01/08: base32: Work around (ash x N) miscompilation at '-O1' and below.

[guix-commits]

civodul pushed a commit to branch master
in repository guix.

commit baa0aa1348aa5fe526425efbf11707d8e02a2640
Author: Ludovic Courtès <ludo@gnu.org>
AuthorDate: Tue Sep 21 09:43:53 2021 +0200

    base32: Work around (ash x N) miscompilation at '-O1' and below.
    
    Fixes <https://issues.guix.gnu.org/50696>.
    Reported by Marius Bakke <marius@gnu.org>.
    
    * guix/base32.scm (bit-field): Introduce 'minus-start' syntax and use it.
---
 guix/base32.scm | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/guix/base32.scm b/guix/base32.scm
index d6c8a02..8f097d4 100644
--- a/guix/base32.scm
+++ b/guix/base32.scm
@@ -53,7 +53,15 @@
        (let* ((s    (syntax->datum #'start))
               (e    (syntax->datum #'end))
               (mask (- (expt 2 (- e s)) 1)))
-         #`(logand (ash n (- start)) #,mask))))))
+         ;; The baseline compiler in Guile <= 3.0.7 miscompiles (ash x N) as
+         ;; (ash x (- N)) when N is a literal: <https://bugs.gnu.org/50696>.
+         ;; Here we take advantage of another bug in the baseline compiler,
+         ;; fixed in Guile commit 330c6ea83f492672578b62d0683acbb532d1a5d9: we
+         ;; introduce 'minus-start' such that it has a different source
+         ;; location, which in turn means that the baseline compiler pattern
+         ;; for (ash x N) doesn't match, thus avoiding the bug (!).
+         (with-syntax ((minus-start (datum->syntax #'start (- s))))
+           #`(logand (ash n minus-start) #,mask)))))))
 
 (define bytevector-quintet-ref
   (let* ((ref  bytevector-u8-ref)