03/09: base32: Provide an open-coded 'bit-field'.

[guix-commits]

civodul pushed a commit to branch master
in repository guix.

commit cb06f7c61e4b8393abf38f1f5891e03c33d53b9b
Author: Ludovic Courtès <ludo@gnu.org>
AuthorDate: Thu Sep 9 23:22:10 2021 +0200

    base32: Provide an open-coded 'bit-field'.
    
    This improves the throughput of 'bytevector->base32-string' a bit.
    
    * guix/base32.scm (bit-field): New macro.
---
 guix/base32.scm | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/guix/base32.scm b/guix/base32.scm
index 49f191b..d6c8a02 100644
--- a/guix/base32.scm
+++ b/guix/base32.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2015, 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2015, 2017, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -42,6 +42,19 @@
 ;;;
 ;;; Code:
 
+(define-syntax bit-field
+  (lambda (s)
+    ;; This inline version of 'bit-field' assumes that START and END are
+    ;; literals and pre-computes the mask.  In an ideal world, using 'define'
+    ;; or 'define-inlinable' would be enough, but as of 3.0.7, peval doesn't
+    ;; expand calls to 'expt' (and 'bit-field' is a subr.)
+    (syntax-case s ()
+      ((_ n start end)
+       (let* ((s    (syntax->datum #'start))
+              (e    (syntax->datum #'end))
+              (mask (- (expt 2 (- e s)) 1)))
+         #`(logand (ash n (- start)) #,mask))))))
+
 (define bytevector-quintet-ref
   (let* ((ref  bytevector-u8-ref)
          (ref+ (lambda (bv offset)