Bison "comparison is always true due to limited range of data type"

[Paul Eggert]

When running "make check" with the latest CVS version of Bison, I
discovered the following failure:

   30. synclines.at:91: testing Prologue synch line...
   synclines.at:91: $CC $CFLAGS $CPPFLAGS -c syncline.c || (exit 1)
   stderr:
   syncline.c:1:2: #error "1"
   synclines.at:91: sed 's/^\([^:]*:[^:.]*\)[.:][^:]*:\(.*\)$/\1:\2/' stderr
   stdout:
   syncline.c:1: #error "1"
   synclines.at:91: test "`cat stdout`" = 'syncline.c:1: #error "1"' || exit 77
   synclines.at:91: bison input.y -o input.c
   synclines.at:91: $CC $CFLAGS $CPPFLAGS -c input.c || (exit 1)
   stderr:
   input.y:2:2: #error "2"
   input.c: In function `yyparse':
   input.c:807: warning: comparison is always true due to limited range of data 
type
   ...

In a way, it's nice that GCC discovers that the comparison is useless
in a toy example, but in real code I don't think it's worth worrying
about any optimization here since the comparison won't be useless.  So
I installed the following patch to work around this problem.

2002-10-13  Paul Eggert  <address@hidden>

        * data/yacc.c (yyparse): Rewrite to avoid "comparison is always
        true due to limited range of data type" warning from GCC.

diff -p -u -r1.17 -r1.18
--- yacc.c      13 Oct 2002 14:25:14 -0000      1.17
+++ yacc.c      14 Oct 2002 06:54:41 -0000      1.18
@@ -918,27 +918,19 @@ yybackup:
       YYDPRINTF ((stderr, "\n"));
     }
 
-  /* Set YYN to the action to take in STATE on seeing token YYCHAR1.
-     Result YYN means
-     - YYN < 0:  Reduce on rule -YYN.
-     - YYN = 0:  Error.
-     - YYN > 0:  Shift to state YYN.  */
+  /* If the proper action on seeing token YYCHAR1 is to reduce or to
+     detect an error, take that action.  */
   yyn += yychar1;
   if (yyn < 0 || YYLAST < yyn || yycheck[yyn] != yychar1)
-    /* Defaulted action (reduction).  */
-    yyn = -yydefact[yystate];
-  else if (yytable[yyn] != YYTABLE_NINF)
-    yyn = yytable[yyn];
-  else
-    yyn = 0;
-
-  if (yyn < 0)
+    goto yydefault;
+  yyn = yytable[yyn];
+  if (yyn <= 0)
     {
+      if (yyn == 0 || yyn == YYTABLE_NINF)
+       goto yyerrlab;
       yyn = -yyn;
       goto yyreduce;
     }
-  else if (yyn == 0)
-    goto yyerrlab;
 
   if (yyn == YYFINAL)
     YYACCEPT;