zstd: Add branchless filling #550
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Do fills without random branching. Avoids 1 fill, since we can guarantee 56 bits. ``` λ benchcmp before.txt after.txt benchmark old ns/op new ns/op delta Benchmark_seqdec_decode/n-12286-lits-13914-prev-9869-1990358-3296656-win-4194304.blk-32 88467 69674 -21.24% Benchmark_seqdec_decode/n-12485-lits-6960-prev-976039-2250252-2463561-win-4194304.blk-32 89402 72286 -19.14% Benchmark_seqdec_decode/n-14746-lits-14461-prev-209-8-1379909-win-4194304.blk-32 80374 88479 +10.08% Benchmark_seqdec_decode/n-1525-lits-1498-prev-2009476-797934-2994405-win-4194304.blk-32 8823 9632 +9.17% Benchmark_seqdec_decode/n-3478-lits-3628-prev-895243-2104056-2119329-win-4194304.blk-32 22177 21437 -3.34% Benchmark_seqdec_decode/n-8422-lits-5840-prev-168095-2298675-433830-win-4194304.blk-32 60001 49542 -17.43% Benchmark_seqdec_decode/n-1000-lits-1057-prev-21887-92-217-win-8388608.blk-32 5676 6190 +9.06% Benchmark_seqdec_decode/n-15134-lits-20798-prev-4882976-4884216-4474622-win-8388608.blk-32 116433 96040 -17.51% Benchmark_seqdec_decode/n-2-lits-0-prev-620601-689171-848-win-8388608.blk-32 45.4 45.6 +0.40% Benchmark_seqdec_decode/n-90-lits-67-prev-19498-23-19710-win-8388608.blk-32 536 582 +8.59% Benchmark_seqdec_decode/n-931-lits-1179-prev-36502-1526-1518-win-8388608.blk-32 5307 5830 +9.85% Benchmark_seqdec_decode/n-2898-lits-4062-prev-335-386-751-win-8388608.blk-32 17262 17753 +2.84% Benchmark_seqdec_decode/n-4056-lits-12419-prev-10792-66-309849-win-8388608.blk-32 26685 25027 -6.21% Benchmark_seqdec_decode/n-8028-lits-4568-prev-917-65-920-win-8388608.blk-32 57524 46995 -18.30% ``` Better, but not too clear a win.
# Conflicts: # zstd/seqdec_amd64.s
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Do fills without random branching.
Avoids 1 fill, since we can guarantee 56 bits.
Add specialized version for when we only need at most 56 bits per loop to only have one fill. ~9% faster when hit.
Better, but not too clear a win.