We use Barrett tables (misspelled in the code unfortunately) to fold
crc computations of multiple buffers into a single crc. This is important
because it turns out to be faster to compute crc of three different buffers
in parallel rather than compute the crc of one large buffer, since the crc
instruction has latency 3.
Currently, we have a separate code generation step to compute the
fold tables. The step generates a new C++ source files with the tables.
But modern C++ allows us to do this computation at compile time, avoiding
the code generation step. This simplifies the build.
This series does that. There is some complication in that the code uses
compiler intrinsics for the computation, and these are not constexpr friendly.
So we first introduce constexpr-friendly alternatives and use them.
To prove the transformation is correct, I compared the generated code from
before the series and from just before the last step (where we use constexpr
evaluation but still retain the generated file) and saw no difference in the values.
Note that constexpr is not strictly needed - we could have run the code in the
global variables' initializer. But that would cause a crash if we run on a pre-clmul
machine, and is not as fun.
Closes#11957
* github.com:scylladb/scylladb:
test: crc: add unit tests for constexpr clmul and barrett fold
utils: crc combine table: generate at compile time
utils: barrett: inline functions in header
utils: crc combine table: generate tables at compile time
utils: crc combine table: extract table generation into a constexpr function
utils: crc combine table: extract "pow table" code into constexpr function
utils: crc combine table: store tables std::arrray rather than C array
utils: barrett: make the barrett reduction constexpr friendly
utils: clmul: add 64-bit constexpr clmul
utils: barrett: extract barrett reduction constants
utils: barrett: reorder functions
utils: make clmul() constexpr
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