First Codegen Result

I tidied up my initial draft of incremental code generation so that it no longer gratuitously lowers functions which are not being recompiled. This was enough to get some results — results which are semi-bogus, due to not relinking, but which nevertheless give some idea of what can be expected.

Compiler Seconds
Trunk 33
Incremental, no server 33
Server, first run 27
Server, second run 14
Preprocess 4

So, the results are a bit odd. Recompiling is fast, as we’d expect — about twice as fast as a plain build. However, it still falls far short of the time used by the preprocessor. What is going on in there?

A look with oprofile seems to indicate that the excess is spread around. About 10% of the total time is spent in the GC; another 7% is used computing MD5s. Other than that… if I add up the top 40 or so non-cpp functions, I get about 5 seconds worth, and there is a long tail after that. That’s a bummer since that kind of problem is hard to fix.

2 Comments

  • So, just using the incremental server I get a speedup of 20% for a clean compile? Cool!

    The Server, second run is after a make clean or equivalent I presume.

    What code base do you use for this test?

    How much time does relinking take approximately?

  • My test case was building zenity, from Gnome. I use that as my canonical “small test” since I have it sitting here and it isn’t very big.

    The 20% number appears to be a best case. If I try other programs the benefit drops or vanishes. I thought that maybe this had to do with hunk sizes, but I tested this theory yesterday with bad results. So, I don’t really know the cause :-(

    Yes, the second run is after “make clean”. This is, in theory, the best possible case for incremental codegen — no code has actually changed, but the resulting objects should not either.

    The bogus part of this is that I haven’t implemented relinking. You just get bad results right now; that’s why the patch isn’t committed. I’ve been poking at objdump and ld trying to find a way to script this part without too much work.

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