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From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.lang.c
Subject: Re: Command line globber/tokenizer library for C?
Date: Wed, 18 Sep 2024 02:46:11 +0300
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On Tue, 17 Sep 2024 22:34:33 -0000 (UTC)
antispam@fricas.org wrote:

> Michael S <already5chosen@yahoo.com> wrote:
> > On Fri, 13 Sep 2024 09:05:04 -0700
> > Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
> >   
> >> Michael S <already5chosen@yahoo.com> writes:
> >> 
> >> [..iterate over words in a string..]
> >> 
> >> I couldn't resist writing some code along similar lines.  The
> >> entry point is words_do(), which returns one on success and
> >> zero if the end of string is reached inside double quotes.
> >> 
> >> 
> >> typedef struct gopher_s *Gopher;
> >> struct gopher_s { void (*f)( Gopher, const char *, const char * );
> >> };
> >> 
> >> static  _Bool   collect_word( const char *, const char *, _Bool,
> >> Gopher ); static  _Bool   is_space( char );
> >> 
> >> 
> >> _Bool
> >> words_do( const char *s, Gopher go ){
> >>   char   c      =  *s;
> >> 
> >>     return
> >>       is_space(c)       ?  words_do( s+1, go )
> >>   : c                 ?  collect_word( s, s, 1, go )
> >> : /***************/    1;
> >> }
> >> 
> >> _Bool
> >> collect_word( const char *s, const char *r, _Bool w, Gopher go ){
> >>   char   c      =  *s;
> >> 
> >>     return
> >>       c == 0            ?  go->f( go, r, s ),  w
> >>   : is_space(c) && w  ?  go->f( go, r, s ),  words_do( s, go )
> >> : /***************/    collect_word( s+1, r, w ^ c == '"', go );
> >> }
> >> 
> >> _Bool
> >> is_space( char c ){
> >>     return  c == ' '  ||  c == '\t';
> >> }  
> > 
> >  
> 
> <snip>
>  
> > Tested on godbolt.
> > gcc -O2 turns it into iteration starting from v.4.4
> > clang -O2 turns it into iteration starting from v.4.0
> > Latest icc still does not turn it into iteration at least along one
> > code paths.
> > Latest MSVC implements it as written, 100% recursion.  
> 
> I tested using gcc 12.  AFAICS calls to 'go->f' in 'collect_word'
> are not tail calls and gcc 12 compiles them as normal call.

Naturally.

> The other calls are compiled to jumps.  But call to 'collect_word'
> in 'words_do' is not "sibicall" and dependig in calling convention
> compiler may treat it narmal call.  Two other calls, that is
> call to 'words_do' in 'words_do' and call to 'collect_word' in
> 'collect_word' are clearly tail self recursion and compiler
> should always optimize them to a jump.
> 

"Should" or not, MSVC does not eliminate them.

The funny thing is that it does eliminate all four calls after I rewrote
the code in more boring style.

_Bool
words_do( const char *s, Gopher go ){
  char   c      =  *s;
#if 1  
  if (is_space(c))
    return  words_do( s+1, go );
  if (c)
    return collect_word( s, s, 1, go );
  return 1;
#else
 return
  is_space(c)       ?  words_do( s+1, go )        :
  c                 ?  collect_word( s, s, 1, go ):
  /***************/    1;
#endif
}

static
_Bool
collect_word( const char *s, const char *r, _Bool w, Gopher go ){
  char   c      =  *s;
#if 1  
  if (c == 0) {
    go->f( go, r, s );
    return  w;
  }
  if (is_space(c) && w) {
    go->f( go, r, s );
    return words_do( s, go );
  }
  return collect_word( s+1, r, w ^ c == '"', go );
#else
 return
  c == 0            ?  go->f( go, r, s ),  w                 :
  is_space(c) && w  ?  go->f( go, r, s ),  words_do( s, go ) :
  /***************/    collect_word( s+1, r, w ^ c == '"', go );
#endif
}