exact BigFloat to IEEE FP conversion in pure Julia
#50691
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| Original file line number | Diff line number | Diff line change |
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| # This file is a part of Julia. License is MIT: https://julialang.org/license | ||
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| """ | ||
| Segment of raw words of bits interpreted as a big integer. Less | ||
| significant words come first. Each word is in machine-native bit-order. | ||
| """ | ||
| struct RawBigInt{T<:Unsigned} | ||
| d::Ptr{T} | ||
| word_count::Int | ||
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| function RawBigInt{T}(d::Ptr{T}, word_count::Int) where {T<:Unsigned} | ||
| new{T}(d, word_count) | ||
| end | ||
| end | ||
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| RawBigInt(d::Ptr{T}, word_count::Int) where {T<:Unsigned} = RawBigInt{T}(d, word_count) | ||
| elem_count(x::RawBigInt, ::Val{:words}) = x.word_count | ||
| elem_count(x::Unsigned, ::Val{:bits}) = sizeof(x) * 8 | ||
| word_length(::RawBigInt{T}) where {T} = elem_count(zero(T), Val(:bits)) | ||
| elem_count(x::RawBigInt{T}, ::Val{:bits}) where {T} = word_length(x) * elem_count(x, Val(:words)) | ||
| reversed_index(n::Int, i::Int) = n - i - 1 | ||
| reversed_index(x, i::Int, v::Val) = reversed_index(elem_count(x, v), i)::Int | ||
| split_bit_index(x::RawBigInt, i::Int) = divrem(i, word_length(x), RoundToZero) | ||
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| """ | ||
| `i` is the zero-based index of the wanted word in `x`, starting from | ||
| the less significant words. | ||
| """ | ||
| function get_elem(x::RawBigInt, i::Int, ::Val{:words}, ::Val{:ascending}) | ||
| unsafe_load(x.d, i + 1) | ||
| end | ||
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| function get_elem(x, i::Int, v::Val, ::Val{:descending}) | ||
| j = reversed_index(x, i, v) | ||
| get_elem(x, j, v, Val(:ascending)) | ||
| end | ||
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| word_is_nonzero(x::RawBigInt, i::Int, v::Val) = !iszero(get_elem(x, i, Val(:words), v)) | ||
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| word_is_nonzero(x::RawBigInt, v::Val) = let x = x | ||
| i -> word_is_nonzero(x, i, v) | ||
| end | ||
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| """ | ||
| Returns a `Bool` indicating whether the `len` least significant words | ||
| of `x` are nonzero. | ||
| """ | ||
| function tail_is_nonzero(x::RawBigInt, len::Int, ::Val{:words}) | ||
| any(word_is_nonzero(x, Val(:ascending)), 0:(len - 1)) | ||
| end | ||
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| """ | ||
| Returns a `Bool` indicating whether the `len` least significant bits of | ||
| the `i`-th (zero-based index) word of `x` are nonzero. | ||
| """ | ||
| function tail_is_nonzero(x::RawBigInt, len::Int, i::Int, ::Val{:word}) | ||
| !iszero(len) && | ||
| !iszero(get_elem(x, i, Val(:words), Val(:ascending)) << (word_length(x) - len)) | ||
| end | ||
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| """ | ||
| Returns a `Bool` indicating whether the `len` least significant bits of | ||
| `x` are nonzero. | ||
| """ | ||
| function tail_is_nonzero(x::RawBigInt, len::Int, ::Val{:bits}) | ||
| if 0 < len | ||
| word_count, bit_count_in_word = split_bit_index(x, len) | ||
| tail_is_nonzero(x, bit_count_in_word, word_count, Val(:word)) || | ||
| tail_is_nonzero(x, word_count, Val(:words)) | ||
| else | ||
| false | ||
| end::Bool | ||
| end | ||
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| """ | ||
| Returns a `Bool` that is the `i`-th (zero-based index) bit of `x`. | ||
| """ | ||
| function get_elem(x::Unsigned, i::Int, ::Val{:bits}, ::Val{:ascending}) | ||
| (x >>> i) % Bool | ||
| end | ||
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| """ | ||
| Returns a `Bool` that is the `i`-th (zero-based index) bit of `x`. | ||
| """ | ||
| function get_elem(x::RawBigInt, i::Int, ::Val{:bits}, v::Val{:ascending}) | ||
| vb = Val(:bits) | ||
| if 0 ≤ i < elem_count(x, vb) | ||
| word_index, bit_index_in_word = split_bit_index(x, i) | ||
| word = get_elem(x, word_index, Val(:words), v) | ||
| get_elem(word, bit_index_in_word, vb, v) | ||
| else | ||
| false | ||
| end::Bool | ||
| end | ||
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| """ | ||
| Returns an integer of type `R`, consisting of the `len` most | ||
| significant bits of `x`. | ||
| """ | ||
| function truncated(::Type{R}, x::RawBigInt, len::Int) where {R<:Integer} | ||
| ret = zero(R) | ||
| if 0 < len | ||
| word_count, bit_count_in_word = split_bit_index(x, len) | ||
| k = word_length(x) | ||
| vals = (Val(:words), Val(:descending)) | ||
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| for w ∈ 0:(word_count - 1) | ||
| ret <<= k | ||
| word = get_elem(x, w, vals...) | ||
| ret |= R(word) | ||
| end | ||
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| if !iszero(bit_count_in_word) | ||
| ret <<= bit_count_in_word | ||
| wrd = get_elem(x, word_count, vals...) | ||
| ret |= R(wrd >>> (k - bit_count_in_word)) | ||
| end | ||
| end | ||
| ret::R | ||
| end | ||
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| struct RawBigIntRoundingIncrementHelper{T<:Unsigned} | ||
| n::RawBigInt{T} | ||
| trunc_len::Int | ||
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| final_bit::Bool | ||
| round_bit::Bool | ||
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| function RawBigIntRoundingIncrementHelper{T}(n::RawBigInt{T}, len::Int) where {T<:Unsigned} | ||
| vals = (Val(:bits), Val(:descending)) | ||
| f = get_elem(n, len - 1, vals...) | ||
| r = get_elem(n, len , vals...) | ||
| new{T}(n, len, f, r) | ||
| end | ||
| end | ||
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| function RawBigIntRoundingIncrementHelper(n::RawBigInt{T}, len::Int) where {T<:Unsigned} | ||
| RawBigIntRoundingIncrementHelper{T}(n, len) | ||
| end | ||
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| (h::RawBigIntRoundingIncrementHelper)(::Rounding.FinalBit) = h.final_bit | ||
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| (h::RawBigIntRoundingIncrementHelper)(::Rounding.RoundBit) = h.round_bit | ||
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| function (h::RawBigIntRoundingIncrementHelper)(::Rounding.StickyBit) | ||
| v = Val(:bits) | ||
| n = h.n | ||
| tail_is_nonzero(n, elem_count(n, v) - h.trunc_len - 1, v) | ||
| end |
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Do these functions need to exist?
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You mean why is
exponent_minnecessary in addition toieee754_exponent_min?The idea was to have
exponent_minbe consistent with functions likeexponent_max, in that it only has methods that are strictly required. To be honest, I don't really understand the significance of that, I was just trying to follow the discussion in #50654, the "incorrectly switches the position / ordering ofUnionandType" part, in particular.Apart from that the formula in the definition of
ieee754_exponent_minis defined in the IEEE 754 standard, and it's only known to be valid for IEEE 754 floating-point. So theoretically we could have some non-IEEE floating-point types with different definitions ofexponent_min.