[lto] Pass ordering and the different lto1 personalities

[Diego Novillo <dnovillo at google dot com>]

We were chatting on IRC about the problem Simon found with IPCP
getting confused because during -fwhopr builds, the inliner
always runs first, so when LTRANS starts up, we run all the IPA
passes before applying inlining decisions.  This confuses IPCP.

For lto1 we have 3 different modes of operation:

LTO (-flto)

  This does the usual link-time operations.  All the IL files are
  loaded in memory (callgraph and bodies) and then
  cgraph_optimize() is invoked to do IPA, GIMPLE, RTL
  optimizations and generate final code.


WPA (-fwpa)

  This is stage 2 of WHOPR.  We load the whole callgraph sans
  the function bodies and run all the IPA passes capable of
  operating with summaries only (currently, the inliner).  We then
  partition the callgraph into separate sub-graphs which are
  written out to new IL files.


LTRANS (-fltran)

  This is stage 3 of WHOPR.  We execute as many LTRANS processes
  as IL files we wrote out during WPA.  Each of these will read
  a single IL file and compile it exactly as LTO.  After this
  stage, all the output files are ready for final linking.


The problem here is that LTRANS will run the standard pipeline
over a callgraph that hasn't been "settled" (i.e., no inlining
decisions have been applied yet).  Perhaps the first thing LTRANS
should do is just call execute_all_ipa_transforms() and then
proceed with the regular pipeline.

Honza, these three stages are all controlled by lto.c:lto_main.


Diego.