Add unit-stride fast path to _mapreduce_kernel!

src/mapreduce.jl

@@ -340,6 +340,17 @@ function _mapreduce_kernel_expr(f, op, initop, N::Int, M::Int)
         outerreturnstrideex[i] = returnex
     end
 
+    # Unit-stride fast path for the innermost (vectorized) loop dimension.
+    # We special-case for contiguous loads/stores to avoid SIMD gather/scatter
+    # in favor of SIMD load/store, which streams memory more efficiently.
+    unitstep1ex = :($(Ivars[1]) += 1)
+    unitstep2ex = Expr(:block)
+    for j in 2:M
+        push!(unitstep2ex.args, :($(Ivars[j]) += 1))
+    end
+    firststrides = Expr(:tuple, (stridevars[1, j] for j in 1:M)...)
+    unitstridecond = :(all(==(1), $firststrides))
+
     if op == Nothing
         ex = Expr(:(=), lhsex, fcallex)
         exa = Expr(:(=), :a, fcallex)
@@ -358,6 +369,14 @@ function _mapreduce_kernel_expr(f, op, initop, N::Int, M::Int)
                 end
                 $lhsex = a
                 $(returnstride2ex[i])
+            elseif $unitstridecond
+                @simd for $(innerloopvars[i]) in Base.OneTo($(blockdimvars[i]))
+                    $ex
+                    $unitstep1ex
+                    $unitstep2ex
+                end
+                $(returnstride1ex[i])
+                $(returnstride2ex[i])
             else
                 @simd for $(innerloopvars[i]) in Base.OneTo($(blockdimvars[i]))
                     $ex