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24
25#ifndef CPU_X86_CRC32C_H
26#define CPU_X86_CRC32C_H
27
28enum {
29 // S. Gueron / Information Processing Letters 112 (2012) 184
30 // shows than anything above 6K and below 32K is a good choice
31 // 32K does not deliver any further performance gains
32 // 6K=8*256 (*3 as we compute 3 blocks together)
33 //
34 // Thus selecting the smallest value so it could apply to the largest number
35 // of buffer sizes.
36 CRC32C_HIGH = 8 * 256,
37
38 // empirical
39 // based on ubench study using methodology described in
40 // V. Gopal et al. / Fast CRC Computation for iSCSI Polynomial Using CRC32 Instruction April 2011 8
41 //
42 // arbitrary value between 27 and 256
43 CRC32C_MIDDLE = 8 * 86,
44
45 // V. Gopal et al. / Fast CRC Computation for iSCSI Polynomial Using CRC32 Instruction April 2011 9
46 // shows that 240 and 1024 are equally good choices as the 216==8*27
47 //
48 // Selecting the smallest value which resulted in a significant performance improvement over
49 // sequential version
50 CRC32C_LOW = 8 * 27,
51
52 CRC32C_NUM_ChunkSizeInBytes = 3,
53
54 // We need to compute powers of 64N and 128N for each "chunk" size
55 CRC32C_NUM_PRECOMPUTED_CONSTANTS = ( 2 * CRC32C_NUM_ChunkSizeInBytes )
56};
57// Notes:
58// 1. Why we need to choose a "chunk" approach?
59// Overhead of computing a powers and powers of for an arbitrary buffer of size N is significant
60// (implementation approaches a library perf.)
61// 2. Why only 3 "chunks"?
62// Performance experiments results showed that a HIGH+LOW was not delivering a stable speedup
63// curve.
64//
65// Disclaimer:
66// If you ever decide to increase/decrease number of "chunks" be sure to modify
67// a) constants table generation (hotspot/src/cpu/x86/vm/stubRoutines_x86.cpp)
68// b) constant fetch from that table (macroAssembler_x86.cpp)
69// c) unrolled for loop (macroAssembler_x86.cpp)
70
71#endif /* !CPU_X86_CRC32C_H */
72