Re-work fast path that emits copies in zippy compression.
The primary motivation for the change is that FindMatchLength is likely to discover a difference in the first 8 bytes it compares. If that occurs then we know the length of the match is less than 12, because FindMatchLength is invoked after a 4-byte match is found. When emitting a copy, it is useful to know that the length is less than 12 because the two-byte variant of an emitted copy requires that. This is a performance-tuning change that should not affect the library's behavior. With FDO on perflab/Haswell the geometric mean for ZFlat/* went from 47,290ns to 45,741ns, an improvement of 3.4%. SAMPLE (before) BM_ZFlat/0 102824 102650 40691 951.4MB/s html (22.31 %) BM_ZFlat/1 1293512 1290442 3225 518.9MB/s urls (47.78 %) BM_ZFlat/2 10373 10353 417959 11.1GB/s jpg (99.95 %) BM_ZFlat/3 268 268 15745324 712.4MB/s jpg_200 (73.00 %) BM_ZFlat/4 12137 12113 342462 7.9GB/s pdf (83.30 %) BM_ZFlat/5 430672 429720 9724 909.0MB/s html4 (22.52 %) BM_ZFlat/6 420541 419636 9833 345.6MB/s txt1 (57.88 %) BM_ZFlat/7 373829 373158 10000 319.9MB/s txt2 (61.91 %) BM_ZFlat/8 1119014 1116604 3755 364.5MB/s txt3 (54.99 %) BM_ZFlat/9 1544203 1540657 2748 298.3MB/s txt4 (66.26 %) BM_ZFlat/10 91041 90866 46002 1.2GB/s pb (19.68 %) BM_ZFlat/11 332766 331990 10000 529.5MB/s gaviota (37.72 %) BM_ZFlat/12 39960 39886 100000 588.3MB/s cp (48.12 %) BM_ZFlat/13 14493 14465 287181 735.1MB/s c (42.47 %) BM_ZFlat/14 4447 4440 947927 799.3MB/s lsp (48.37 %) BM_ZFlat/15 1316362 1313350 3196 747.7MB/s xls (41.23 %) BM_ZFlat/16 312 311 10000000 613.0MB/s xls_200 (78.00 %) BM_ZFlat/17 388471 387502 10000 1.2GB/s bin (18.11 %) BM_ZFlat/18 65 64 64838208 2.9GB/s bin_200 (7.50 %) BM_ZFlat/19 65900 65787 63099 554.3MB/s sum (48.96 %) BM_ZFlat/20 6188 6177 681951 652.6MB/s man (59.21 %) SAMPLE (after) Benchmark Time(ns) CPU(ns) Iterations -------------------------------------------- BM_ZFlat/0 99259 99044 42428 986.0MB/s html (22.31 %) BM_ZFlat/1 1257039 1255276 3341 533.4MB/s urls (47.78 %) BM_ZFlat/2 10044 10030 405781 11.4GB/s jpg (99.95 %) BM_ZFlat/3 268 267 15732282 713.3MB/s jpg_200 (73.00 %) BM_ZFlat/4 11675 11657 358629 8.2GB/s pdf (83.30 %) BM_ZFlat/5 420951 419818 9739 930.5MB/s html4 (22.52 %) BM_ZFlat/6 415460 414632 10000 349.8MB/s txt1 (57.88 %) BM_ZFlat/7 367191 366436 10000 325.8MB/s txt2 (61.91 %) BM_ZFlat/8 1098345 1096036 3819 371.3MB/s txt3 (54.99 %) BM_ZFlat/9 1508701 1505306 2758 305.3MB/s txt4 (66.26 %) BM_ZFlat/10 87195 87031 47289 1.3GB/s pb (19.68 %) BM_ZFlat/11 322338 321637 10000 546.5MB/s gaviota (37.72 %) BM_ZFlat/12 36739 36668 100000 639.9MB/s cp (48.12 %) BM_ZFlat/13 13646 13618 304009 780.9MB/s c (42.47 %) BM_ZFlat/14 4249 4240 992456 837.0MB/s lsp (48.37 %) BM_ZFlat/15 1262925 1260012 3314 779.4MB/s xls (41.23 %) BM_ZFlat/16 308 308 10000000 619.8MB/s xls_200 (78.00 %) BM_ZFlat/17 379750 378944 10000 1.3GB/s bin (18.11 %) BM_ZFlat/18 62 62 67443280 3.0GB/s bin_200 (7.50 %) BM_ZFlat/19 61706 61587 67645 592.1MB/s sum (48.96 %) BM_ZFlat/20 5968 5958 698974 676.6MB/s man (59.21 %)
This commit is contained in:
Geoff Pike
committed by
Alkis Evlogimenos
Alkis Evlogimenos
parent
094c67de88
commit
38a5ec5fca
@@ -70,11 +70,12 @@ char* CompressFragment(const char* input,
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uint16* table,
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const int table_size);
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// Return the largest n such that
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// Find the largest n such that
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//
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// s1[0,n-1] == s2[0,n-1]
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// and n <= (s2_limit - s2).
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//
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// Return make_pair(n, n < 8).
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// Does not read *s2_limit or beyond.
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// Does not read *(s1 + (s2_limit - s2)) or beyond.
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// Requires that s2_limit >= s2.
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@@ -82,11 +83,27 @@ char* CompressFragment(const char* input,
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// Separate implementation for x86_64, for speed. Uses the fact that
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// x86_64 is little endian.
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#if defined(ARCH_K8)
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static inline int FindMatchLength(const char* s1,
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const char* s2,
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const char* s2_limit) {
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static inline pair<size_t, bool> FindMatchLength(const char* s1,
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const char* s2,
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const char* s2_limit) {
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assert(s2_limit >= s2);
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int matched = 0;
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size_t matched = 0;
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// This block isn't necessary for correctness; we could just start looping
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// immediately. As an optimization though, it is useful. It creates some not
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// uncommon code paths that determine, without extra effort, whether the match
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// length is less than 8. In short, we are hoping to avoid a conditional
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// branch, and perhaps get better code layout from the C++ compiler.
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if (PREDICT_TRUE(s2 <= s2_limit - 8)) {
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uint64 a1 = UNALIGNED_LOAD64(s1);
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uint64 a2 = UNALIGNED_LOAD64(s2);
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if (a1 != a2) {
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return pair<size_t, bool>(Bits::FindLSBSetNonZero64(a1 ^ a2) >> 3, true);
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} else {
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matched = 8;
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s2 += 8;
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}
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}
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// Find out how long the match is. We loop over the data 64 bits at a
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// time until we find a 64-bit block that doesn't match; then we find
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@@ -97,14 +114,11 @@ static inline int FindMatchLength(const char* s1,
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s2 += 8;
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matched += 8;
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} else {
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// On current (mid-2008) Opteron models there is a 3% more
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// efficient code sequence to find the first non-matching byte.
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// However, what follows is ~10% better on Intel Core 2 and newer,
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// and we expect AMD's bsf instruction to improve.
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uint64 x = UNALIGNED_LOAD64(s2) ^ UNALIGNED_LOAD64(s1 + matched);
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int matching_bits = Bits::FindLSBSetNonZero64(x);
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matched += matching_bits >> 3;
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return matched;
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assert(matched >= 8);
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return pair<size_t, bool>(matched, false);
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}
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}
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while (PREDICT_TRUE(s2 < s2_limit)) {
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@@ -112,15 +126,15 @@ static inline int FindMatchLength(const char* s1,
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++s2;
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++matched;
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} else {
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return matched;
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return pair<size_t, bool>(matched, matched < 8);
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}
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}
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return matched;
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return pair<size_t, bool>(matched, matched < 8);
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}
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#else
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static inline int FindMatchLength(const char* s1,
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const char* s2,
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const char* s2_limit) {
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static inline pair<size_t, bool> FindMatchLength(const char* s1,
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const char* s2,
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const char* s2_limit) {
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// Implementation based on the x86-64 version, above.
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assert(s2_limit >= s2);
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int matched = 0;
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@@ -140,7 +154,7 @@ static inline int FindMatchLength(const char* s1,
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++matched;
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}
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}
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return matched;
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return pair<size_t, bool>(matched, matched < 8);
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}
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#endif
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