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Anything speaking against this simplification?


From: David Kastrup
Subject: Anything speaking against this simplification?
Date: Sat, 30 Jul 2011 22:19:00 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.50 (gnu/linux)

Hi,

music_list in parser.yy temporarily maintains an awkward
half-self-referential data structure in order to have "fast append".  It
makes more sense in my opinion to use prepend and reverse afterwards.
"Fast append" in theory may show minimally better cache coherency at the
cost of uglier code and uglier data structures.  So I'd just like to do
the following (no full regtest yet).

Comments?

diff --git a/lily/parser.yy b/lily/parser.yy
index 2a4032e..27b158c 100644
--- a/lily/parser.yy
+++ b/lily/parser.yy
@@ -928,25 +928,14 @@ tempo_event:
        ;
 
 /*
-The representation of a  list is the
-
-  (LIST . LAST-CONS)
-
- to have efficient append.  */
+The representation of a  list is reversed to have efficient append.  */
 
 music_list:
        /* empty */ {
-               $$ = scm_cons (SCM_EOL, SCM_EOL);
+               $$ = SCM_EOL;
        }
        | music_list music {
-               SCM s = $$;
-               SCM c = scm_cons ($2, SCM_EOL);
-
-               if (scm_is_pair (scm_cdr (s)))
-                       scm_set_cdr_x (scm_cdr (s), c); /* append */
-               else
-                       scm_set_car_x (s, c); /* set first cons */
-               scm_set_cdr_x (s, c);  /* remember last cell */
+               $$ = scm_cons ($2, $1);
        }
        | music_list embedded_scm {
 
@@ -955,15 +944,8 @@ music_list:
                Music *m = MY_MAKE_MUSIC("Music", @$);
                // ugh. code dup
                m->set_property ("error-found", SCM_BOOL_T);
-               SCM s = $$;
-               SCM c = scm_cons (m->self_scm (), SCM_EOL);
+               $$ = scm_cons (m->self_scm (), $1);
                m->unprotect (); /* UGH */
-
-               if (scm_is_pair (scm_cdr (s)))
-                       scm_set_cdr_x (scm_cdr (s), c); /* append */
-               else
-                       scm_set_car_x (s, c); /* set first cons */
-               scm_set_cdr_x (s, c);  /* remember last cell */
        }
        ;
 
@@ -978,7 +960,7 @@ alternative_music:
                $$ = SCM_EOL;
        }
        | ALTERNATIVE '{' music_list '}' {
-               $$ = scm_car ($3);
+               $$ = scm_reverse_x ($3, SCM_EOL);
        }
        ;
 
@@ -992,19 +974,19 @@ repeated_music:
 
 sequential_music:
        SEQUENTIAL '{' music_list '}'           {
-               $$ = MAKE_SYNTAX ("sequential-music", @$, scm_car ($3));
+               $$ = MAKE_SYNTAX ("sequential-music", @$, scm_reverse_x ($3, 
SCM_EOL));
        }
        | '{' music_list '}'            {
-               $$ = MAKE_SYNTAX ("sequential-music", @$, scm_car ($2));
+               $$ = MAKE_SYNTAX ("sequential-music", @$, scm_reverse_x ($2, 
SCM_EOL));
        }
        ;
 
 simultaneous_music:
        SIMULTANEOUS '{' music_list '}'{
-               $$ = MAKE_SYNTAX ("simultaneous-music", @$, scm_car ($3));
+               $$ = MAKE_SYNTAX ("simultaneous-music", @$, scm_reverse_x ($3, 
SCM_EOL));
        }
        | DOUBLE_ANGLE_OPEN music_list DOUBLE_ANGLE_CLOSE       {
-               $$ = MAKE_SYNTAX ("simultaneous-music", @$, scm_car ($2));
+               $$ = MAKE_SYNTAX ("simultaneous-music", @$, scm_reverse_x ($2, 
SCM_EOL));
        }
        ;
 

-- 
David Kastrup

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