Description

This PR contains some simple performance improvements to the ldm matcher, together with some other cosmetic changes that I made "en passant" (variable renames, typo fixes, ...).

The changes to the ldm matcher only affect performance and the new code should produce precisely the same output as the baseline. Two changes helped improve the performance relatively uniformly:

• Instead of computing the tag with a branch and a fair amount of arithmetic operations in the hot loop of ZSTD_ldm_generateSequences we now compute a tag mask once before entering the loop and use it to check if the rolling hash satisfies the "tag condition". The tag mask is computed in such a way that the new tag condition is equivalent to the original one.
• When the tag condition is met but we did not find a match in the ldm hash table (it is the common case), we insert a new entry in the hash table using ZSTD_ldm_insertEntry instead of ZSTD_ldm_makeEntryAndInsertByTag which performs some redundant checks.

Benchmarks

The following files were used to assess the impact of the change:

• hhvm-rt.tar (5.9G) - an image of production version of HHVM
• l1m.tar (2.0G) - two snapshots of the linux kernel source code taken a month apart
• l1y.tar (1.9G) - two snapshots of the linux kernel source code taken a year apart
• l5.tar (544M) - the first 544M of an archive containing the linux kernel source code

The results are compiled in the table below. All times are computed as the best over a set of 5 runs.