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Re: lto

Jan Hubicka wrote:

I have to admit that i had not considered the issue of having the later
stages of compilation of one function feed into the early stages of
another. The problem with this is that it implies a serial ordering of
the compilation and that is not going to yield a usable system for
compiling very large programs.

Yes, this is why I wanted to bring this in. I am not expert for whole program register allocation, but my understanding is that it can really help, especially on machine like x86-64, where saving prologues/epilogues are relatively expensive. It seems to me that it can't scale to really big programs, but it don't have to if we subdivide the programs sanely (ie do equivalent of region based compilation for large function). As long as the calls crossing boundaries are not very frequent, we should be all happy.

Vladimir, Andrew, I am sure you know a lot more about global RA.
Is it resonable to expect that RA needs to propagate things just down in
topological order and can we split the program to resonably sized chunks
where the RA can be handled locally?

IP RA as currenly implemented in IRA does propagate info only down in topological order. But a good IP RA (e.g. Minimal cost inter-procedural regiter allocator needs to propagate info up and down.

But I am quite skeptical about IP RA. To be sucessfull it needs that the called function is small and uses few registers. Such functions should be just inlined. It solves IP RA besides other problems. Intel/Sun/Pathscale compilers make very aggressive function inlining during LTO. Therefore the code generated by the compilers with LTO is much bigger (e.g. LTO in pathscale results in 30% bigger code for x86_64 SPECINT2000 and by the way improves the code by 4% and makes the compiler 50% slower).

In overall I don't think that IP RA is important thing and worth to be implemented.

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