Fix mutable allocations in n-ary multiplication (#326)

Author: odow

src/rewrite_generic.jl

@@ -216,11 +216,8 @@ function _rewrite_generic(stack::Expr, expr::Expr)
         push!(stack.args, :($root = $rhs))
         for i in 4:length(expr.args)
             arg, _ = _rewrite_generic(stack, expr.args[i])
-            rhs = if is_mutable
-                Expr(:call, operate!!, *, root, arg)
-            else
-                Expr(:call, operate, *, root, arg)
-            end
+            # It is always safe to modify `root` here
+            rhs = Expr(:call, operate!!, *, root, arg)
             root = gensym()
             push!(stack.args, :($root = $rhs))
         end

test/rewrite_generic.jl

@@ -498,6 +498,18 @@ function test_allocations_rewrite_unary_minus()
     return
 end
 
+function test_allocations_rewrite_mult()
+    x = BigFloat[1, 2, 3]
+    y = MA.@rewrite(x[1] * x[2] * x[3], move_factors_into_sums = false)
+    @test y == BigFloat(6)
+    @test x == BigFloat[1, 2, 3]
+    total = @allocated (x[1] * x[2]) * x[3]
+    @test @allocated(
+        MA.@rewrite(x[1] * x[2] * x[3], move_factors_into_sums = false),
+    ) < total
+    return
+end
+
 end  # module
 
 TestRewriteGeneric.runtests()