inline-insns-auto to see if they could actually get any 
benefit out of it?

It seems to me that as a function grows beyond a certain size, the value 
of inlining it diminishes rapidly.  Only when the function-call overhead 
is a significant part of the over-all run-time of the rest of the 
function does it really help to inline.  Well, if the kernel isn't 
getting benefit from the defaults for -O3, then perhaps the defaults are 
wrong and need to be tweaked.

Anyone experiment with this?  Any thoughts?


I doubt this would apply well right off to the kernel, but right now, 
I'm doing gentoo emerges of GCC with varying CFLAGS settings.  First, I 
emerge GCC with the experimental values of CFLAGS.  Then I change the 
CFLAGS to a standard setting and time it (the timed run must always have 
the same parameters for the target).  My objective is to determine the 
MINIMUM value of max-inline-insns-auto which yields an improvement over -O2.