Re: Beginner's question:Are there any inter-procedural optimization in GCC backend ?

[Daniel Berlin <dberlin at dberlin dot org>]

> Two are known to me:
>
> 1) PR 9972 has been concluded to be impossible to fix so long as some
>    front ends use both inliners.
>
> 2) The RTL inliner is a major obstacle to making all RTL be
>    transiently allocated, and therefore to removing RTL from the
>    purview of the garbage collector.  Daniel Berlin has numbers
>    indicating that allocation of RTL from the GC heap causes huge
>    amounts of memory to be wasted, with consequent compile-time
>    performance hit.

Note that this is mainly due to the extremely variant lifetimes of RTL. 
 Some of it lives long time, some of it lives very short time.  Since 
we have no idea which is which, we end up with pages that would be 
freed, except for some small number of pieces of RTL on them (ie 
100-200 bytes out of 4096).   This happens quite frequently.  Trees 
have variant lifetimes, but nowhere near the variation you see with 
RTL.
One of the reasons the zone allocator does better is because it doesn't 
let RTL allocations interfere with trees or other allocated things.

> zw