diff --git a/base/Base.jl b/base/Base.jl
index 10a8dd1532f926..23633f0b5138bb 100644
--- a/base/Base.jl
+++ b/base/Base.jl
@@ -306,7 +306,6 @@ end
 include("hashing.jl")
 include("rounding.jl")
 include("div.jl")
-include("rawbigints.jl")
 include("float.jl")
 include("twiceprecision.jl")
 include("complex.jl")
diff --git a/base/mpfr.jl b/base/mpfr.jl
index d393469aa26a17..bea1bd4862d2fd 100644
--- a/base/mpfr.jl
+++ b/base/mpfr.jl
@@ -18,12 +18,10 @@ import
         setrounding, maxintfloat, widen, significand, frexp, tryparse, iszero,
         isone, big, _string_n, decompose, minmax, _precision_with_base_2,
         sinpi, cospi, sincospi, tanpi, sind, cosd, tand, asind, acosd, atand,
-        uinttype, exponent_max, exponent_min, ieee754_representation, significand_mask,
-        RawBigIntRoundingIncrementHelper, truncated, RawBigInt
-
+        uinttype, exponent_max, exponent_min, ieee754_representation, significand_mask
 
 using .Base.Libc
-import ..Rounding:
+import ..Rounding: Rounding,
     rounding_raw, setrounding_raw, rounds_to_nearest, rounds_away_from_zero,
     tie_breaker_is_to_even, correct_rounding_requires_increment
 
@@ -39,7 +37,6 @@ else
     const libmpfr = "libmpfr.so.6"
 end
 
-
 version() = VersionNumber(unsafe_string(ccall((:mpfr_get_version,libmpfr), Ptr{Cchar}, ())))
 patches() = split(unsafe_string(ccall((:mpfr_get_patches,libmpfr), Ptr{Cchar}, ())),' ')
 
@@ -120,44 +117,111 @@ const mpfr_special_exponent_zero = typemin(Clong) + true
 const mpfr_special_exponent_nan = mpfr_special_exponent_zero + true
 const mpfr_special_exponent_inf = mpfr_special_exponent_nan + true
 
+struct BigFloatLayout
+    prec::Clong
+    sign::Cint
+    exp::Clong
+    d::Ptr{Limb}
+    p::Limb # Tuple{Vararg{Limb}}
+end
+const offset_prec = fieldoffset(BigFloatLayout, 1)
+const offset_sign = fieldoffset(BigFloatLayout, 2)
+const offset_exp = fieldoffset(BigFloatLayout, 3)
+const offset_d = fieldoffset(BigFloatLayout, 4)
+const offset_p = fieldoffset(BigFloatLayout, 5)
+const offset_p_limbs = (offset_p ÷ sizeof(Limb)) % Int
+
 """
     BigFloat <: AbstractFloat
 
 Arbitrary precision floating point number type.
 """
-mutable struct BigFloat <: AbstractFloat
-    prec::Clong
-    sign::Cint
-    exp::Clong
-    d::Ptr{Limb}
-    # _d::Buffer{Limb} # Julia gc handle for memory @ d
-    _d::String # Julia gc handle for memory @ d (optimized)
+struct BigFloat <: AbstractFloat
+    d::Memory{Limb}
 
     # Not recommended for general use:
     # used internally by, e.g. deepcopy
-    global function _BigFloat(prec::Clong, sign::Cint, exp::Clong, d::String)
-        # ccall-based version, inlined below
-        #z = new(zero(Clong), zero(Cint), zero(Clong), C_NULL, d)
-        #ccall((:mpfr_custom_init,libmpfr), Cvoid, (Ptr{Limb}, Clong), d, prec) # currently seems to be a no-op in mpfr
-        #NAN_KIND = Cint(0)
-        #ccall((:mpfr_custom_init_set,libmpfr), Cvoid, (Ref{BigFloat}, Cint, Clong, Ptr{Limb}), z, NAN_KIND, prec, d)
-        #return z
-        return new(prec, sign, exp, pointer(d), d)
-    end
+    global _BigFloat(d::Memory{Limb}) = new(d)
 
     function BigFloat(; precision::Integer=_precision_with_base_2(BigFloat))
         precision < 1 && throw(DomainError(precision, "`precision` cannot be less than 1."))
         nb = ccall((:mpfr_custom_get_size,libmpfr), Csize_t, (Clong,), precision)
-        nb = (nb + Core.sizeof(Limb) - 1) ÷ Core.sizeof(Limb) # align to number of Limb allocations required for this
-        #d = Vector{Limb}(undef, nb)
-        d = _string_n(nb * Core.sizeof(Limb))
-        EXP_NAN = mpfr_special_exponent_nan
-        return _BigFloat(Clong(precision), one(Cint), EXP_NAN, d) # +NAN
+        nl = (nb + Core.sizeof(BigFloatLayout) - 1) ÷ Core.sizeof(Limb) # align to number of Limb allocations required for this
+        d = Memory{Limb}(undef, nl % Int)
+        # ccall-based version, inlined below
+        z = _BigFloat(d) # initialize to +NAN
+        #ccall((:mpfr_custom_init,libmpfr), Cvoid, (Ptr{Limb}, Clong), BigFloatData(d), prec) # currently seems to be a no-op in mpfr
+        #NAN_KIND = Cint(0)
+        #ccall((:mpfr_custom_init_set,libmpfr), Cvoid, (Ref{BigFloat}, Cint, Clong, Ptr{Limb}), z, NAN_KIND, prec, BigFloatData(d))
+        z.prec = Clong(precision)
+        z.sign = one(Cint)
+        z.exp = mpfr_special_exponent_nan
+        return z
     end
 end
 
-# The rounding mode here shouldn't matter.
-significand_limb_count(x::BigFloat) = div(sizeof(x._d), sizeof(Limb), RoundToZero)
+"""
+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 BigFloatData{Limb}
+    d::Memory{Limb}
+end
+
+# BigFloat interface
+@inline function Base.getproperty(x::BigFloat, s::Symbol)
+    d = getfield(x, :d)
+    p = Base.unsafe_convert(Ptr{Limb}, d)
+    if s === :prec
+        return GC.@preserve d unsafe_load(Ptr{Clong}(p) + offset_prec)
+    elseif s === :sign
+        return GC.@preserve d unsafe_load(Ptr{Cint}(p) + offset_sign)
+    elseif s === :exp
+        return GC.@preserve d unsafe_load(Ptr{Clong}(p) + offset_exp)
+    elseif s === :d
+        return BigFloatData(d)
+    else
+        return throw(FieldError(typeof(x), s))
+    end
+end
+
+@inline function Base.setproperty!(x::BigFloat, s::Symbol, v)
+    d = getfield(x, :d)
+    p = Base.unsafe_convert(Ptr{Limb}, d)
+    if s === :prec
+        return GC.@preserve d unsafe_store!(Ptr{Clong}(p) + offset_prec, v)
+    elseif s === :sign
+        return GC.@preserve d unsafe_store!(Ptr{Cint}(p) + offset_sign, v)
+    elseif s === :exp
+        return GC.@preserve d unsafe_store!(Ptr{Clong}(p) + offset_exp, v)
+    #elseif s === :d # not mutable
+    else
+        return throw(FieldError(x, s))
+    end
+end
+
+# Ref interface: make sure the conversion to C is done properly
+Base.unsafe_convert(::Type{Ref{BigFloat}}, x::Ptr{BigFloat}) = error("not compatible with mpfr")
+Base.unsafe_convert(::Type{Ref{BigFloat}}, x::Ref{BigFloat}) = error("not compatible with mpfr")
+Base.cconvert(::Type{Ref{BigFloat}}, x::BigFloat) = x.d # BigFloatData is the Ref type for BigFloat
+function Base.unsafe_convert(::Type{Ref{BigFloat}}, x::BigFloatData)
+    d = getfield(x, :d)
+    p = Base.unsafe_convert(Ptr{Limb}, d)
+    GC.@preserve d unsafe_store!(Ptr{Ptr{Limb}}(p) + offset_d, p + offset_p, :monotonic) # :monotonic ensure that TSAN knows that this isn't a data race
+    return Ptr{BigFloat}(p)
+end
+Base.unsafe_convert(::Type{Ptr{Limb}}, fd::BigFloatData) = Base.unsafe_convert(Ptr{Limb}, getfield(fd, :d)) + offset_p
+function Base.setindex!(fd::BigFloatData, v, i)
+    getfield(fd, :d)[i + offset_p_limbs] = v
+    return fd
+end
+function Base.getindex(fd::BigFloatData, i)
+    return getfield(fd, :d)[i + offset_p_limbs]
+end
+Base.length(fd::BigFloatData) = length(getfield(fd, :d)) - offset_p_limbs
+Base.copyto!(fd::BigFloatData, limbs) = copyto!(getfield(fd, :d), offset_p_limbs + 1, limbs) # for Random
+
+include("rawbigfloats.jl")
 
 rounding_raw(::Type{BigFloat}) = something(Base.ScopedValues.get(CURRENT_ROUNDING_MODE), ROUNDING_MODE[])
 setrounding_raw(::Type{BigFloat}, r::MPFRRoundingMode) = ROUNDING_MODE[]=r
@@ -165,24 +229,12 @@ function setrounding_raw(f::Function, ::Type{BigFloat}, r::MPFRRoundingMode)
     Base.ScopedValues.@with(CURRENT_ROUNDING_MODE => r, f())
 end
 
-
 rounding(::Type{BigFloat}) = convert(RoundingMode, rounding_raw(BigFloat))
 setrounding(::Type{BigFloat}, r::RoundingMode) = setrounding_raw(BigFloat, convert(MPFRRoundingMode, r))
 setrounding(f::Function, ::Type{BigFloat}, r::RoundingMode) =
     setrounding_raw(f, BigFloat, convert(MPFRRoundingMode, r))
 
 
-# overload the definition of unsafe_convert to ensure that `x.d` is assigned
-# it may have been dropped in the event that the BigFloat was serialized
-Base.unsafe_convert(::Type{Ref{BigFloat}}, x::Ptr{BigFloat}) = x
-@inline function Base.unsafe_convert(::Type{Ref{BigFloat}}, x::Ref{BigFloat})
-    x = x[]
-    if x.d == C_NULL
-        x.d = pointer(x._d)
-    end
-    return convert(Ptr{BigFloat}, Base.pointer_from_objref(x))
-end
-
 """
     BigFloat(x::Union{Real, AbstractString} [, rounding::RoundingMode=rounding(BigFloat)]; [precision::Integer=precision(BigFloat)])
 
@@ -283,17 +335,18 @@ function BigFloat(x::Float64, r::MPFRRoundingMode=rounding_raw(BigFloat); precis
     nlimbs = (precision + 8*Core.sizeof(Limb) - 1) ÷ (8*Core.sizeof(Limb))
 
     # Limb is a CLong which is a UInt32 on windows (thank M$) which makes this more complicated and slower.
+    zd = z.d
     if Limb === UInt64
         for i in 1:nlimbs-1
-            unsafe_store!(z.d, 0x0, i)
+            setindex!(zd, 0x0, i)
         end
-        unsafe_store!(z.d, val, nlimbs)
+        setindex!(zd, val, nlimbs)
     else
         for i in 1:nlimbs-2
-            unsafe_store!(z.d, 0x0, i)
+            setindex!(zd, 0x0, i)
         end
-        unsafe_store!(z.d, val % UInt32, nlimbs-1)
-        unsafe_store!(z.d, (val >> 32) % UInt32, nlimbs)
+        setindex!(zd, val % UInt32, nlimbs-1)
+        setindex!(zd, (val >> 32) % UInt32, nlimbs)
     end
     z
 end
@@ -440,12 +493,12 @@ function to_ieee754(::Type{T}, x::BigFloat, rm) where {T<:AbstractFloat}
     ret_u = if is_regular & !rounds_to_inf & !rounds_to_zero
         if !exp_is_huge_p
             # significand
-            v = RawBigInt{Limb}(x._d, significand_limb_count(x))
+            v = x.d::BigFloatData
             len = max(ieee_precision + min(exp_diff, 0), 0)::Int
             signif = truncated(U, v, len) & significand_mask(T)
 
             # round up if necessary
-            rh = RawBigIntRoundingIncrementHelper(v, len)
+            rh = BigFloatDataRoundingIncrementHelper(v, len)
             incr = correct_rounding_requires_increment(rh, rm, sb)
 
             # exponent
@@ -1193,10 +1246,8 @@ set_emin!(x) = check_exponent_err(ccall((:mpfr_set_emin, libmpfr), Cint, (Clong,
 
 function Base.deepcopy_internal(x::BigFloat, stackdict::IdDict)
     get!(stackdict, x) do
-        # d = copy(x._d)
-        d = x._d
-        d′ = GC.@preserve d unsafe_string(pointer(d), sizeof(d)) # creates a definitely-new String
-        y = _BigFloat(x.prec, x.sign, x.exp, d′)
+        d′ = copy(getfield(x, :d))
+        y = _BigFloat(d′)
         #ccall((:mpfr_custom_move,libmpfr), Cvoid, (Ref{BigFloat}, Ptr{Limb}), y, d) # unnecessary
         return y
     end::BigFloat
@@ -1210,7 +1261,8 @@ function decompose(x::BigFloat)::Tuple{BigInt, Int, Int}
     s.size = cld(x.prec, 8*sizeof(Limb)) # limbs
     b = s.size * sizeof(Limb)            # bytes
     ccall((:__gmpz_realloc2, libgmp), Cvoid, (Ref{BigInt}, Culong), s, 8b) # bits
-    memcpy(s.d, x.d, b)
+    xd = x.d
+    GC.@preserve xd memcpy(s.d, Base.unsafe_convert(Ptr{Limb}, xd), b)
     s, x.exp - 8b, x.sign
 end
 
diff --git a/base/rawbigints.jl b/base/rawbigfloats.jl
similarity index 59%
rename from base/rawbigints.jl
rename to base/rawbigfloats.jl
index 6508bea05be0ff..6a4523af2bb9ba 100644
--- a/base/rawbigints.jl
+++ b/base/rawbigfloats.jl
@@ -1,34 +1,21 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
-"""
-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::String
-    word_count::Int
-
-    function RawBigInt{T}(d::String, word_count::Int) where {T<:Unsigned}
-        new{T}(d, word_count)
-    end
-end
+# Some operations on BigFloat can be done more directly by treating the data portion ("BigFloatData") as a BigInt
 
-elem_count(x::RawBigInt, ::Val{:words}) = x.word_count
+elem_count(x::BigFloatData, ::Val{:words}) = length(x)
 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))
+word_length(::BigFloatData{T}) where {T} = elem_count(zero(T), Val(:bits))
+elem_count(x::BigFloatData{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)
+split_bit_index(x::BigFloatData, i::Int) = divrem(i, word_length(x), RoundToZero)
 
 """
 `i` is the zero-based index of the wanted word in `x`, starting from
 the less significant words.
 """
-function get_elem(x::RawBigInt{T}, i::Int, ::Val{:words}, ::Val{:ascending}) where {T}
-    # `i` must be non-negative and less than `x.word_count`
-    d = x.d
-    (GC.@preserve d unsafe_load(Ptr{T}(pointer(d)), i + 1))::T
+Base.@propagate_inbounds function get_elem(x::BigFloatData{T}, i::Int, ::Val{:words}, ::Val{:ascending}) where {T}
+    return x[i + 1]::T
 end
 
 function get_elem(x, i::Int, v::Val, ::Val{:descending})
@@ -36,9 +23,9 @@ function get_elem(x, i::Int, v::Val, ::Val{:descending})
     get_elem(x, j, v, Val(:ascending))
 end
 
-word_is_nonzero(x::RawBigInt, i::Int, v::Val) = !iszero(get_elem(x, i, Val(:words), v))
+word_is_nonzero(x::BigFloatData, i::Int, v::Val) = !iszero(get_elem(x, i, Val(:words), v))
 
-word_is_nonzero(x::RawBigInt, v::Val) = let x = x
+word_is_nonzero(x::BigFloatData, v::Val) = let x = x
     i -> word_is_nonzero(x, i, v)
 end
 
@@ -46,7 +33,7 @@ end
 Returns a `Bool` indicating whether the `len` least significant words
 of `x` are nonzero.
 """
-function tail_is_nonzero(x::RawBigInt, len::Int, ::Val{:words})
+function tail_is_nonzero(x::BigFloatData, len::Int, ::Val{:words})
     any(word_is_nonzero(x, Val(:ascending)), 0:(len - 1))
 end
 
@@ -54,7 +41,7 @@ end
 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})
+function tail_is_nonzero(x::BigFloatData, len::Int, i::Int, ::Val{:word})
     !iszero(len) &&
     !iszero(get_elem(x, i, Val(:words), Val(:ascending)) << (word_length(x) - len))
 end
@@ -63,7 +50,7 @@ end
 Returns a `Bool` indicating whether the `len` least significant bits of
 `x` are nonzero.
 """
-function tail_is_nonzero(x::RawBigInt, len::Int, ::Val{:bits})
+function tail_is_nonzero(x::BigFloatData, 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)) ||
@@ -83,7 +70,7 @@ end
 """
 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})
+function get_elem(x::BigFloatData, 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)
@@ -98,7 +85,7 @@ end
 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}
+function truncated(::Type{R}, x::BigFloatData, len::Int) where {R<:Integer}
     ret = zero(R)
     if 0 < len
         word_count, bit_count_in_word = split_bit_index(x, len)
@@ -120,14 +107,14 @@ function truncated(::Type{R}, x::RawBigInt, len::Int) where {R<:Integer}
     ret::R
 end
 
-struct RawBigIntRoundingIncrementHelper{T<:Unsigned}
-    n::RawBigInt{T}
+struct BigFloatDataRoundingIncrementHelper{T<:Unsigned}
+    n::BigFloatData{T}
     trunc_len::Int
 
     final_bit::Bool
     round_bit::Bool
 
-    function RawBigIntRoundingIncrementHelper{T}(n::RawBigInt{T}, len::Int) where {T<:Unsigned}
+    function BigFloatDataRoundingIncrementHelper{T}(n::BigFloatData{T}, len::Int) where {T<:Unsigned}
         vals = (Val(:bits), Val(:descending))
         f = get_elem(n, len - 1, vals...)
         r = get_elem(n, len    , vals...)
@@ -135,15 +122,15 @@ struct RawBigIntRoundingIncrementHelper{T<:Unsigned}
     end
 end
 
-function RawBigIntRoundingIncrementHelper(n::RawBigInt{T}, len::Int) where {T<:Unsigned}
-    RawBigIntRoundingIncrementHelper{T}(n, len)
+function BigFloatDataRoundingIncrementHelper(n::BigFloatData{T}, len::Int) where {T<:Unsigned}
+    BigFloatDataRoundingIncrementHelper{T}(n, len)
 end
 
-(h::RawBigIntRoundingIncrementHelper)(::Rounding.FinalBit) = h.final_bit
+(h::BigFloatDataRoundingIncrementHelper)(::Rounding.FinalBit) = h.final_bit
 
-(h::RawBigIntRoundingIncrementHelper)(::Rounding.RoundBit) = h.round_bit
+(h::BigFloatDataRoundingIncrementHelper)(::Rounding.RoundBit) = h.round_bit
 
-function (h::RawBigIntRoundingIncrementHelper)(::Rounding.StickyBit)
+function (h::BigFloatDataRoundingIncrementHelper)(::Rounding.StickyBit)
     v = Val(:bits)
     n = h.n
     tail_is_nonzero(n, elem_count(n, v) - h.trunc_len - 1, v)
diff --git a/stdlib/Random/src/generation.jl b/stdlib/Random/src/generation.jl
index d8bb48d2764d21..b605dff9e5d803 100644
--- a/stdlib/Random/src/generation.jl
+++ b/stdlib/Random/src/generation.jl
@@ -66,7 +66,7 @@ function _rand!(rng::AbstractRNG, z::BigFloat, sp::SamplerBigFloat)
         limbs[end] |= Limb_high_bit
     end
     z.sign = 1
-    GC.@preserve limbs unsafe_copyto!(z.d, pointer(limbs), sp.nlimbs)
+    copyto!(z.d, limbs)
     randbool
 end
 
diff --git a/test/dict.jl b/test/dict.jl
index 13c60d5a6a053e..909afb3607907f 100644
--- a/test/dict.jl
+++ b/test/dict.jl
@@ -1049,7 +1049,7 @@ Dict(1 => rand(2,3), 'c' => "asdf") # just make sure this does not trigger a dep
 
     # issue #26939
     d26939 = WeakKeyDict()
-    (@noinline d -> d[big"1.0" + 1.1] = 1)(d26939)
+    (@noinline d -> d[big"1" + 1] = 1)(d26939)
     GC.gc() # primarily to make sure this doesn't segfault
     @test count(d26939) == 0
     @test length(d26939.ht) == 1