Q: Profile-directed optimizations

[Andrew Huang]

This might be a good question for the FAQ.  I looked around but
couldn't find any information.

I was wondering if egcs/gcc supports profile-directed optimizations.
A grep through the source codes returned paragraphs on profiling 
basic blocks and control arcs.  But this seems to be for generating
profile information only.

Is there support for profile-directed optimization?  If not, is it
mostly because

1) it is difficult to do,
2) it wouldn't buy much performance, or
3) just that no one has gotten around to it?  

A week ago, there was a thread about egcs's SPEC95 number on the DEC
Alpha being 60% of what it should be.  Any idea on what portion of
the missing 40% is due to lack of profile-directed optimization?
I'm not sure that the DEC compiler supports it.  But it seems that
many compilers (e.g. the xlC under AIX and the Moto compiler for the
PowerPC) support it.

-Andrew