rt: pull upstream ISAAC code for consistency between 32/64 bit platforms #6058
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The "unsigned 4 byte" `ub4`s are actually 8 bytes on 64-bit platforms which mean that some bits > 2**32 were retained in calculations, these would then "reappear" after a shift and so the stream of random numbers would differ on 32 bit vs 64 bit platforms.
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@graydon: r? |
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Nice catch. |
This means that `ub4`s are always 4 bytes, rather than being 8 bytes on x64. (Suggested but not implemented by upstream: "Porting it to a 64-bit machine [...] may just need an adjustment of the definition of ub4")
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@graydon: I just read the source web page in more detail and it suggests:
So I added a commit that does this change (while retaining the old one). Without it, half the seed was unused. |
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The "unsigned 4 byte" `ub4`s are actually 8 bytes on 64-bit platforms which mean that some bits > 2**32 were retained in calculations, these would then "reappear" after a right shift and so the stream of random numbers would differ on 32 bit vs 64 bit platforms. http://burtleburtle.net/bob/c/randport.c
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Apr 30, 2013
This replaces the wrapper around the runtime RNG with a pure Rust implementation of the same algorithm. This is much faster (up to 5x), and is hopefully safer. There is still (a little) room for optimisation: testing by summing 100,000,000 random `u32`s indicates this is about ~~40-50%~~ 10% slower than the pure C implementation (running as standalone executable, not in the runtime). (Only 6d50d55 is part of this PR, the first two are from #6058, but are required for the rt rng to be correct to compare against in the tests.)
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The "unsigned 4 byte"
ub4s are actually 8 bytes on 64-bit platformswhich mean that some bits > 2**32 were retained in calculations, these
would then "reappear" after a right shift and so the stream of random numbers
would differ on 32 bit vs 64 bit platforms.
http://burtleburtle.net/bob/c/randport.c