Global analysis of RTL

[Geoff Wozniak]

My team and I are working on a procedural abstraction (PA) 
optimization for GCC based on the ARM Embedded Toolchain 
distribution that uses GCC 6.2.1. Our initial attempt to get 
something functional has been successful, but it is a very messy 
solution because we don't see a reasonable way to do an analysis 
of a compilation unit or whole program at the level or RTL.   From 
what we have been able to gleen (note: we're all relative 
newcomers to the GCC code base), all RTL transformation and 
analysis is done at the level of functions only. Once pass_final 
runs, the RTL has been output to assembler, meaning no more 
changes to RTL is possible.   We have not been able to find a 
reasonable way to collect all the RTL for a compilation unit to do 
an analysis across functions. Any solution we have come up with 
involves some serious changes to cgraphunit.c and final.c, among 
others.   While we're willing to put in the effort to make these 
changes, we're wondering if anyone knows of something that would 
allow global RTL analysis without making large-scale changes. It 
seems as though the structure of the pass manager does not allow 
for this. The code structure suggests that we should either do an 
IPA pass on GIMPLE or we work as an assembler pass, thus forgoing 
work on RTL at all. Is this is what we should be doing?   If 
anyone has some guidance here, it would be appreciated.   -- Geoff