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

[Jim Wilson <wilson at specifixinc dot com>]

Chunhua Liao wrote:
> I am new in GCC development. I am wondering if there are any 
> inter-procedural optimizations
> on RTL representation in GCC backend.  If yes, what are they(or function 
> names)? Thanks

There isn't much.

There are political problems that have obstructed our implementation of 
inter procedural optimizations, as FSF policy forbids us from having a 
way to store/load RTL to/from files.  This policy exists to prevent 
people from subverting the GPL, but as side effect is that it prevents 
us from having inter procedural optimizations, except within files.

We do have function inlining in RTL, and this has been around for a long 
time, but this is on the way out, because RTL is too low level for this 
optimization.  It is being replaced with a tree level function inliner. 
 The RTL function inliner is in integrate.c.

There is code in cgraph* that is enabled by the -funit-at-a-time option, 
which is now the default at -O2.  This causes us to parse all functions 
before emitting any output, thus allowing some interprocedural 
optimizations within a file.  However, this only does a few limited 
things.  It can identify functions that are unused.  It can identify 
functions which are good candidates for inlining.  It probably doesn't 
do what you are looking for.

You might be interested in the tree-ssa project.  This is adding a new 
higher level intermediate format to gcc, which is better for higher 
level optimizations.  This might be a better place for interprocedural 
optimizations.

You might also be interested in LLVM, which has link time 
interprocedural optimizations.  http://llvm.cs.uiuc.edu/
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com