diff --git a/src/FLoops.jl b/src/FLoops.jl
index 4358b886c..3a12572c9 100644
--- a/src/FLoops.jl
+++ b/src/FLoops.jl
@@ -20,7 +20,7 @@ module FLoops
     doc
 end FLoops
 
-export @floop, @init, @reduce, DistributedEx, SequentialEx, ThreadedEx
+export @floop, @init, @combine, @reduce, DistributedEx, SequentialEx, ThreadedEx
 
 using BangBang.Extras: broadcast_inplace!!
 using BangBang: materialize!!, push!!
diff --git a/src/reduce.jl b/src/reduce.jl
index 807a7efe0..29ef514cb 100644
--- a/src/reduce.jl
+++ b/src/reduce.jl
@@ -86,13 +86,38 @@ macro reduce(args...)
 end
 # TODO: detect free variables in `do` blocks
 
-struct ReduceOpSpec
+abstract type OpSpec end
+
+struct ReduceOpSpec <: OpSpec
     args::Vector{Any}
     visible::Vector{Symbol}
 end
 
 ReduceOpSpec(args::Vector{Any}) = ReduceOpSpec(args, Symbol[])
 
+"""
+    @combine() do (acc₁ [= init₁]; x₁), ..., (accₙ [= initₙ]; xₙ)
+        ...
+    end
+    @combine(acc₁ ⊗₁= x₁, ..., accₙ ⊗ₙ= xₙ)
+    @combine(acc₁ .⊗₁= x₁, ..., accₙ .⊗ₙ= xₙ)
+    @combine(acc₁ = ⊗₁(other₁, x₁), ..., accₙ = ⊗ₙ(otherₙ, xₙ))
+    @combine(acc₁ .= (⊗₁).(other₁, x₁), ..., accₙ = (⊗ₙ).(otherₙ, xₙ))
+
+Declare how accumulators from two basecases are combined.  Unlike `@reduce`, the
+reduction for the basecase is not defined by this macro.
+"""
+macro combine(args...)
+    :(throw($(CombineOpSpec(collect(Any, args)))))
+end
+
+struct CombineOpSpec <: OpSpec
+    args::Vector{Any}
+    visible::Vector{Symbol}
+end
+
+CombineOpSpec(args::Vector{Any}) = CombineOpSpec(args, Symbol[])
+
 """
     @init begin
         pv₁ = init₁
@@ -148,7 +173,7 @@ end
 copyexpr(expr::Expr) = Expr(expr.head, (copyexpr(a) for a in expr.args)...)
 copyexpr(@nospecialize x) = x
 
-function analyze_loop_local_variables!(spec::Union{ReduceOpSpec,InitSpec}, scopes)
+function analyze_loop_local_variables!(spec::Union{OpSpec,InitSpec}, scopes)
     @assert isempty(spec.visible)
     append!(spec.visible, (var.name for sc in scopes for var in sc.bounds))
     unique!(spec.visible)
@@ -217,16 +242,18 @@ function unpack_kwargs(;
     otherwise = donothing,
     on_expr = otherwise,
     on_init = otherwise,
+    on_combine = otherwise,
     kwargs...,
 )
     @assert isempty(kwargs)
-    return (otherwise, on_expr, on_init)
+    return (otherwise, on_expr, on_init, on_combine)
 end
 
 function on_reduce_op_spec(on_spec, ex; kwargs...)
-    (otherwise, on_expr, on_init) = unpack_kwargs(; kwargs...)
+    (otherwise, on_expr, on_init, on_combine) = unpack_kwargs(; kwargs...)
     @match ex begin
         Expr(:call, throw′, spec::ReduceOpSpec) => on_spec(spec)
+        Expr(:call, throw′, spec::CombineOpSpec) => on_combine(spec)
         Expr(:call, throw′, spec::InitSpec) => on_init(spec)
         Expr(head, args...) => begin
             new_args = map(args) do x
@@ -238,8 +265,8 @@ function on_reduce_op_spec(on_spec, ex; kwargs...)
     end
 end
 
-on_reduce_op_spec_reconstructing(on_spec, ex; otherwise = identity, on_init = otherwise) =
-    on_reduce_op_spec(on_spec, ex; on_expr = Expr, otherwise = otherwise, on_init = on_init)
+on_reduce_op_spec_reconstructing(on_spec, ex; kwargs...) =
+    on_reduce_op_spec(on_spec, ex; otherwise = identity, on_expr = Expr, kwargs...)
 
 function floop_parallel(ctx::MacroContext, ex::Expr, simd, executor = nothing)
     if !isexpr(ex, :for, 2)
@@ -439,7 +466,7 @@ end
 ```
 """
 function process_reduce_op_spec(
-    spec::ReduceOpSpec,
+    spec::OpSpec,
 )::NamedTuple{(:inits, :accs, :inputs, :pre_updates, :initializers, :updaters)}
     opspecs = spec.args::Vector{Any}
 
@@ -662,14 +689,26 @@ function as_parallel_loop(ctx::MacroContext, rf_arg, coll, body0::Expr, simd, ex
             $ScratchSpace($init_allocator, ($(accs...),))
         end
 
+        @gensym value_field
+        allocated_value = quote
+            # Just in case the value is deallocated:
+            $grouped_private_states = $allocate($grouped_private_states)
+            # Get the `.value` field but let the compiler know that field is allocated:
+            let $value_field = $grouped_private_states.value
+                if $value_field isa $Cleared
+                    $unreachable_floop()
+                else
+                    $value_field
+                end
+            end
+        end
+
         if isempty(intersect(spec.visible, unbound_rhs(spec.expr)))
             # Hoisting out `@init`, since it is not accessing variables used
             # inside the loop body.
             push!(init_exprs, scratch)
             return quote
-                # Just in case it is demoted to `EmptyScratchSpace`:
-                $grouped_private_states = $allocate($grouped_private_states)
-                ($(accs...),) = $grouped_private_states.value
+                ($(accs...),) = $allocated_value
             end
         else
             push!(init_exprs, _FLoopInit())
@@ -679,16 +718,54 @@ function as_parallel_loop(ctx::MacroContext, rf_arg, coll, body0::Expr, simd, ex
                     # Assigning to `grouped_private_states` for reusing it next time.
                     $grouped_private_states = $scratch
                 else
-                    # Just in case it is demoted to `EmptyScratchSpace`:
-                    $grouped_private_states = $allocate($grouped_private_states)
                     # After the initialization, just carry it over to the next iteration:
-                    ($(accs...),) = $grouped_private_states.value
+                    ($(accs...),) = $allocated_value
                 end
             end
         end
     end
 
-    body1 = on_reduce_op_spec_reconstructing(body0; on_init = on_init) do spec
+    function on_combine(spec::CombineOpSpec)
+        @gensym grouped_accs grouped_inputs
+        push!(accs_symbols, grouped_accs)
+        push!(inputs_symbols, grouped_inputs)
+        (_inits, accs, inputs, pre_updates, _initializers, updaters) =
+            process_reduce_op_spec(spec)
+        # TODO: check `init`
+        # TODO: check `initializers`
+
+        push!(is_init, false)
+        push!(init_exprs, _FLoopInit())
+        push!(all_rf_inits, nothing)
+        push!(all_rf_accs, accs)
+        push!(all_rf_inputs, inputs)
+        verify_unique_symbols(accs, "accumulator")
+        verify_unique_symbols(inputs, "input")
+
+        combine_body = quote
+            if $grouped_inputs isa $_FLoopInit
+            else
+                if $grouped_accs isa $_FLoopInit
+                else
+                    ($(inputs...),) = $grouped_inputs
+                    ($(accs...),) = $grouped_accs
+                    $(updaters...)
+                end
+                $grouped_accs = ($(accs...),)
+            end
+        end
+        push!(combine_bodies, combine_body)
+        return quote
+            $(pre_updates...)
+            $grouped_accs = ($(inputs...),)
+        end
+    end
+
+    body1 = on_reduce_op_spec_reconstructing(
+        body0;
+        on_init = on_init,
+        on_combine = on_combine,
+    ) do spec
         spec = spec::ReduceOpSpec
         @gensym grouped_accs grouped_inputs
         push!(accs_symbols, grouped_accs)
@@ -858,6 +935,12 @@ function Base.showerror(io::IO, opspecs::ReduceOpSpec)
     print(io, ")` used outside `@floop`")
 end
 
+function Base.showerror(io::IO, opspecs::CombineOpSpec)
+    print(io, "`@combine(")
+    join(io, opspecs.args, ", ")
+    print(io, ")` used outside `@floop`")
+end
+
 function Base.showerror(io::IO, spec::InitSpec)
     ex = spec.expr
     print(io, "`@init", ex, "` used outside `@floop`")
diff --git a/test/FLoopsTests/src/test_combine.jl b/test/FLoopsTests/src/test_combine.jl
new file mode 100644
index 000000000..9d1b8ac26
--- /dev/null
+++ b/test/FLoopsTests/src/test_combine.jl
@@ -0,0 +1,44 @@
+module TestCombine
+
+using FLoops
+using MicroCollections
+using Test
+
+function countmap_two_pass(indices, ex = nothing)
+    l, h = extrema(indices)
+    n = h - l + 1
+    @floop ex for i in indices
+        @init b = zeros(Int, n)
+        b[i-l+1] += 1
+        @combine h .+= b
+    end
+    return h
+end
+
+function test_countmap_two_pass()
+    @testset "$(repr(ex))" for ex in [SequentialEx(), nothing, ThreadedEx(basesize = 1)]
+        @test countmap_two_pass(1:3, ex) == [1, 1, 1]
+        @test countmap_two_pass([1, 2, 4, 1], ex) == [2, 1, 0, 1]
+    end
+end
+
+#=
+using FillArrays
+function countmap_one_pass(indices)
+    @floop for i in indices
+        @init l = nothing
+        @init b = [0]
+        if l === nothing
+            l = i
+        elseif i < l
+            splice!(b, 1:0, Zeros(l - i + 1))
+            l = i
+        end
+        b[i - l + 1] += 1
+        @combine() do (h; b), (l; l2)
+        end
+    end
+end
+=#
+
+end  # module