From: "Ludovic Courtès" <ludo@gnu.org>
To: bug-Guile@gnu.org
Cc: wingo@igalia.com
Subject: bug#39573: [3.0.0] Compiler fails to optimize out side-effect-free expression
Date: Wed, 12 Feb 2020 12:50:44 +0100 [thread overview]
Message-ID: <87k14svxbf.fsf@gnu.org> (raw)
Hello!
Consider this loop:
(let loop ((n (expt 2 18))
(i 1))
(unless (zero? n)
(loop (- n 1)
(logior 0 (ash i 1)))))
Guile 2.2 strips away the computation of ‘i’ (it cannot throw, has no
side effects, and the result is unused):
--8<---------------cut here---------------start------------->8---
0 (assert-nargs-ee/locals 1 4) ;; 5 slots (0 args) at (unknown file):16:10
1 (current-module 4)
2 (static-set! 4 74) ;; #f
4 (toplevel-box 4 84 72 82 #t) ;; `expt'
9 (box-ref 2 4)
10 (make-short-immediate 1 10) ;; 2
11 (make-short-immediate 0 74) ;; 18
12 (handle-interrupts) at (unknown file):16:24
13 (call 2 3)
15 (receive 0 2 5)
17 (load-u64 3 0 0) at (unknown file):18:11
20 (br-if-u64-=-scm 3 4 #f 12) ;; -> L2
23 (sub/immediate 4 4 1) at (unknown file):21:11
24 (br-if-u64-=-scm 3 4 #f 8) ;; -> L2 at (unknown file):18:11
L1:
27 (handle-interrupts) at (unknown file):21:11
28 (sub/immediate 4 4 1)
29 (br-if-u64-=-scm 3 4 #t -2) ;; -> L1 at (unknown file):18:11
L2:
32 (make-short-immediate 3 2052) ;; #<unspecified> at (unknown file):21:11
33 (handle-interrupts)
34 (return-values 2) ;; 1 value
--8<---------------cut here---------------end--------------->8---
However, 3.0.0 keeps the computation of ‘i’:
--8<---------------cut here---------------start------------->8---
0 (instrument-entry 192) at (unknown file):383:9
2 (assert-nargs-ee/locals 1 5) ;; 6 slots (0 args)
3 (call-scm<-thread 5 62)
5 (static-set! 5 147) ;; #f
7 (static-ref 5 147) ;; #f
9 (immediate-tag=? 5 7 0) ;; heap-object?
11 (je 19) ;; -> L2
12 (static-ref 5 140) ;; #f
14 (static-ref 4 150) ;; expt
16 (call-scm<-scm-scm 5 5 4 40)
18 (immediate-tag=? 5 7 0) ;; heap-object?
20 (jne 8) ;; -> L1
21 (scm-ref/immediate 3 5 1)
22 (immediate-tag=? 3 4095 2308) ;; undefined?
24 (je 4) ;; -> L1
25 (static-set! 5 129) ;; #f
27 (j 3) ;; -> L2
L1:
28 (throw/value 4 156) ;; #(unbound-variable #f "Unbound variable: ~S")
L2:
30 (scm-ref/immediate 2 5 1)
31 (make-short-immediate 1 10) ;; 2
32 (make-short-immediate 0 74) ;; 18
33 (handle-interrupts) at (unknown file):383:23
34 (call 3 3)
36 (receive 0 3 6)
38 (make-short-immediate 4 2) ;; 0 at (unknown file):385:11
39 (=? 5 4)
40 (je 24) ;; -> L4
41 (call-scm<-scm-uimm 5 5 1 3) at (unknown file):388:11
43 (load-s64 3 0 0) at (unknown file):389:11
46 (load-s64 2 0 2)
49 (ulogior 3 3 2)
50 (tag-fixnum 3 3)
51 (=? 5 4) at (unknown file):385:11
52 (je 12) ;; -> L4
L3:
53 (instrument-loop 139)
55 (handle-interrupts)
56 (call-scm<-scm-uimm 5 5 1 3) at (unknown file):388:11
58 (call-scm<-scm-uimm 3 3 1 34) at (unknown file):389:21
60 (call-scm<-scm-scm 3 4 3 10) at (unknown file):389:11
62 (=? 5 4) at (unknown file):385:11
63 (jne -10) ;; -> L3
L4:
64 (make-short-immediate 5 2052) ;; #<unspecified> at (unknown file):388:5
65 (reset-frame 1) ;; 1 slot
66 (handle-interrupts)
67 (return-values)
--8<---------------cut here---------------end--------------->8---
I’m not sure where the optimization should be taking place. Perhaps
it’s just a matter of amount-of-work threshold somewhere?
Thanks,
Ludo’.
next reply other threads:[~2020-02-12 11:50 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-12 11:50 Ludovic Courtès [this message]
2020-02-12 14:57 ` bug#39573: [3.0.0] Compiler fails to optimize out side-effect-free expression Andy Wingo
2020-02-13 11:35 ` Ludovic Courtès
2020-02-13 15:11 ` Mikael Djurfeldt
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