m4-1.4.2: speed up parsing

[Andreas Schwab]

This patch speeds up m4 by 20% in its typical use by autoconf.  It was
tested with the configure script of libtool 1.9b.  Before the patch we
have the following top 5 functions:

  %   cumulative   self              self     total           
 time   seconds   seconds    calls   s/call   s/call  name    
 50.33     26.54    26.54 833006410     0.00     0.00  next_char
 43.97     49.73    23.19 12563045     0.00     0.00  next_token
  2.09     50.83     1.10   413305     0.00     0.00  expand_token
  0.86     51.28     0.46 19684516     0.00     0.00  peek_input
  0.75     51.68     0.39   251578     0.00     0.00  expand_user_macro

After the patch we have this profile:

  %   cumulative   self              self     total           
 time   seconds   seconds    calls   s/call   s/call  name    
 91.77     33.86    33.86 12563045     0.00     0.00  next_token
  3.01     34.97     1.11   413305     0.00     0.00  expand_token
  1.25     35.43     0.46 19684516     0.00     0.00  peek_input
  1.15     35.86     0.43   251578     0.00     0.00  expand_user_macro
[...]
  0.03     36.79     0.01   290882     0.00     0.00  next_char_1

As you can see, 99.9% of all calls to next_char are reading from a string,
so we want that case to be fast.  In the test case the total runtime
decreased from 56 to 44 seconds.

Andreas.

2004-09-05  Andreas Schwab  <address@hidden>

        * src/input.c (next_char_1): Renamed from next_char.
        (next_char): New macro.

--- src/input.c.~1.1.1.1.~      2000-02-17 04:03:19.000000000 +0100
+++ src/input.c 2004-09-05 17:44:00.622015557 +0200
@@ -424,10 +424,17 @@ peek_input (void)
 | they do not get wrong, due to lookahead.  The token consisting of a     |
 | newline alone is taken as belonging to the line it ends, and the current |
 | line number is not incremented until the next character is read.        |
+| 99.9% of all calls will read from a string, so factor that out into a    |
+| macro for speed.                                                        |
 `-------------------------------------------------------------------------*/
 
+#define next_char() \
+  (isp && isp->type == INPUT_STRING && isp->u.u_s.string[0] \
+   ? *isp->u.u_s.string++                                  \
+   : next_char_1 ())
+
 static int
-next_char (void)
+next_char_1 (void)
 {
   register int ch;
 

-- 
Andreas Schwab, SuSE Labs, address@hidden
SuSE Linux AG, Maxfeldstraße 5, 90409 Nürnberg, Germany
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"And now for something completely different."