From fa7e79c4df60e4184f452f6404b085d6b7f4cb9b Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Thu, 10 Feb 2022 07:57:50 -0500
Subject: [PATCH] use Val(dims) and Iterators.peel for _dim_stack

---
 base/abstractarray.jl | 49 ++++++++++++++++++++-----------------------
 test/abstractarray.jl | 14 +++++++++++++
 2 files changed, 37 insertions(+), 26 deletions(-)

diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index 932beb2a268fc4..42753a44ef98dc 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -2662,44 +2662,41 @@ _typed_stack(dims::Integer, ::Type{T}, ::Type{S}, ::IteratorSize, A) where {T,S}
 _vec_axis(A, ax=_axes(A)) = length(ax) == 1 ? only(ax) : OneTo(prod(length, ax; init=1))
 
 function _dim_stack(dims::Integer, ::Type{T}, ::Type{S}, A) where {T,S}
-    xit = iterate(A)
+    xit = Iterators.peel(A)
     nothing === xit && return _empty_stack(dims, T, S, A)
-    x1, _ = xit
+    x1, xrest = xit
     ax1 = _axes(x1)
     N1 = length(ax1)+1
     dims in 1:N1 || throw(ArgumentError("cannot stack slices ndims(x) = $(N1-1) along dims = $dims"))
 
     newaxis = _vec_axis(A)
-    outax = ntuple(d -> d==dims ? newaxis : _axes(x1)[d - (d>dims)], N1)
+    outax = ntuple(d -> d==dims ? newaxis : ax1[d - (d>dims)], N1)
     B = similar(_first_array(x1, A), T, outax...)
 
-    iit = iterate(newaxis)
-    while xit !== nothing
-        x, state = xit
-        i, istate = iit
-        _stack_size_check(x, ax1)
-        @inbounds if dims==1
-            inds1 = ntuple(d -> d==1 ? i : Colon(), N1)
-            if x isa AbstractArray
-                B[inds1...] = x
-            else
-                copyto!(view(B, inds1...), x)
-            end
-        else
-            inds = ntuple(d -> d==dims ? i : Colon(), N1)
-                if x isa AbstractArray
-                B[inds...] = x
-            else
-                # This is where the type-instability of inds hurts, but it is pretty exotic:
-                copyto!(view(B, inds...), x)
-            end
-        end
-        xit = iterate(A, state)
-        iit = iterate(newaxis, istate)
+    if dims == 1
+        _dim_stack!(Val(1), B, x1, xrest)
+    elseif dims == 2
+        _dim_stack!(Val(2), B, x1, xrest)
+    else
+        _dim_stack!(Val(dims), B, x1, xrest)
     end
     B
 end
 
+function _dim_stack!(::Val{dims}, B::AbstractArray, x1, xrest) where {dims}
+    before = ntuple(d -> Colon(), dims - 1)
+    after = ntuple(d -> Colon(), ndims(B) - dims)
+
+    i = firstindex(B, dims)
+    copyto!(view(B, before..., i, after...), x1)
+
+    for x in xrest
+        _stack_size_check(x, _axes(x1))
+        i += 1
+        @inbounds copyto!(view(B, before..., i, after...), x)
+    end
+end
+
 @inline function _stack_size_check(x, ax1::Tuple)
     if _axes(x) != ax1
         uax1 = UnitRange.(ax1)
diff --git a/test/abstractarray.jl b/test/abstractarray.jl
index fef44da4628ef5..b0dd7407feb299 100644
--- a/test/abstractarray.jl
+++ b/test/abstractarray.jl
@@ -1606,6 +1606,20 @@ end
     # Trivial, because numbers are iterable:
     @test stack(abs2, 1:3) == [1, 4, 9] == collect(Iterators.flatten(abs2(x) for x in 1:3))
 
+    # Allocation tests
+    xv = [rand(10) for _ in 1:100]
+    xt = Tuple.(xv)
+    for dims in (1, 2, :)
+        @test stack(xv; dims) == stack(xt; dims)
+        @test 9000 > @allocated stack(xv; dims)
+        @test 9000 > @allocated stack(xt; dims)
+    end
+    xr = (reshape(1:1000,10,10,10) for _ = 1:1000)
+    for dims in (1, 2, 3, :)
+        stack(xr; dims)
+        @test 8.1e6 > @allocated stack(xr; dims)
+    end
+
     # Mismatched sizes
     @test_throws DimensionMismatch stack([1:2, 1:3])
     @test_throws DimensionMismatch stack([1:2, 1:3]; dims=1)