While trying to use the -flto and -fwhole-program flags I ran into problems
understanding what they do. I would like to update the documentation but I
still don't understand these flags enough to be able to describe their
behaviour. Here is the document section I would like to fix but don't
have enough information to do so.
From lto.texi:
| @subsection LTO modes of operation
|
| One of the main goals of the GCC link-time infrastructure was to allow
| effective compilation of large programs. For this reason GCC implements two
| link-time compilation modes.
|
| @enumerate
| @item @emph{LTO mode}, in which the whole program is read into the
| compiler at link-time and optimized in a similar way as if it
| were a single source-level compilation unit.
|
| @item @emph{WHOPR or partitioned mode}, designed to utilize multiple
| CPUs and/or a distributed compilation environment to quickly link
| large applications. WHOPR stands for WHOle Program optimizeR (not to
| be confused with the semantics of @option{-fwhole-program}). It
| partitions the aggregated callgraph from many different @code{.o}
| files and distributes the compilation of the sub-graphs to different
| CPUs.
What flag(s) do I use (or not use) to enable @emph{LTO mode}?
I am guessing that if I use -flto but not -flto-partition on a
link, this is what I get. Is that correct?
What flag(s) do I use to enable @emph{WHOPR or partitioned mode}?
I am guessing that this is -flto-partition? Do I also need -flto if I
am using -flto-partition? I don't see any description in lto.texi or in
common.opt of exactly what the various values for -flto-partition
(none, one, balanced, 1to1, max) do. Does such a description exist
anywhere?
Steve Ellcey
sell...@marvell.com
