[ccache] [Patch] Faster direct-mode hash

[Justin Lebar]

> check_for_temporal_macros could stop searching if both macros have been found?
> I cannot tell if doing that would make any real difference.

We could certainly do that, although I don't think it would help much.
 If your code has lots of __TIME__s, you're screwed anyway.  :)

On Sun, Nov 7, 2010 at 1:47 PM, Anders Furuhed
<anders.furuhed at pantor.com> wrote:
> Hi,
>
> check_for_temporal_macros could stop searching if both macros have been found?
> I cannot tell if doing that would make any real difference.
>
> Regards,
> Anders
>
> 7 nov 2010 kl. 22.13 skrev Justin Lebar:
>
>> This patch is a followup to the discussion in "Questions about two hot
>> functions in ccache".
>>
>> On my machine, the patch speeds up direct mode cache hits by about a
>> factor of 1.7 for the c++_includes.cc test file.  My benchmark of
>> |make clean && time make| in Mozilla's docshell/base went from 1.04s
>> (git master) to 0.64s, a 1.63x speedup.
>>
>> Full output from ../perf.py on c++_includes.cc is included below.
>>
>> I suspect we could use the fast_hash function for preprocessor mode
>> without much work.  I also suspect that switching to a smarter
>> algorithm for searching for "#include" would decrease the cost of
>> cache misses.  But I haven't profiled either of these cases.
>>
>> I'm a bit concerned about the fact that I had to change the reported
>> file lengths in the manifest test (in test.sh).  I'm not sure what's
>> going on here; I may well have messed something up.  Hopefully not.
>> :)
>>
>> -Justin
>>
>> $ ../perf.py -n10 --hit-factor=10 --ccache=../ccache gcc-4.5 c++_includes.cc
>> Compilation command: gcc-4.5 c++_includes.cc -c -o c++_includes.o
>> Compilercheck: mtime
>> Compression: off
>> Hardlink: off
>> Nostats: off
>>
>> * git master (9cdd1154)
>> Without ccache:                               3.55 s (100.00 %) ( 1.00 x)
>> With ccache, preprocessor mode, cache miss:   4.16 s (117.10 %) ( 0.85 x)
>> With ccache, preprocessor mode, cache hit:    0.87 s ( 24.51 %) ( 4.08 x)
>> With ccache, direct mode, cache miss:         4.22 s (118.98 %) ( 0.84 x)
>> With ccache, direct mode, cache hit:          0.15 s (  4.36 %) (22.92 x)
>>
>> * patched
>> Without ccache:                               3.53 s (100.00 %) ( 1.00 x)
>> With ccache, preprocessor mode, cache miss:   4.13 s (116.90 %) ( 0.86 x)
>> With ccache, preprocessor mode, cache hit:    0.86 s ( 24.25 %) ( 4.12 x)
>> With ccache, direct mode, cache miss:         4.15 s (117.55 %) ( 0.85 x)
>> With ccache, direct mode, cache hit:          0.09 s (  2.47 %) (40.43 x)
>>
>> * Speedup:  = .15 / .09 = 1.7x
>> <diff>_______________________________________________
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>
> Anders Furuhed
> Pantor Engineering AB
> +46 8 412 9781
>
>