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

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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.
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Ludovic Courtès 2021-09-21 09:43:53 +02:00
parent 39414b78d8
commit baa0aa1348
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1 changed files with 9 additions and 1 deletions
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guix/base32.scm
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@ -53,7 +53,15 @@ (define-syntax bit-field
(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)