From a3173d17581efda4db96b19ade771f63096a84db Mon Sep 17 00:00:00 2001
From: Andy Wingo <wingo@pobox.com>
Date: Tue, 21 Nov 2017 21:46:08 +0100
Subject: [PATCH] DCE eliminates effect-free branches to the same continuation

* module/language/cps/dce.scm (compute-live-code): Eliminate some
  branches.
---
 module/language/cps/dce.scm | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/module/language/cps/dce.scm b/module/language/cps/dce.scm
index efead7544..cbd12c160 100644
--- a/module/language/cps/dce.scm
+++ b/module/language/cps/dce.scm
@@ -173,8 +173,11 @@ sites."
           (or
            ;; No defs; perhaps continuation is $ktail.
            (not defs)
-           ;; We don't remove branches.
-           (match exp (($ $branch) #t) (_ #f))
+           ;; We don't remove branches, unless both branches go to the
+           ;; same place.
+           (match exp
+             (($ $branch kt) (not (eqv? k kt)))
+             (_ #f))
            ;; Do we have a live def?
            (any-var-live? defs live-vars)
            ;; Does this expression cause all effects?  If so, it's