Speed up decompression by caching ip_.
It is seemingly hard for the compiler to understand that ip_, the current input
pointer into the compressed data stream, can not alias on anything else, and
thus using it directly will incur memory traffic as it cannot be kept in a
register. The code already knew about this and cached it into a local
variable, but since Step() only decoded one tag, it had to move ip_ back into
place between every tag. This seems to have cost us a significant amount of
performance, so changing Step() into a function that decodes as much as it can
before it saves ip_ back and returns. (Note that Step() was already inlined,
so it is not the manual inlining that buys the performance here.)
The wins are about 3-6% for Core 2, 6-13% on Core i7 and 5-12% on Opteron
(for plain array-to-array decompression, in 64-bit opt mode).
There is a tiny difference in the behavior here; if an invalid literal is
encountered (ie., the writer refuses the Append() operation), ip_ will now
point to the byte past the tag byte, instead of where the literal was
originally thought to end. However, we don't use ip_ for anything after
DecompressAllTags() has returned, so this should not change external behavior
in any way.
Microbenchmark results for Core i7, 64-bit (Opteron results are similar):
Benchmark Time(ns) CPU(ns) Iterations
---------------------------------------------------
BM_UFlat/0 79134 79110 8835 1.2GB/s html [ +6.2%]
BM_UFlat/1 786126 786096 891 851.8MB/s urls [+10.0%]
BM_UFlat/2 9948 9948 69125 11.9GB/s jpg [ -1.3%]
BM_UFlat/3 31999 31998 21898 2.7GB/s pdf [ +6.5%]
BM_UFlat/4 318909 318829 2204 1.2GB/s html4 [ +6.5%]
BM_UFlat/5 31384 31390 22363 747.5MB/s cp [ +9.2%]
BM_UFlat/6 14037 14034 49858 757.7MB/s c [+10.6%]
BM_UFlat/7 4612 4612 151395 769.5MB/s lsp [ +9.5%]
BM_UFlat/8 1203174 1203007 582 816.3MB/s xls [+19.3%]
BM_UFlat/9 253869 253955 2757 571.1MB/s txt1 [+11.4%]
BM_UFlat/10 219292 219290 3194 544.4MB/s txt2 [+12.1%]
BM_UFlat/11 672135 672131 1000 605.5MB/s txt3 [+11.2%]
BM_UFlat/12 902512 902492 776 509.2MB/s txt4 [+12.5%]
BM_UFlat/13 372110 371998 1881 1.3GB/s bin [ +5.8%]
BM_UFlat/14 50407 50407 10000 723.5MB/s sum [+13.5%]
BM_UFlat/15 5699 5701 100000 707.2MB/s man [+12.4%]
BM_UFlat/16 83448 83424 8383 1.3GB/s pb [ +5.7%]
BM_UFlat/17 256958 256963 2723 684.1MB/s gaviota [ +7.9%]
BM_UValidate/0 42795 42796 16351 2.2GB/s html [+25.8%]
BM_UValidate/1 490672 490622 1427 1.3GB/s urls [+22.7%]
BM_UValidate/2 237 237 2950297 499.0GB/s jpg [+24.9%]
BM_UValidate/3 14610 14611 47901 6.0GB/s pdf [+26.8%]
BM_UValidate/4 171973 171990 4071 2.2GB/s html4 [+25.7%]
git-svn-id: https://snappy.googlecode.com/svn/trunk@38 03e5f5b5-db94-4691-08a0-1a8bf15f6143
1 file changed
