From 6bb4c0b712788c9cec9a1721ceb3c3a5c89c4181 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Thu, 9 Oct 2014 13:08:53 +0200
Subject: [PATCH] mkList: Scrub better
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Clearing v.app.right was not enough, because the length field of a
list only takes 32 bits, so the most significant 32 bits of v.app.left
(a.k.a. v.thunk.env) would remain. This could cause Boehm GC to
interpret it as a valid pointer.

This change reduces maximum RSS for evaluating the ‘tested’ job in
nixos/release-small.nix from 1.33 GiB to 0.80 GiB, and runtime by
about 8%.
---
 src/libexpr/eval.cc  | 1 +
 src/libexpr/value.hh | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 5122bd145..5b9db6eea 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -455,6 +455,7 @@ Bindings * EvalState::allocBindings(Bindings::size_t capacity)
 
 void EvalState::mkList(Value & v, unsigned int length)
 {
+    clearValue(v);
     v.type = tList;
     v.list.length = length;
     v.list.elems = length ? (Value * *) GC_MALLOC(length * sizeof(Value *)) : 0;
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 408b5e087..5f18f629e 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -96,7 +96,7 @@ struct Value
    Value to ensure that the target isn't kept alive unnecessarily. */
 static inline void clearValue(Value & v)
 {
-    v.app.right = 0;
+    v.app.left = v.app.right = 0;
 }
 
 
@@ -118,8 +118,8 @@ static inline void mkBool(Value & v, bool b)
 
 static inline void mkNull(Value & v)
 {
+    clearValue(v);
     v.type = tNull;
-    v.app.left = v.app.right = 0; // scrub
 }