Improve performance of generic dot products (#27678)

* improve performance of generic dot products

* update generic dot as suggested by @haampie

Author: ranocha

stdlib/LinearAlgebra/src/generic.jl

@@ -621,21 +621,6 @@ opnorm(v::TransposeAbsVec) = norm(v.parent)
 
 norm(v::Union{TransposeAbsVec,AdjointAbsVec}, p::Real) = norm(v.parent, p)
 
-function dot(x::AbstractArray, y::AbstractArray)
-    lx = _length(x)
-    if lx != _length(y)
-        throw(DimensionMismatch("first array has length $(lx) which does not match the length of the second, $(_length(y))."))
-    end
-    if lx == 0
-        return dot(zero(eltype(x)), zero(eltype(y)))
-    end
-    s = zero(dot(first(x), first(y)))
-    for (Ix, Iy) in zip(eachindex(x), eachindex(y))
-        @inbounds s += dot(x[Ix], y[Iy])
-    end
-    s
-end
-
 """
     dot(x, y)
     x ⋅ y
@@ -678,14 +663,13 @@ function dot(x, y) # arbitrary iterables
     while true
         ix = iterate(x, xs)
         iy = iterate(y, ys)
-        if (ix == nothing) || (iy == nothing)
-            break
-        end
+        ix === nothing && break
+        iy === nothing && break
         (vx, xs), (vy, ys) = ix, iy
         s += dot(vx, vy)
     end
-    if !(iy == nothing && ix == nothing)
-            throw(DimensionMismatch("x and y are of different lengths!"))
+    if !(iy === nothing && ix === nothing)
+        throw(DimensionMismatch("x and y are of different lengths!"))
     end
     return s
 end
@@ -709,24 +693,19 @@ julia> dot([im; im], [1; 1])
 0 - 2im
 ```
 """
-function dot(x::AbstractVector, y::AbstractVector)
-    if length(LinearIndices(x)) != length(LinearIndices(y))
-        throw(DimensionMismatch("dot product arguments have unequal lengths $(length(LinearIndices(x))) and $(length(LinearIndices(y)))"))
+function dot(x::AbstractArray, y::AbstractArray)
+    lx = _length(x)
+    if lx != _length(y)
+        throw(DimensionMismatch("first array has length $(lx) which does not match the length of the second, $(_length(y))."))
     end
-    ix = iterate(x)
-    if ix === nothing
-        # we only need to check the first vector, since equal lengths have been asserted
+    if lx == 0
         return dot(zero(eltype(x)), zero(eltype(y)))
     end
-    iy = iterate(y)
-    s = dot(ix[1], iy[1])
-    ix, iy = iterate(x, ix[2]), iterate(y, iy[2])
-    while ix != nothing
-        s += dot(ix[1], iy[1])
-        ix = iterate(x, ix[2])
-        iy = iterate(y, iy[2])
+    s = zero(dot(first(x), first(y)))
+    for (Ix, Iy) in zip(eachindex(x), eachindex(y))
+        @inbounds s += dot(x[Ix], y[Iy])
     end
-    return s
+    s
 end
 
 

stdlib/LinearAlgebra/test/matmul.jl

@@ -232,16 +232,18 @@ end
     @test dot(Z, Z) == convert(elty, 34.0)
 end
 
-dot_(x,y) = invoke(dot, Tuple{Any,Any}, x,y)
+dot1(x,y) = invoke(dot, Tuple{Any,Any}, x,y)
+dot2(x,y) = invoke(dot, Tuple{AbstractArray,AbstractArray}, x,y)
 @testset "generic dot" begin
     AA = [1+2im 3+4im; 5+6im 7+8im]
     BB = [2+7im 4+1im; 3+8im 6+5im]
     for A in (copy(AA), view(AA, 1:2, 1:2)), B in (copy(BB), view(BB, 1:2, 1:2))
-        @test dot(A,B) == dot(vec(A),vec(B)) == dot_(A,B) == dot(float.(A),float.(B))
-        @test dot(Int[], Int[]) == 0 == dot_(Int[], Int[])
+        @test dot(A,B) == dot(vec(A),vec(B)) == dot1(A,B) == dot2(A,B) == dot(float.(A),float.(B))
+        @test dot(Int[], Int[]) == 0 == dot1(Int[], Int[]) == dot2(Int[], Int[])
         @test_throws MethodError dot(Any[], Any[])
-        @test_throws MethodError dot_(Any[], Any[])
-        for n1 = 0:2, n2 = 0:2, d in (dot, dot_)
+        @test_throws MethodError dot1(Any[], Any[])
+        @test_throws MethodError dot2(Any[], Any[])
+        for n1 = 0:2, n2 = 0:2, d in (dot, dot1, dot2)
             if n1 != n2
                 @test_throws DimensionMismatch d(1:n1, 1:n2)
             else