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bugfixes in Libc.rand for Windows #32895

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Sep 4, 2019
Merged

bugfixes in Libc.rand for Windows #32895

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bugfixes in Libc.rand for Windows
stevengj Aug 14, 2019
bfae1dd
typo
stevengj Aug 14, 2019
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12 changes: 9 additions & 3 deletions base/libc.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -382,9 +382,15 @@ Interface to the C `rand()` function. If `T` is provided, generate a value of ty
by composing two calls to `rand()`. `T` can be `UInt32` or `Float64`.
"""
rand() = ccall(:rand, Cint, ())
# RAND_MAX at least 2^15-1 in theory, but we assume 2^16-1 (in practice, it's 2^31-1)
rand(::Type{UInt32}) = ((rand() % UInt32) << 16) ⊻ (rand() % UInt32)
rand(::Type{Float64}) = rand(UInt32) / 2^32
@static if Sys.iswindows()
# Windows RAND_MAX is 2^15-1
rand(::Type{UInt32}) = ((rand() % UInt32) << 17) ⊻ ((rand() % UInt32) << 8) ⊻ (rand() % UInt32)
else
# RAND_MAX is at least 2^15-1 in theory, but we assume 2^16-1
# on non-Windows systems (in practice, it's 2^31-1)
rand(::Type{UInt32}) = ((rand() % UInt32) << 16) ⊻ (rand() % UInt32)
end
rand(::Type{Float64}) = rand(UInt32) * 2.0^-32
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Use ldexp?

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@stevengj stevengj Aug 14, 2019 •
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@code_native g(UInt32(3)) reports an enormous amount of code for g(x) = ldexp(Float64(x), -32) for some reason…

(ldexp(x, n) only currently supports floating-point x.)

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I think ldexp is probably slower here here because it doesn't seem to constant-propagate the exponent?


"""
srand([seed])
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7 changes: 7 additions & 0 deletions test/misc.jl
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Expand Up @@ -733,5 +733,12 @@ end
@test sort([a, b]) == [b, a]
end

@testset "Libc.rand" begin
low, high = extrema(Libc.rand(Float64) for i=1:10^4)
# these fail with probability 2^(-10^4) ≈ 5e-3011
@test 0 ≤ low < 0.5
@test 0.5 < high < 1
end

# Pointer 0-arg constructor
@test Ptr{Cvoid}() == C_NULL