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
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