gcc is a skyscraper [LWN.net]

gcc is a skyscraper

Posted Nov 1, 2007 10:40 UTC (Thu) by jfj (guest, #37917)
Parent article: A potential competitor for GCC: pcc

It is no little program!

gcc is probably bigger than linux. Sure linux has more lines of code but most of that is
device drivers, so a user at a given time only uses about 5% of the total of the kernel.

Also, the kernel's job is to talk to the hardware. The job of a compiler is to find the
optimal machine code representation of a C (C++, etc) program. This is a rather unsolvable
problem and it is simply constantly improved over the years.

It would be probably easier to write an operating system better than linux/BSD than a compiler
better then gcc.

OK. Some times gcc devs are too concerned with the SPEC benchmarks (which is a red herring
imho), but at least they believe in free software.

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gcc is a skyscraper

Posted Nov 1, 2007 21:16 UTC (Thu) by nix (subscriber, #2304) [Link]

The SPEC benchmarks are only benchmarks, but *sudden worsenings* in SPEC 
output are a good clue that something's just been broken. It's also useful 
to keep an eye on C compilation speed (bootstrap-and-test time is also 
somewhat useful, but the compiler and testsuite are growing, so that is 
expected to rise even if the compiler got no slower).

gcc is a skyscraper

Posted Nov 2, 2007 11:45 UTC (Fri) by jfj (guest, #37917) [Link]

Yes. But take for example, auto-vectorization. icc does it and it gets some amazing results in
SPEC. But the fact is that *any real program that really cares about speed cannot depend on
the compiler to auto-vectorize*; its developers will provide custom assembly with MMX for the
critical parts.  Now that is a red herring. Maybe LTO as well.

Also, I suspect that commercial compilers spend a lot of effort to look good in SPEC. Maybe
they even identify those benchmark programs and generate predefined assembly, I don't know :)

I generally think that timings on real programs is far more useful. There could be a testsuite
that runs some timings on bzip, gzip, ffmpeg encoding/decoding, etc. SPEC's little programs
are to sensitive to small changes.

Finally, I think that open source would benefit more from new gcc extensions and special
attributes.

gcc is a skyscraper

Posted Nov 2, 2007 13:04 UTC (Fri) by nix (subscriber, #2304) [Link]

LTO definitely isn't a red herring: most programs consist of more than one 
object file and can't easily use --combine (which is far too memory hungry 
to be useful anyway: I tried compiling GCC 4.2.1 with --combine and it ran 
my 1.5Gb Athlon out of memory...)