Compute next highest power of 2 function #374
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Compute next highest power of 2 to fix issues when using code (in different parts): const size_t npoints = pow(2, ceil(log2(depth))); (see scopehal\TestWaveformSource.cpp => TestWaveformSource::DegradeSerialData()...) The original code (pow(2, ceil(log2())) has some border effects when built with MSYS2 mingw64 "Release" mode as it does not compute correctly the highest power of 2 for example with parameter 100000 it was returning 131071 (instead of 131072) which crashed ffts. This implementation fix issue ngscopeclient/scopehal-apps#295 The function next_pow2() shall replace pow(2, ceil(log2())) in following code: scopehal\TestWaveformSource.cpp Line 282: //const size_t npoints = pow(2, ceil(log2(depth))); scopeprotocols\DeEmbedFilter.cpp Line 230: const size_t npoints = pow(2, ceil(log2(npoints_raw))); scopeprotocols\FFTFilter.cpp Line 179: const size_t npoints = pow(2, ceil(log2(npoints_raw))); scopeprotocols\JitterSpectrumFilter.cpp Line 195: const size_t npoints = pow(2, ceil(log2(npoints_raw)));
miek
reviewed
Dec 7, 2020
| v |= v >> 2; | ||
| v |= v >> 4; | ||
| v |= v >> 8; | ||
| v |= v >> 16; |
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This needs an extra shift by 32 to work properly for 64bit inputs. The input 0x100000001 would fail currently, for example.
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I have done a fast test and actual version seems to work fine up to value 9223372036854775807
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I confirm your fix is good
see small tests here https://www.onlinegdb.com/mw3ZHYZsQ
bvernoux
commented
Dec 7, 2020
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That is a variant of https://jameshfisher.com/2018/03/30/round-up-power-2/ see uint64_t next_pow2m1(uint64_t x) |
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Compute next highest power of 2 to fix issues when using code (in different parts):
const size_t npoints = pow(2, ceil(log2(depth))); (see scopehal\TestWaveformSource.cpp => TestWaveformSource::DegradeSerialData()...)
The original code (pow(2, ceil(log2())) has some border effects when built with MSYS2 mingw64 "Release" mode as it does not compute correctly the highest power of 2 for example with parameter 100000 it was returning 131071 (instead of 131072) which crashed ffts.
This implementation fix issue ngscopeclient/scopehal-apps#295
The function next_pow2() shall replace pow(2, ceil(log2())) in following code:
scopehal\TestWaveformSource.cpp
Line 282: //const size_t npoints = pow(2, ceil(log2(depth)));
scopeprotocols\DeEmbedFilter.cpp
Line 230: const size_t npoints = pow(2, ceil(log2(npoints_raw)));
scopeprotocols\FFTFilter.cpp
Line 179: const size_t npoints = pow(2, ceil(log2(npoints_raw)));
scopeprotocols\JitterSpectrumFilter.cpp
Line 195: const size_t npoints = pow(2, ceil(log2(npoints_raw)));