Re: (C++) High memory usage with templates

[Jeffrey A Law <law at cygnus dot com>]

  In message <Pine.LNX.3.96.981203072754.204E-100000@oar3remote.oarcorp.com>you
 write:
  > I don't know if it is relevant but there are a couple of files in the
  > powerpc simulator included with gdb that are (AFAIK) plain C that take an
  > enormous amount of resources to compile.  I have seen a Linux box with 256
  > Mbytes RAM thrash trying to compile this file with optimization on.  This
  > occurs when you configure psim to --enable-sim-inline.  Perhaps this
  > indicates a problem that is not entirely C++ specific.  Someone more
  > knowledgeable about psim and gcc can tell me if I am way off base here.
When a function is declared inlined, the entire internal representation for the
function is saved for integration into callers.

When -finline-functions is in effect (including -O3), then *every* function's
internal representation is saved.  That alone costs a lot of memory.

When integration occurs, the resulting code is also much larger, with many more
expressions/registers/insns.  This in turn causes a significant increase in the
amount of memory to compile a particular function.  Particularly since some
of the data structuers are 2X based on insns, regs or whatever.  And gcc tries
just as hard to optimize big functions as little ones...  So it does not say
"gee, this is a big function, maybe I shouldn't do optimization xyz..."

I looked at one case not long ago where the tables & bitmaps to implement
global cse alone were 200+ meg for a heavily inlined/templated hunk of code.

jeff