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Re: Criteria for GCC 4.0


Phil Edwards wrote:
The recurring 3.5-vs-4.0 thing is sure to come up, so I thought I'd
generate some discussion by providing my opinion, and talking points,
on why we don't have 4.0 material yet.

http://www.jaj.com/space/phil/40.html

Since my work, Acovea, is mentioned in your article, and I've been a participant in the debates, I'll respond.


As to your main point: There may be additional reasons to consider a 4.0 designation for the next major release of GCC. Chief among these is the inclusion of gfortran, which completely replaced the older g77. This will be a significant moment for many users, and something that needs to be clearly denoted in the next release.

As for what should be in a 4.0:

The compile speed issue must be addressed more fully. I have encountered a perception, in many places, that the GCC developers are insulated from the needs of the user community. This is likely a matter of both focus and funding; most funding comes from system vendors, so the focus is on what's needed for building distributions and working on the compiler, and not on documentation, warnings, and other issues you've raised.

That is *not* an indictment of the GCC developers! I am merely stating a fact of life -- development goes where the money is, whether it's free or closed software.

Based on Acovea experiments, I believe that optimizations need to be enabled on a platform basis. The current settings for -O1 don't appear to be detrimental, and should be kept; -O2 would implement a set of general- and platform-specific optimizations, while -O3 would throw the usual kitchen sink at code.

One problem is that "minor" differences in processors can lead to major differences in the efficacy of different optimizations. I'm working on a model for deciding how optimizations should be chosen.

On a related issue: The -ffast-math documentation is very misleading. I'm writing a major article for publication this week, looking at accuracy and precision in GCC. Using -ffast-math with GCC produces code that performs *better* on many precision and accuracy tests.

To the horror of some, I have compiled an entire Gentoo Linux system with -ffast-math. ;) Just mentioning this on Slashdot brought down a rather interesting pile of e-mail...

Acovea is undergoing another metamorphosis; a new version will be posted later this week, with new test data (for GCC "3.5") and minor refinements. At that point, I'll be opening a discussion as to the future of Acovea, particularly in terms of GCC.

> Thought I'd post something inflammatory and controversial just before
> leaving for a week.  (And then encountering all the offended people in
> person in Ottawa, yay!)

I'd love to be there, but other concerns will keep me in Florida. Have a good time...

..Scott

--
Scott Robert Ladd
Coyote Gulch Productions (http://www.coyotegulch.com)
Software Invention for High-Performance Computing


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