bug#39573: [3.0.0] Compiler fails to optimize out side-effect-free expression

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’.