From ccd2a1d4ab60a9539472df45fc4f9ec7b7fdc7b7 Mon Sep 17 00:00:00 2001
From: Paul Khuong <pvk@pvk.ca>
Date: Sat, 20 Apr 2013 13:50:52 +0200
Subject: [PATCH] Substitute constants with modular equivalents more safely
* Modular arithmetic sometimes lets us narrow constants down,
especially with signed arithmetic. We now update the receiving
LVAR's type conservatively when there are multiple uses; otherwise,
conflicting type information results in spurious dead code
elimination.
* Test case by Eric Marsden.
* Reported by Eric Marsden on sbcl-devel (2013-04-18).
---
NEWS | 3 +++
src/compiler/srctran.lisp | 5 ++++-
tests/compiler.pure.lisp | 14 ++++++++++++++
3 files changed, 21 insertions(+), 1 deletion(-)
diff --git a/NEWS b/NEWS
index c2cf5f2..7ba1204 100644
--- a/NEWS
+++ b/NEWS
@@ -1,5 +1,8 @@
;;;; -*- coding: utf-8; fill-column: 78 -*-
changes relative to sbcl-1.1.6
+ * bug fix: modular arithmetic involving large constants and conditionals
+ should no longer result in spurious dead code elimination. Reported by
+ Eric Marsden on sbcl-devel.
* bug fix: our mach exception handler can seemingly called very early in
the program execution process on OS X 10.8.0. Try and handle that case
robustly, without potentially leaking mach ports too much.
diff --git a/src/compiler/srctran.lisp b/src/compiler/srctran.lisp
index 5858da9..f6369e6 100644
--- a/src/compiler/srctran.lisp
+++ b/src/compiler/srctran.lisp
@@ -3060,7 +3060,10 @@
(ldb (byte width 0) constant-value))))
(unless (= constant-value new-value)
(change-ref-leaf node (make-constant new-value))
- (setf (lvar-%derived-type (node-lvar node)) (make-values-type :required (list (ctype-of new-value))))
+ (let ((lvar (node-lvar node)))
+ (setf (lvar-%derived-type lvar)
+ (and (lvar-has-single-use-p lvar)
+ (make-values-type :required (list (ctype-of new-value))))))
(setf (block-reoptimize (node-block node)) t)
(reoptimize-component (node-component node) :maybe)
(return-from cut-node t))))
diff --git a/tests/compiler.pure.lisp b/tests/compiler.pure.lisp
index 1a4fc7d..0e7c53c 100644
--- a/tests/compiler.pure.lisp
+++ b/tests/compiler.pure.lisp
@@ -4383,3 +4383,17 @@
(compile nil `(lambda (x)
(symbol-macrolet ((sv x))
(values (svref sv 0) (setf (svref sv 0) 99))))))
+
+;; The compiler used to update the receiving LVAR's type too
+;; aggressively when converting a large constant to a smaller
+;; (potentially signed) one, causing other branches to be
+;; inferred as dead.
+(with-test (:name :modular-cut-constant-to-width)
+ (let ((test (compile nil
+ `(lambda (x)
+ (logand 254
+ (case x
+ ((3) x)
+ ((2 2 0 -2 -1 2) 9223372036854775803)
+ (t 358458651)))))))
+ (assert (= (funcall test -10470605025) 26))))
--
1.7.10.4