optimization/10155: [3.3/3.4 regression] gcc -O2/-O3 uses excessive amount of memory

[Jan Hubicka]

> http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10155
> 
> How much of this can be explained with Kaveh's physmem
> patch?  IIRC that patch is not in 3.2,  and the increase
> in memory consumption at -O2 may be a result of that
> patch.
> 
> The increase in memory at -O3 is a result of unit at a
> time compilation (which is why I CC you, Honza).  You
> can check that by compiling with -O2 + all flags enabled
> at -O3 except -funit-at-a-time:
> 
> ./cc1 10155.c -quiet -ftime-report -O2
> TOTAL                 :  24.74             0.74            26.24
> 
> ./cc1 10155.c -quiet -ftime-report -O2 -funswitch-loops 
> -frename-registers -finline-functions
> TOTAL                 :  31.49             0.59            33.87
> 
> Loop unswitching is responsible for most of the compile
Zdenek, this really ought not to happen, what is going on?
> time increase.
> Now add -funit-at-a-time, and kabooooom! you lose.
> 
> Apparently unit-at-a-time should still honor some size
> constraints, and it does not in its current form.

It should be more problem of inlining heuristics, than unit-at-a-time
(ie unit-at-a-time enables more inlining oppurtunities but it is
inlining heuristic mistake to take so many of them).
Or perhaps we manage to flatten functions called once into one
extraordinarily large function body and give up on it.  I will try to
investigate it, but my current priority is to get unit-at-a-time working
on C++.  Fixing this testcase should be easy then :)

Honza
> 
> Greetz
> Steven
> 
>