[LTO merge][0/15] Description of the final 15 patches

Diego Novillo dnovillo@google.com
Tue Sep 29 00:58:00 GMT 2009

I will be sending the final 15 patches to bring all the
functionality from the LTO branch.  I tried to divide the patches
along maintainer lines, but there are some overlaps

- C FE
	Adds processing of -flto and -fwhopr.

- C++ FE
	Adds a langhook used by need_assembler_name_p to handle
	templates.  This implements Jason's suggestion from

- Fortran FE
	Adds processing of -flto and -fwhopr.  Note that the
	Fortran testsuite still shows ~140 failures with LTO
	enabled.  IIUC, most of these have corresponding bugs
	filed and they are not LTO-related (in that LTO exposes
	some bugs in the FE).  We should discuss whether we want
	to include Fortran support for LTO in 4.5.

- Java FE
	There is no LTO support for Java, but since LTO uses
	zlib, Java does not need to include it.

	This is the new front end for GIMPLE.

- Configury
	Detection of libelf, LTO support, etc.

- Contrib
	A minor patch to add lto-plugin to the timestamps.

- Documentation
	Documentation for the different LTO options.

- Driver
	The somewhat ugly changes to collect2 and the main gcc
	driver to support spawning lto1 when we detect a .o file
	with LTO sections in it.

- Streamer
	The code needed to support the on-disk representation of
	GIMPLE, CFG and cgraph.

- Langhooks
	New langhooks to support manipulating ELF sections and
	the new langhook for asking the FE if a DECL needs to
	have an assembler name set.

- libiberty
	I need help with this one.  When the linker plugin is
	enabled (if GCC is configured to use gold), LTO can
	detect LTO objects inside archives via the callbacks it
	gets from the linker.  Since the linker plugin is a
	shared object, and it uses libiberty functions, it needs
	to use a shared libiberty.

	Currently, we just force --enable-shared on libiberty,
	but I would only want to do that if gold and lto are
	enabled.  We detect gold and lto support in the top
	configure script, but how do I send that down to
	libiberty's configure?

- Linker plugin
	This is the set of callbacks that gold uses to talk with
	lto1 and resolve symbols out of object files in the link.

- Middle end
	All sorts of changes, mostly in the callgraph, IPA and
	pass manager.

- Testsuite
	New tests for LTO and new dejagnu support for writing LTO
	tests.  Most of this has already been reviewed by Janis.

Major outstanding work:

- Support for debugging information.  Currently, -flto -g does
  not work (no debug information is generated).  This is because
  free_lang_data does not preserve enough information for the
  debug hooks to emit it.  I will fix this during stage 3.

- Outstanding bugzilla (http://tinyurl.com/yarz5q9).

- Some 'FIXME lto' markers.  These are cleanup markers, several
  related to WHOPR.  We could probably debate whether to include
  the WHOPR functionality in 4.5.  It certainly is not very
  different from LTO, but it does need some TLC.

Once I get all the approvals I need, I will request trunk to be
frozen for a day or so to give me time to commit the whole thing.
I already have a merged local tree and this exact same code is in
the LTO branch.

Thanks.  Diego.

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