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[Guile-commits] branch stable-2.2 updated: Fix fixpoint computation in c
[Guile-commits] branch stable-2.2 updated: Fix fixpoint computation in compute-significant-bits
Mon, 23 Mar 2020 09:48:04 -0400
This is an automated email from the git hooks/post-receive script.
wingo pushed a commit to branch stable-2.2
in repository guile.
The following commit(s) were added to refs/heads/stable-2.2 by this push:
new a58758e Fix fixpoint computation in compute-significant-bits
a58758e is described below
Author: Andy Wingo <address@hidden>
AuthorDate: Mon Mar 23 14:45:29 2020 +0100
Fix fixpoint computation in compute-significant-bits
* module/language/cps/specialize-numbers.scm (preserve-eq?): New
(sigbits-union): Use the new helper. Fixes bugs.gnu.org/38486.
Thanks to Zack Marvel for the bug report and Matt Wette for tracking
module/language/cps/specialize-numbers.scm | 25 +++++++++++++++++++++++--
1 file changed, 23 insertions(+), 2 deletions(-)
diff --git a/module/language/cps/specialize-numbers.scm
index d558703..822ef70 100644
@@ -1,6 +1,6 @@
;;; Continuation-passing style (CPS) intermediate language (IL)
-;; Copyright (C) 2015, 2016 Free Software Foundation, Inc.
+;; Copyright (C) 2015, 2016, 2020 Free Software Foundation, Inc.
;;;; This library is free software; you can redistribute it and/or
;;;; modify it under the terms of the GNU Lesser General Public
@@ -159,8 +159,29 @@
($continue kunbox-b src
($primcall 'scm->f64 (a)))))))
+;; compute-significant-bits solves a flow equation to compute a
+;; least-fixed-point over the lattice VAR -> BITMASK, where X > Y if
+;; X[VAR] > Y[VAR] for any VAR. Adjoining VAR -> BITMASK to X results
+;; in a distinct value X' (in the sense of eq?) if and only if X' > X.
+;; This property is used in compute-significant-bits to know when to
+;; stop iterating, and is ensured by intmaps, provided that the `meet'
+;; function passed to `intmap-add' and so on also preserves this
+;; The meet function for adding bits is `sigbits-union'; the first
+;; argument is the existing value, and the second is the bitmask to
+;; adjoin. For fixnums, BITMASK' will indeed be distinct if and only if
+;; bits were added. However for bignums it's possible that (= X' X) but
+;; not (eq? X' X). This preserve-eq? helper does the impedance matching
+;; for bignums, returning the first value if the values are =.
+(define (preserve-eq? x x*)
+ (if (= x x*)
(define (sigbits-union x y)
- (and x y (logior x y)))
+ (and x y
+ (preserve-eq? x (logior x y))))
(define (sigbits-intersect x y)
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