perf improvements for zstd decode (#1668)

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