Make non-accumulators local

src/macro.jl

@@ -157,7 +157,125 @@ end
 # https://github.com/tkf/ThreadsX.jl/pull/106.  But this should be
 # done automatically in Transducers.jl.
 
+"""
+    localize(body::Expr) -> body′::Expr
+
+Add `local`s to make non-accumulators local in the loop `body`.
+
+It adds `local v` to the inner most scope-creating lexical block that contain
+the outer most lexical block that has an assignment to `v`.
+"""
+localize(body::Expr) = localize!(body)
+
+struct VariableEnv
+    current::Vector{Symbol}
+    outer::Union{VariableEnv,Nothing}
+end
+
+function Base.in(v::Symbol, env::VariableEnv)
+    v in env.current && return true
+    outer = env.outer
+    return outer !== nothing && v in outer
+end
+
+function Base.push!(env::VariableEnv, v::Symbol)
+    v in env || push!(env.current, v)
+    return env
+end
+
+Base.append!(env::VariableEnv, variables) = foldl(push!, variables; init = env)
+
+localize!(@nospecialize(body), ::VariableEnv) = body
+function localize!(body::Expr, env::VariableEnv = VariableEnv(Symbol[], nothing))
+    add_new_assignments!(env, body)
+    body = localize_nested(body, env)
+    if isempty(env.current)
+        return body
+    else
+        return Expr(:block, Expr(:local, env.current...), body)
+    end
+end
+
+# Not that this doesn't handle `local lhs = rhs` but it's OK since it doesn't
+# matter exactly which scope the variable comes from.
+function add_new_assignments!(env::VariableEnv, ex::Expr)
+    @match ex begin
+        Expr(:meta, _...) => nothing
+        Expr(:loopinfo, _...) => nothing
+
+        Expr(:function, Expr(:call, f::Symbol, _...), _...) => push!(env, f)
+        Expr(:(=), lhs, rhs) => begin
+            @match lhs begin
+                Expr(:call, f::Symbol, _...) => push!(env, f)
+                # TODO: handle where
+                _ => begin
+                    if rhs isa Expr
+                        add_new_assignments!(env, rhs)
+                    end
+                    append!(env, vars_in(lhs))
+                end
+            end
+        end
+
+        # Scope-creating
+        Expr(:let, _...) => nothing
+        Expr(:function, _...) => nothing
+        Expr(:->, _...) => nothing
+
+        Expr(_, args...) => begin
+            for x in args
+                if x isa Expr
+                    add_new_assignments!(env, x)
+                end
+            end
+        end
+    end
+end
+
+localize_nested(@nospecialize(body), ::VariableEnv) = body
+function localize_nested(body::Expr, env::VariableEnv)
+    @match body begin
+        Expr(:let, let_bindings_, let_body) => begin
+            let_bindings = @match let_bindings_ begin
+                Expr(:block, args...) => collect(args)
+                b => [b]
+            end
+            let_vars = mapfoldl(vars_in, append!, let_bindings; init = Symbol[])
+            localize!(let_body, VariableEnv(let_vars, env))
+        end
+        Expr(:(=), lhs, rhs) => begin
+            if isexpr(lhs, :call)
+                Expr(:(=), lhs, localize!(rhs, VariableEnv(Symbol[], env)))
+            else
+                Expr(:(=), lhs, localize_nested(rhs, env))
+            end
+        end
+        Expr(:function, call, def) => Expr(
+            :function,
+            localize_nested(call, env),
+            localize!(def, VariableEnv(Symbol[], env)),
+        )
+        Expr(:->, lhs, rhs) =>
+            Expr(:->, localize_nested(lhs, env), localize!(rhs, VariableEnv(Symbol[], env)))
+
+        Expr(:meta, _...) => body
+        Expr(:loopinfo, _...) => body
+
+        Expr(head, args...) => begin
+            args = mapfoldl(push!, args; init = []) do x
+                if x isa Expr
+                    localize_nested(x, env)
+                else
+                    x
+                end
+            end
+            Expr(head, args...)
+        end
+    end
+end
+
 function transform_loop_body(body, state_vars)
+    # body = localize(body)  # TODO: enable this for sequential case as well
     external_labels::Vector{Symbol} = setdiff(gotos_in(body), labels_in(body))
     # state_vars = extract_state_vars(body)
     pack_state = :(($(state_vars...),))
@@ -236,7 +354,7 @@ end
 vars_in(x::Symbol) = [x]
 function vars_in(ex)
     @match ex begin
-        Expr(:tuple, vars...) => vars
+        Expr(:tuple, vars...) => mapfoldl(vars_in, append!, vars; init = Symbol[])
         _ => Symbol[]
     end
 end

src/reduce.jl

@@ -763,7 +763,7 @@ function as_parallel_loop(ctx::MacroContext, rf_arg, coll, body0::Expr, simd, ex
     end
     check_invariance()
 
-    body2, info = transform_loop_body(body1, accs_symbols)
+    body2, info = transform_loop_body(localize(body1), accs_symbols)
 
     @gensym oninit_function reducing_function combine_function result context_function
     if ctx.module_ === Main