getfield for Node ptr

[chris-b1]

This is the same issue/approach as #142

I'm doing some profiling on XLSX.jl and this picks up a nice little win

# master
julia> @btime XLSX.readtable(f, "Sheet1")
  341.240 ms (3018190 allocations: 98.87 MiB

# PR
julia> @btime XLSX.readtable(f, "Sheet1")
  284.024 ms (1940230 allocations: 54.22 MiB)

[codecov]

Codecov Report

Merging #144 into master will decrease coverage by 0.24%.
The diff coverage is 88.57%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
- Coverage   93.55%   93.30%   -0.25%     
==========================================
  Files           7        7              
  Lines         760      762       +2     
==========================================
  Hits          711      711              
- Misses         49       51       +2     

Impacted Files	Coverage Δ
src/node.jl	93.46% <88.57%> (-0.48%)	⬇️

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[chris-b1]

gentle ping @bicycle1885 - any changes you'd like to see here?

[aminya]

@chris-b1 He does maintain the package on GitHub. You should email him in person and let him know that there is a pull request. I wish we had moved this repository to an organization to let others continue developing the package.

[bicycle1885]

@chris-b1 I appreciate your suggestion. However, I discovered this will be fixed by the improved compiler and what we need to do is just waiting for the next release of Julia (i.e., Julia 1.6). For example, lots of instructions related to getproperty in Julia 1.5.3 are optimized out in the latest nightly build of Julia as follows:

julia> code_native(hasparentnode, (EzXML.Node,))
        .text
; ┌ @ node.jl:616 within `hasparentnode'
; │┌ @ node.jl:398 within `getproperty'
        movq    (%rdi), %rax
; │└
; │┌ @ operators.jl:204 within `!='
; ││┌ @ pointer.jl:155 within `==' @ promotion.jl:410
        cmpq    $0, 40(%rax)
        setne   %al
; │└└
        retq
        nopl    (%rax)
; └

julia> versioninfo()
Julia Version 1.7.0-DEV.6
Commit 5f1b21b357* (2020-12-09 16:06 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: AMD Ryzen 9 3950X 16-Core Processor
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.0 (ORCJIT, znver2)
Environment:
  JULIA_PROJECT = @.

julia> code_native(hasparentnode, (EzXML.Node,))
        .text
; ┌ @ node.jl:615 within `hasparentnode'
        pushq   %r15
        pushq   %r14
        pushq   %rbx
        subq    $48, %rsp
        vxorps  %xmm0, %xmm0, %xmm0
        movq    $0, 32(%rsp)
        movq    %fs:0, %rbx
        leaq    16(%rsp), %rdi
        movq    %rsp, %r14
        movabsq $140301301986384, %rcx  # imm = 0x7F9A71443450
; │ @ node.jl:616 within `hasparentnode'

... and many instructions!

julia> versioninfo()
Julia Version 1.5.3
Commit 788b2c77c1 (2020-11-09 13:37 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: AMD Ryzen 9 3950X 16-Core Processor
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, znver2)
Environment:
  JULIA_EDITOR = "/usr/share/code/code"
  JULIA_NUM_THREADS = 

Given that the current performance is not devastating and the suggested change affects many lines, I think it would be better to stay and wait for Julia 1.6.