From e7744efb7d36aafa35dc0ac678dd6038e46804a1 Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Thu, 14 Mar 2019 06:22:19 -0600
Subject: [PATCH] expander: avoid unneeded namespace reference

This change avoids the stair-step effect that is depicted in the
"current Racket -M" build plot from the January 2019 blog post about
Racket on Chez Scheme.

The stair step in that plot is a result of a combination of effects,
but one key part is that the `.set-transformer!` linklet import (to
support macro definitions) has a reference back to the namespace.
While `.set-transformer!` normally would not be captured in any
closure, `db/private/generic/prepared` creates a thread that causes
the "prefix" part of a closure to be moved to a thread's runstack
before it can be pruned by the GC. The stair-step problem happens only
when running directly from machine-independent form, because that form
is recompiled in a way that doesn't optimize away the unused
`.set-transformer!` import. The change in this commit avoids a
reference to the namespace in some cases where it will not be useful,
which turns out to be sufficient to address the build problem.

A more complete repair would be to change the compiler to pair a
closure prefix on the runstack with a liveness mask. An even more
complete repair is to switch to Racket CS. Racket CS is immune to the
problem, even when running from machine-independent bytecode, because
its closures do not keep extra references (with the tradeoff that
there's less sharing).
---
 racket/src/expander/eval/module.rkt | 12 ++++++++++--
 racket/src/racket/src/startup.inc   | 25 ++++++++++++++++++++++++-
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/racket/src/expander/eval/module.rkt b/racket/src/expander/eval/module.rkt
index ca9254551d..220676f861 100644
--- a/racket/src/expander/eval/module.rkt
+++ b/racket/src/expander/eval/module.rkt
@@ -190,8 +190,16 @@
 
                                    (define module-body-instance-instance
                                      (make-module-body-instance-instance
-                                      #:set-transformer! (lambda (name val)
-                                                           (namespace-set-transformer! ns (sub1 phase-level) name val))))
+                                      #:set-transformer! (cond
+                                                          [(zero-phase? phase-level)
+                                                           (lambda (name val)
+                                                             (error 'define-syntax "should not happen at phase level 0"))]
+                                                          [(zero-phase? (phase+ phase-shift phase-level))
+                                                           ;; No use for phase -1 bindings
+                                                           (lambda (name val) (void))]
+                                                          [else
+                                                           (lambda (name val)
+                                                             (namespace-set-transformer! ns (sub1 phase-level) name val))])))
 
                                    (define (instantiate-body)
                                      (instantiate-linklet phase-linklet
diff --git a/racket/src/racket/src/startup.inc b/racket/src/racket/src/startup.inc
index 51e3106534..2cdd0218a6 100644
--- a/racket/src/racket/src/startup.inc
+++ b/racket/src/racket/src/startup.inc
@@ -37864,14 +37864,37 @@ static const char *startup_source =
 "(values))))"
 "(let-values(((module-body-instance-instance_0)"
 "(let-values(((temp52_0)"
+"(if(zero-phase?"
+" phase-level_0)"
+"(let-values()"
 "(lambda(name_0"
 " val_0)"
+"(begin"
+" 'temp52"
+"(error"
+" 'define-syntax"
+"                                                                                                                                                                         \"should not happen at phase level 0\"))))"
+"(if(zero-phase?"
+"(phase+"
+" phase-shift_0"
+" phase-level_0))"
+"(let-values()"
+"(lambda(name_0"
+" val_0)"
+"(begin"
+" 'temp52"
+"(void))))"
+"(let-values()"
+"(lambda(name_0"
+" val_0)"
+"(begin"
+" 'temp52"
 "(namespace-set-transformer!"
 " ns_2"
 "(sub1"
 " phase-level_0)"
 " name_0"
-" val_0))))"
+" val_0))))))))"
 "(make-module-body-instance-instance18.1"
 " temp52_0))))"
 "(let-values(((instantiate-body_0)"