Forward length to parent

[jishnub]

This might lead to simpler code, especially if the length of the parent is easy to compute without evaluating a product (e.g. for StaticArrays where it is known at compile time, and may be constant-propagated).

On nightly with this PR

julia> A = OffsetArray(reshape(SA[1,2,3,4],2,2), 3, 4)
2×2 OffsetArray(reshape(::SVector{4, Int64}, 2, 2), 4:5, 5:6) with eltype Int64 with indices 4:5×5:6:
 1  3
 2  4

julia> @code_typed length(A)
CodeInfo(
1 ─     return 4
) => Int64

[codecov]

Codecov Report

Merging #304 (b0cff2c) into master (9b018f0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #304   +/-   ##
=======================================
  Coverage   96.43%   96.44%           
=======================================
  Files           5        5           
  Lines         449      450    +1     
=======================================
+ Hits          433      434    +1     
  Misses         16       16           

Impacted Files	Coverage Δ
src/OffsetArrays.jl	98.29% <100.00%> (+<0.01%)	⬆️

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[johnnychen94]

I believe Julia compiler can optimize this well, but it's harmless to add this.

[jishnub]

Without this, I find the code_native to be

julia> @code_native debuginfo=:none length(A)
	.text
	.file	"length"
	.globl	julia_length_2616               # -- Begin function julia_length_2616
	.p2align	4, 0x90
	.type	julia_length_2616,@function
julia_length_2616:                      # @julia_length_2616
	.cfi_startproc
# %bb.0:                                # %top
	movq	40(%rdi), %rax
	imulq	32(%rdi), %rax
	retq
.Lfunc_end0:
	.size	julia_length_2616, .Lfunc_end0-julia_length_2616
	.cfi_endproc
                                        # -- End function
	.section	".note.GNU-stack","",@progbits

whereas with this

julia> @code_native debuginfo=:none length(A)
	.text
	.file	"length"
	.globl	julia_length_2618               # -- Begin function julia_length_2618
	.p2align	4, 0x90
	.type	julia_length_2618,@function
julia_length_2618:                      # @julia_length_2618
	.cfi_startproc
# %bb.0:                                # %top
	movl	$4, %eax
	retq
.Lfunc_end0:
	.size	julia_length_2618, .Lfunc_end0-julia_length_2618
	.cfi_endproc
                                        # -- End function
	.section	".note.GNU-stack","",@progbits

so it seems that the imulq is avoided. I'm uncertain if the compiler can do this by itself, maybe it can, but I guess this is harmless, as you say.