diff --git a/docs/adr/0007-symbolic-bounds.md b/docs/adr/0007-symbolic-bounds.md
index d64b7e45..49a73726 100644
--- a/docs/adr/0007-symbolic-bounds.md
+++ b/docs/adr/0007-symbolic-bounds.md
@@ -87,7 +87,9 @@ Replace the fact model with one representation:
   The compiler is at about +29% of its time before step 1, past the
   ceiling; the next step cuts first. The copy analysis's one pass per arm
   (`defk-o08.3`, 6.39) took 9% off: +17.6% of the work before step 1, still
-  past the ceiling.
+  past the ceiling. Each statement's reads walked once (`defk-o08.4`, 6.40)
+  took 8.6% more off: +7.4% of the work before step 1 (-1.0% without
+  `SatEx.Kernels`), under the ceiling again.
 - **Raising guards become the way to state facts about arguments** in a
   kernel, which the language docs would describe.
 - **Out of scope:** degree 3 and nested products, facts across loop
diff --git a/docs/performance.md b/docs/performance.md
index f9e449d1..1d2f4d10 100644
--- a/docs/performance.md
+++ b/docs/performance.md
@@ -2577,6 +2577,30 @@ else: 0))`; it now says `if first < 0 and n > 0, do: raise(ArgumentError,
 - **Source:** `defk-o08.3`; `test/copies_test.exs`.
 - **Status:** measured, provisional.
 
+### 6.40 Each statement's reads walked once: another -9% of the compiler's work
+
+- **Numbers:** entering a statement computed its reads three times: in
+  `Copies.live_after/1` (`stmt_reads/1`), in the compiler's `hoist/3` and
+  in `Copies.enter_stmt/4` (`reads/1`, the same set: `read_counts/1` skips a
+  match's left side as `stmt_reads/1` does). `Copies.block_liveness/1` keeps
+  each statement's reads beside its live set, and `enter/3` hands them on.
+  Over the 108 modules of `mix dump_c`, two rounds each, interleaved with
+  main: 125.9 -> 115.0 M reductions (-8.6%, the same without
+  `SatEx.Kernels`), 987 -> 934 ms (-5.4%); `HashChain.Kernels` and its
+  Elixir-mode twin -2.2 M reductions each, `SatEx.Kernels` -3.6 M. Against
+  the compiler before step 1 (6.38's baseline, by reductions): +7.4%
+  (-1.0% without `SatEx.Kernels`), from +17.6%: under ADR-0007's ceiling.
+- **Same C:** `mix dump_c` byte-identical for all 108 modules; a property
+  test checks `reads/1 == stmt_reads/1` and that `block_liveness/1` agrees
+  with `live_after/1` on generated blocks.
+- **Not done:** a statement nested d blocks deep is still walked once per
+  enclosing statement (its reads are part of theirs); building a
+  statement's reads from its blocks' would need the walk to follow the
+  lowering's order.
+- **Conditions:** M-series Mac on battery, no quiet check; `cc` not run.
+- **Source:** `defk-o08.4`; `test/copies_test.exs`.
+- **Status:** measured, provisional.
+
 ---
 
 ## 7. When a hand-written or wrapper NIF is still the answer
@@ -2884,4 +2908,4 @@ example or close a workload ticket, add its entries and its line here.
   `defk-sve`, `defk-sve.1` ([`state-copy-cost.md`](https://github.com/DanielLuning/defk/blob/main/docs/research/state-copy-cost.md)), `defk-ulk`, `defk-ulk.1` ([`simd.md`](https://github.com/DanielLuning/defk/blob/main/docs/research/simd.md)), `defk-33b`,
   `defk-ulk.4`, `defk-52v.15` ([`cdcl_sat.exs`](https://github.com/DanielLuning/defk/blob/main/examples/cdcl_sat.exs)), `defk-tif`, `defk-gqk`, `defk-6ag`,
   `defk-7dm` ([`bench/port_roundtrip.exs`](https://github.com/DanielLuning/defk/blob/main/bench/port_roundtrip.exs)), `defk-1ub`,
-  `defk-o08.2`, `defk-o08.3`.
+  `defk-o08.2`, `defk-o08.3`, `defk-o08.4`.
diff --git a/lib/defk/compiler.ex b/lib/defk/compiler.ex
index b5f3a66d..53a646f5 100644
--- a/lib/defk/compiler.ex
+++ b/lib/defk/compiler.ex
@@ -369,7 +369,8 @@ defmodule Defk.Compiler do
     case Code.string_to_quoted(src) do
       # the statements are lowered in the block of the statement env is at
       {:ok, ast} ->
-        probe(fn -> stmt_list(exprs(ast), Copies.live_after(exprs(ast)), env) end) != :error
+        probe(fn -> stmt_list(exprs(ast), Copies.block_liveness(exprs(ast)), env) end) !=
+          :error
 
       _ ->
         false
@@ -906,7 +907,7 @@ defmodule Defk.Compiler do
   defp lower_block_env(ast, env) do
     es = exprs(ast)
     {init, [last]} = Enum.split(es, -1)
-    lives = Copies.live_after(es)
+    lives = Copies.block_liveness(es)
     {s1, env} = stmt_list(init, lives, nested(env, ast))
 
     {s2, v, env} =
@@ -957,10 +958,11 @@ defmodule Defk.Compiler do
   # A block evaluated only for effect, nested like lower_block's.
   defp lower_stmts(ast, env) do
     es = exprs(ast)
-    stmt_list(es, Copies.live_after(es), nested(env, ast))
+    stmt_list(es, Copies.block_liveness(es), nested(env, ast))
   end
 
-  # statements `es` of one block; `lives` has the names read after each
+  # statements `es` of one block; `lives` has, for each, the names read after
+  # it and the names it reads (Copies.block_liveness/1)
   defp stmt_list(es, lives, env) do
     Enum.zip(AllocZeroing.mark(es), lives)
     |> Enum.reduce({[], env}, fn {e, live}, {acc, env} ->
@@ -1349,12 +1351,13 @@ defmodule Defk.Compiler do
   defp array_ids({:arr, _}), do: %{id: make_ref(), buf: make_ref(), fin: false, depth: 1}
   defp array_ids(_t), do: %{}
 
-  # Before statement `e` of a block, `live` the names read after it: copy
-  # what its nested writes need (hoist/3), then note what is live.
-  defp enter(e, live, env) do
+  # Before statement `e` of a block, `live` the names read after it and
+  # `reads` those it reads: copy what its nested writes need (hoist/4), then
+  # note what is live.
+  defp enter(e, {live, reads}, env) do
     names = Copies.live_names(e, live)
-    {sh, env} = hoist(e, names, env)
-    {sh, %{env | scope: Copies.enter_stmt(env.scope, e, names, env.vars)}}
+    {sh, env} = hoist(e, names, reads, env)
+    {sh, %{env | scope: Copies.enter_stmt(env.scope, e, names, reads, env.vars)}}
   end
 
   # A variable bound outside the branches and loop bodies of `e` and written
@@ -1363,8 +1366,8 @@ defmodule Defk.Compiler do
   # only binding of an owned buffer, so its writes are in place, and a loop
   # copies once. An input array written in a branch is copied even when the
   # branch is not taken.
-  defp hoist(e, names, env) do
-    seen = MapSet.union(names, Copies.reads(e))
+  defp hoist(e, names, reads, env) do
+    seen = MapSet.union(names, reads)
 
     Enum.reduce(Copies.nested_writes(e), {[], env}, fn n, {acc, env} ->
       case env.vars do
@@ -5099,7 +5102,7 @@ defmodule Defk.Compiler do
   defp lower_tail(ast, env, leaf) do
     es = exprs(ast)
     {init, [last]} = Enum.split(es, -1)
-    lives = Copies.live_after(es)
+    lives = Copies.block_liveness(es)
     {s, env} = stmt_list(init, lives, nested(env, ast))
 
     [
diff --git a/lib/defk/copies.ex b/lib/defk/copies.ex
index 8f157f06..8778d1fe 100644
--- a/lib/defk/copies.ex
+++ b/lib/defk/copies.ex
@@ -177,10 +177,17 @@ defmodule Defk.Copies do
   def stmt_reads(e), do: reads(e)
 
   # for each statement of a block, the names read after it in the block
-  def live_after(es) do
+  def live_after(es), do: es |> block_liveness() |> Enum.map(&elem(&1, 0))
+
+  # for each statement of a block, {the names read after it in the block,
+  # the names it reads}: what entering it needs (enter_stmt/5), from one
+  # walk of it. stmt_reads/1 is reads/1 of the statement: read_counts/1
+  # skips a match's left side as stmt_reads/1 does.
+  def block_liveness(es) do
     {lives, _} =
       List.foldr(es, {[], MapSet.new()}, fn e, {acc, live} ->
-        {[live | acc], live |> MapSet.difference(binds(e)) |> MapSet.union(stmt_reads(e))}
+        reads = stmt_reads(e)
+        {[{live, reads} | acc], live |> MapSet.difference(binds(e)) |> MapSet.union(reads)}
       end)
 
     lives
@@ -289,9 +296,12 @@ defmodule Defk.Copies do
   # with `vars` the bindings after the copies hoisted before `e`: what is
   # live after `e`, and for the blocks nested in it (which also see what `e`
   # reads, as a later iteration or a later arm does).
-  def enter_stmt(%Scope{} = s, e, names, vars) do
+  # `reads` is reads(e), when the caller has it (block_liveness/1).
+  def enter_stmt(%Scope{} = s, e, names, vars), do: enter_stmt(s, e, names, reads(e), vars)
+
+  def enter_stmt(%Scope{} = s, e, names, reads, vars) do
     live = MapSet.union(pairs(vars, names), s.out)
-    nest = MapSet.union(live, pairs(vars, reads(e)))
+    nest = MapSet.union(live, pairs(vars, reads))
     %{s | stmt: e, live: live, live_names: names, nest: nest}
   end
 
diff --git a/test/copies_test.exs b/test/copies_test.exs
index 1a32eb51..6e95fbca 100644
--- a/test/copies_test.exs
+++ b/test/copies_test.exs
@@ -407,4 +407,33 @@ defmodule Defk.CopiesTest do
       assert Copies.arm_reads(twice, node) == nil
     end
   end
+
+  describe "block_liveness/1, each statement's reads walked once" do
+    property "a statement's reads are its stmt_reads, and the lives are live_after's" do
+      check all(es <- StreamData.list_of(stmt_gen(), min_length: 1, max_length: 5), max_runs: 300) do
+        for e <- es, do: assert(Copies.reads(e) == Copies.stmt_reads(e), Macro.to_string(e))
+
+        liveness = Copies.block_liveness(es)
+        assert Enum.map(liveness, &elem(&1, 0)) == Copies.live_after(es)
+        assert Enum.map(liveness, &elem(&1, 1)) == Enum.map(es, &Copies.reads/1)
+      end
+    end
+
+    test "an array write reads its target, a match does not read its left side" do
+      [{_, write}, {_, match}] =
+        Copies.block_liveness(
+          quote(
+            do:
+              (
+                a[i] = v
+                x = f(y)
+              )
+          )
+          |> elem(2)
+        )
+
+      assert write == MapSet.new([:a, :i, :v])
+      assert match == MapSet.new([:y])
+    end
+  end
 end