Messages in this thread | | | Date | Tue, 16 Mar 2004 11:32:09 -0500 | From | Timothy Miller <> | Subject | -O3.... again |
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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.
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