Re: [PR lto/41528] Add internal documentation in doc/lto.texi

[Diego Novillo <dnovillo at google dot com>]

On 10-11-14 22:57 , Xinliang David Li wrote:

> cc1, cc1plus, f951, and lto1 -- where does '1' here mean? It always
> got me thinking where cc2/lto2/... is.

Lost in the mists of tradition.  IIRC, the '1' used to distinguish it 
from 'cc', the driver (others may remember the true reason, I'm too 
young ;).  We are creatures of habit.

> WHOPR --- the Whole part is not accurate -- it can be any part of the
> whole program. The 'Whole Program' should be 'the set of IL units' to
> be more exact.

Naturally.  This is the usual assumption.  Whole refers to the whole set 
of files presented to the compiler.  If the user is withholding files, 
there's little the compiler can do about that, short of guessing.  I'll 
add a note on the semantics of 'whole' at the start of the document.

> > +Currently, GCC does not support combining LTO object files compiled
> > +with different set of the command line options into a single binary.
> > +At link-time, the options given on the command line and the options
> > +saved on all the files in a link-time set are applied globally.  No
> > +attempt is made at validating the combination of flags (other than the
> > +usual validation done by option processing).  This is implemented in
> > +@file{lto/lto.c}:@code{lto_read_all_file_options}.
>
> This can be a big limiting factor for the wide adoption of LTO. In
> LIPO, incompatible options are detected and modules not safe to
> include are banned.

It's another bug in a long stream of bugs.  We have discussed ways to 
address it (http://gcc.gnu.org/wiki/whopr), but so far it has not 
percolated to the top of anyone's todo list.


Diego.