Floating point hash bug

[jmkuhn]

using InteractiveUtils
using Printf

function myhash(x)
    # the following is valid for x == Inf
    h = UInt(0)
    num, pow, den = Base.decompose(x)
    den == 0 && return hash(ifelse(num > 0, Inf, -Inf), h)
    return nothing
end

versioninfo()
h = hash(Inf)
@printf("%#x\n", h)
h = myhash(Inf)
@printf("%#x\n", h)

With LLVM 9 I get the expected output:

Julia Version 1.6.0-DEV.1014
Commit 2a36f83ce9 (2020-09-22 20:52 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin18.7.0)
  CPU: Intel(R) Core(TM) i7-7567U CPU @ 3.50GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, skylake)
0x807bb202c9cbfc6f
0x807bb202c9cbfc6f

With LLVM 10 I get:

Julia Version 1.6.0-DEV.1015
Commit 9adcdd42a0 (2020-09-23 03:04 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin18.7.0)
  CPU: Intel(R) Core(TM) i7-7567U CPU @ 3.50GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-10.0.1 (ORCJIT, skylake)
0x807bb202c9cbfc6f
0x15d7d083d04ecb90

Edit:
0x15d7d083d04ecb90 is the correct hash for NaN

[yuyichao]

julia> function myhash(b)
           return hash(ifelse(b, Inf, -Inf), UInt(0))
       end
myhash (generic function with 1 method)

julia> myhash(false)
0x15d7d083d04ecb90

julia> myhash(true)
0x15d7d083d04ecb90

julia> hash(-Inf, UInt(0))
0x96e782fc7639e1bc

julia> hash(Inf, UInt(0))
0x807bb202c9cbfc6f

[yuyichao]

Seems as if it evaluated Inf == Inf to false and used the NaN hash value instead...

[yuyichao]

And it seems like a julia bug. The implementation of hash relies on fptoui of the floating point value which produces a poison value if the fp value couldn't fit. It is therefore perfectly legal for LLVM to produce any result for hash(Inf). The implementation was previously relying on fptoui to return a fixed value which is wrong. In fact, it doesn't even return the same result on, say AArch64.

[yuyichao]

Ref 3696968 which was an incomplete fix with an incorrect explaination.

[KristofferC]

Did some archeology and found 3696968. Kinda funny how this comes back 6 years later.

[KristofferC]

Sniped...