perf improvements for zstd decode (#1668)
* perf improvements for zstd decode
tldr: 7.5% average decode speedup on silesia corpus at compression levels 1-3 (sandy bridge)
Background: while investigating zstd perf differences between clang and gcc I noticed that even though gcc is vectorizing the loop in in wildcopy, it was not being done as well as could be done by hand. The sites where wildcopy is invoked have an interesting distribution of lengths to be copied. The loop trip count is rarely above 1, yet long copies are common enough to make their performance important.The code in zstd_decompress.c to invoke wildcopy handles the latter well but the gcc autovectorizer introduces a needlessly expensive startup check for vectorization.
See how GCC autovectorizes the loop here:
https://godbolt.org/z/apr0x0
Here is the code after this diff has been applied: (left hand side is the good one, right is with vectorizer on)
After: https://godbolt.org/z/OwO4F8
Note that autovectorization still does not do a good job on the optimized version, so it's turned off\
via attribute and flag. I found that neither attribute nor command-line flag were entirely successful in turning off vectorization, which is why there were both.
silesia benchmark data - second triad of each file is with the original code:
file orig compressedratio encode decode change
1#dickens 10192446-> 4268865(2.388), 198.9MB/s 709.6MB/s
2#dickens 10192446-> 3876126(2.630), 128.7MB/s 552.5MB/s
3#dickens 10192446-> 3682956(2.767), 104.6MB/s 537MB/s
1#dickens 10192446-> 4268865(2.388), 195.4MB/s 659.5MB/s 7.60%
2#dickens 10192446-> 3876126(2.630), 127MB/s 516.3MB/s 7.01%
3#dickens 10192446-> 3682956(2.767), 105MB/s 479.5MB/s 11.99%
1#mozilla 51220480-> 20117517(2.546), 285.4MB/s 734.9MB/s
2#mozilla 51220480-> 19067018(2.686), 220.8MB/s 686.3MB/s
3#mozilla 51220480-> 18508283(2.767), 152.2MB/s 669.4MB/s
1#mozilla 51220480-> 20117517(2.546), 283.4MB/s 697.9MB/s 5.30%
2#mozilla 51220480-> 19067018(2.686), 225.9MB/s 665MB/s 3.20%
3#mozilla 51220480-> 18508283(2.767), 154.5MB/s 640.6MB/s 4.50%
1#mr 9970564-> 3840242(2.596), 262.4MB/s 899.8MB/s
2#mr 9970564-> 3600976(2.769), 181.2MB/s 717.9MB/s
3#mr 9970564-> 3563987(2.798), 116.3MB/s 620MB/s
1#mr 9970564-> 3840242(2.596), 253.2MB/s 827.3MB/s 8.76%
2#mr 9970564-> 3600976(2.769), 177.4MB/s 655.4MB/s 9.54%
3#mr 9970564-> 3563987(2.798), 111.2MB/s 564.2MB/s 9.89%
1#nci 33553445-> 2849306(11.78), 575.2MB/s , 1335.8MB/s
2#nci 33553445-> 2890166(11.61), 509.3MB/s , 1238.1MB/s
3#nci 33553445-> 2857408(11.74), 431MB/s , 1210.7MB/s
1#nci 33553445-> 2849306(11.78), 565.4MB/s , 1220.2MB/s 9.47%
2#nci 33553445-> 2890166(11.61), 508.2MB/s , 1128.4MB/s 9.72%
3#nci 33553445-> 2857408(11.74), 429.1MB/s , 1097.7MB/s 10.29%
1#ooffice 6152192-> 3590954(1.713), 231.4MB/s , 662.6MB/s
2#ooffice 6152192-> 3323931(1.851), 162.8MB/s , 592.6MB/s
3#ooffice 6152192-> 3145625(1.956), 99.9MB/s , 549.6MB/s
1#ooffice 6152192-> 3590954(1.713), 224.7MB/s , 624.2MB/s 6.15%
2#ooffice 6152192-> 3323931 (1.851), 155MB/s , 564.5MB/s 4.98%
3#ooffice 6152192-> 3145625(1.956), 101.1MB/s , 521.2MB/s 5.45%
1#osdb 10085684-> 3739042(2.697), 271.9MB/s 876.4MB/s
2#osdb 10085684-> 3493875(2.887), 208.2MB/s 857MB/s
3#osdb 10085684-> 3515831(2.869), 135.3MB/s 805.4MB/s
1#osdb 10085684-> 3739042(2.697), 257.4MB/s 793.8MB/s 10.41%
2#osdb 10085684-> 3493875(2.887), 209.7MB/s 776.1MB/s 10.42%
3#osdb 10085684-> 3515831(2.869), 130.6MB/s 727.7MB/s 10.68%
1#reymont 6627202-> 2152771(3.078), 198.9MB/s 696.2MB/s
2#reymont 6627202-> 2071140(3.200), 170MB/s 595.2MB/s
3#reymont 6627202-> 1953597(3.392), 128.5MB/s 609.7MB/s
1#reymont 6627202-> 2152771(3.078), 199.6MB/s 655.2MB/s 6.26%
2#reymont 6627202-> 2071140(3.200), 168.2MB/s 554.4MB/s 7.36%
3#reymont 6627202-> 1953597(3.392), 128.7MB/s 557.4MB/s 9.38%
1#samba 21606400-> 5510994(3.921), 338.1MB/s 1066MB/s
2#samba 21606400-> 5240208(4.123), 258.7MB/s 992.3MB/s
3#samba 21606400-> 5003358(4.318), 200.2MB/s 991.1MB/s
1#samba 21606400-> 5510994(3.921), 330.8MB/s 974MB/s 9.45%
2#samba 21606400-> 5240208(4.123), 257.9MB/s 919.4MB/s 7.93%
3#samba 21606400-> 5003358(4.318), 198.5MB/s 908.9MB/s 9.04%
1#sao 7251944-> 6256401(1.159), 194.6MB/s 602.2MB/s
2#sao 7251944-> 5808761(1.248), 128.2MB/s 532.1MB/s
3#sao 7251944-> 5556318(1.305), 73MB/s 509.4MB/s
1#sao 7251944-> 6256401(1.159), 198.7MB/s 580.7MB/s 3.70%
2#sao 7251944-> 5808761(1.248), 129.1MB/s 502.7MB/s 5.85%
3#sao 7251944-> 5556318(1.305), 74.6MB/s 493.1MB/s 3.31%
1#webster 41458703-> 13692222(3.028), 222.3MB/s 752MB/s
2#webster 41458703-> 12842646(3.228), 157.6MB/s 532.2MB/s
3#webster 41458703-> 12191964(3.400), 124MB/s 468.5MB/s
1#webster 41458703-> 13692222(3.028), 219.7MB/s 697MB/s 7.89%
2#webster 41458703-> 12842646(3.228), 153.9MB/s 495.4MB/s 7.43%
3#webster 41458703-> 12191964(3.400), 124.8MB/s 444.8MB/s 5.33%
1#xml 5345280-> 696652(7.673), 485MB/s , 1333.9MB/s
2#xml 5345280-> 681492(7.843), 405.2MB/s , 1237.5MB/s
3#xml 5345280-> 639057(8.364), 328.5MB/s , 1281.3MB/s
1#xml 5345280-> 696652(7.673), 473.1MB/s , 1232.4MB/s 8.24%
2#xml 5345280-> 681492(7.843), 398.6MB/s , 1145.9MB/s 7.99%
3#xml 5345280-> 639057(8.364), 327.1MB/s , 1175MB/s 9.05%
1#x-ray 8474240-> 6772557(1.251), 521.3MB/s 762.6MB/s
2#x-ray 8474240-> 6684531(1.268), 230.5MB/s 688.5MB/s
3#x-ray 8474240-> 6166679(1.374), 68.7MB/s 478.8MB/s
1#x-ray 8474240-> 6772557(1.251), 502.8MB/s 736.7MB/s 3.52%
2#x-ray 8474240-> 6684531(1.268), 224.4MB/s 662MB/s 4.00%
3#x-ray 8474240-> 6166679(1.374), 67.3MB/s 437.8MB/s 9.37%
7.51%
* makefile changed to only pass -fno-tree-vectorize to gcc
* <Replace this line with a title. Use 1 line only, 67 chars or less>
Don't add "no-tree-vectorize" attribute on clang (which defines __GNUC__)
* fix for warning/error with subtraction of void* pointers
* fix c90 conformance issue - ISO C90 forbids mixed declarations and code
* Fix assert for negative diff, only when there is no overlap
* fix overflow revealed in fuzzing tests
* tweak for small speed increase
812e8f2a16