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Re: Thoughts on LLVM and LTO


On Tue, 22 Nov 2005, Steven Bosscher wrote:
On Tuesday 22 November 2005 17:20, Diego Novillo wrote:
The initial impression I get is that LLVM involves starting from scratch.
I thought it would basically "only" replace the GIMPLE parts of the
compiler.  That is,

FE	-->	GENERIC	-->	LLVM	-->	RTL	--> asm
(trees)		(trees)

In the longer-term, you could maybe cut out the RTL part for targets for which LLVM has its own backend. This is not less evolutionary or revolutionary than tree-ssa was IMHO.

Yes, agreed. For my work at Apple, we will probably end up using the LLVM backends. For the bigger GCC picture, making use of the RTL backends is essential.


With our limited resources, we cannot really afford to go off on a
multi-year tangent nurturing and growing a new technology just to add a
new feature.

It depends on who is going to invest these resources. Would you want to tell Apple they can't do this even though they can? ;-)

:)


But what are the
timelines?  What resources are needed?

Interesting questions. Both projects obviously will take significant effort. But IIUC Chris has bits of the LLVM stuff already going, so he has the head-start (like tree-SSA did when LLVM was introduced to the GCC community, ironically? ;-) so maybe Chris can have a working prototype implementation within, what, months? The GVM plan could take years to get to that point...

That is the plan.


So my dummy prediction would be that the LLVM path would result in a
reasonable product more quickly than the GVM plan -- iff RTL stays.

Yes, totally agreed.


-Chris

--
http://nondot.org/sabre/
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