improve Base.summarysize for BigInt

[MHellmund]

julia> versioninfo()
Julia Version 1.7.2
Commit bf53498635 (2022-02-06 15:21 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i7-10875H CPU @ 2.30GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, skylake)

julia> b=big(17)^300;

julia> Base.summarysize(b)
16

According to the discussion https://discourse.julialang.org/t/how-do-i-get-the-memory-size-of-a-bigint/ ,
the b.alloc field contains the number of limbs allocated by GMP.

Base.summarysize(b) should return this:

julia> div(b.alloc * Base.GMP.BITS_PER_LIMB,   8)
184

[vtjnash]

PRs welcome

[MHellmund]

Just looked into the source of summarysize(). That's way over my head.

The current answer Base.summarysize(a_BigInt) =16 is the size of the Julia struct, isn't it? So let
me correct my first proposal to:

16 + b.alloc * sizeof(Base.GMP.Limb)

[vtjnash]

yes, you potentially want to invoke the current method, then add the internal (malloc) memory

[skleinbo]

Like this (ss::SummarySize)(obj::BigInt) = _summarysize(ss, obj) + obj.alloc*sizeof(Base.GMP.Limb) ?