Re: [RFA] Close PR 150: division by zero [take 2]

[Neil Booth <neil at daikokuya dot demon dot co dot uk>]

Joseph S. Myers wrote:-

> This doesn't test warn_div_by_zero.

Sigh.

> Above you defaulted this option to on, but here you document it as off by
> default and enabled by -Wall (by virtue of its position above the entry
> for -Wall that says it enables the above warning options).

IMO that whole warnings option page is hard to read, and not
self-consistent (I'm not volounteering to fix it, though).  I
originally followed the example of -Wmultichar, so that was another
violator.  I have fixed it below, too, and followed the example and
wording of -Wlong-long instead.  However, I don't think -Wlong-long is
consistent with the rest of the page either.  If you're not happy with
the doc part after this, then I'd ask you to do what you see as
correct rather than us batting this patch backwards and forwards.

Anyway, OK, at last?

Neil.

	* c-common.c (warn_div_by_zero): New.
	* c-common.h (warn_div_by_zero): New.
	* c-decl.c (c_decode_option): Take it on the command line.
 	* c-typeck.c (build_binary_op): Warn about division by zero.
	* doc/invoke.texi: Document the new command line option, fix
	documentation of -Wmultichar.
 	* testsuite/gcc.dg/divbyzero.c: New tests.
 	* testsuite/gcc.dg/noncompile/20010524-1.c: Update.

Index: c-common.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/c-common.c,v
retrieving revision 1.264
diff -u -p -r1.264 c-common.c
--- c-common.c	2001/10/17 09:40:21	1.264
+++ c-common.c	2001/10/22 19:57:25
@@ -194,6 +194,9 @@ int flag_short_wchar;
 
 int warn_sequence_point;
 
+/* Nonzero means to warn about compile-time division by zero.  */
+int warn_div_by_zero = 1;
+
 /* The elements of `ridpointers' are identifier nodes for the reserved
    type names and storage classes.  It is indexed by a RID_... value.  */
 tree *ridpointers;
Index: c-common.h
===================================================================
RCS file: /cvs/gcc/gcc/gcc/c-common.h,v
retrieving revision 1.90
diff -u -p -r1.90 c-common.h
--- c-common.h	2001/10/09 14:03:10	1.90
+++ c-common.h	2001/10/22 19:57:28
@@ -401,6 +401,9 @@ extern int warn_missing_format_attribute
 
 extern int warn_pointer_arith;
 
+/* Nonzero means to warn about compile-time division by zero.  */
+extern int warn_div_by_zero;
+
 /* Nonzero means do some things the same way PCC does.  */
 
 extern int flag_traditional;
Index: c-decl.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/c-decl.c,v
retrieving revision 1.256
diff -u -p -r1.256 c-decl.c
--- c-decl.c	2001/10/20 10:03:49	1.256
+++ c-decl.c	2001/10/22 19:57:49
@@ -750,6 +750,10 @@ c_decode_option (argc, argv)
     warn_multichar = 1;
   else if (!strcmp (p, "-Wno-multichar"))
     warn_multichar = 0;
+  else if (!strcmp (p, "-Wdiv-by-zero"))
+    warn_div_by_zero = 1;
+  else if (!strcmp (p, "-Wno-div-by-zero"))
+    warn_div_by_zero = 0;
   else if (!strcmp (p, "-Wunknown-pragmas"))
     /* Set to greater than 1, so that even unknown pragmas in system
        headers will be warned about.  */
Index: c-typeck.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/c-typeck.c,v
retrieving revision 1.141
diff -u -p -r1.141 c-typeck.c
--- c-typeck.c	2001/10/11 03:15:23	1.141
+++ c-typeck.c	2001/10/22 19:58:05
@@ -1973,6 +1973,11 @@ build_binary_op (code, orig_op0, orig_op
     case FLOOR_DIV_EXPR:
     case ROUND_DIV_EXPR:
     case EXACT_DIV_EXPR:
+      /* Floating point division by zero is a legitimate way to obtain
+	 infinities and NaNs.  */
+      if (warn_div_by_zero && skip_evaluation == 0 && integer_zerop (op1))
+	warning ("division by zero");
+
       if ((code0 == INTEGER_TYPE || code0 == REAL_TYPE
 	   || code0 == COMPLEX_TYPE)
 	  && (code1 == INTEGER_TYPE || code1 == REAL_TYPE
@@ -2026,6 +2031,9 @@ build_binary_op (code, orig_op0, orig_op
 
     case TRUNC_MOD_EXPR:
     case FLOOR_MOD_EXPR:
+      if (warn_div_by_zero && skip_evaluation == 0 && integer_zerop (op1))
+	warning ("division by zero");
+
       if (code0 == INTEGER_TYPE && code1 == INTEGER_TYPE)
 	{
 	  /* Although it would be tempting to shorten always here, that loses
Index: doc/invoke.texi
===================================================================
RCS file: /cvs/gcc/gcc/gcc/doc/invoke.texi,v
retrieving revision 1.65
diff -u -p -r1.65 invoke.texi
--- invoke.texi	2001/10/22 07:42:23	1.65
+++ invoke.texi	2001/10/22 19:58:40
@@ -208,7 +208,7 @@ in the following sections.
 -fsyntax-only  -pedantic  -pedantic-errors @gol
 -w  -W  -Wall  -Waggregate-return @gol
 -Wcast-align  -Wcast-qual  -Wchar-subscripts  -Wcomment @gol
--Wconversion  -Wdisabled-optimization  -Werror @gol
+-Wconversion  -Wdisabled-optimization  -Wdiv-by-zero  -Werror @gol
 -Wfloat-equal  -Wformat  -Wformat=2 @gol
 -Wformat-nonliteral  -Wformat-security @gol
 -Wimplicit  -Wimplicit-int  @gol
@@ -1868,12 +1868,6 @@ int a[2][2] = @{ 0, 1, 2, 3 @};
 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
 @end smallexample
 
-@item -Wmultichar
-@opindex Wmultichar
-Warn if a multicharacter constant (@samp{'FOOF'}) is used.  Usually they
-indicate a typo in the user's code, as they have implementation-defined
-values, and should not be used in portable code.
-
 @item -Wparentheses
 @opindex Wparentheses
 Warn if parentheses are omitted in certain contexts, such
@@ -2128,6 +2122,22 @@ All of the above @samp{-W} options combi
 warnings about constructions that some users consider questionable, and
 that are easy to avoid (or modify to prevent the warning), even in
 conjunction with macros.
+
+@item -Wdiv-by-zero
+@opindex Wno-div-by-zero
+@opindex Wdiv-by-zero
+Warn about compile-time integer division by zero.  This is default.  To
+inhibit the warning messages, use @option{-Wno-div-by-zero}.  Floating
+point division by zero is not warned about, as it can be a legitimate
+way of obtaining infinities and NaNs.
+
+@item -Wmultichar
+@opindex Wno-multichar
+@opindex Wmultichar
+Warn if a multicharacter constant (@samp{'FOOF'}) is used.  This is
+default.  To inhibit the warning messages, use @option{-Wno-multichar}.
+Usually they indicate a typo in the user's code, as they have
+implementation-defined values, and should not be used in portable code.
 
 @item -Wsystem-headers
 @opindex Wsystem-headers
Index: testsuite/gcc.dg/divbyzero.c
===================================================================
RCS file: divbyzero.c
diff -N divbyzero.c
--- /dev/null	Tue May  5 13:32:27 1998
+++ divbyzero.c	Mon Oct 22 12:58:40 2001
@@ -0,0 +1,22 @@
+/* Copyright (C) 2001 Free Software Foundation, Inc.  */
+
+/* { dg-do compile } */
+
+/* Source: Neil Booth, Oct 22 2001.  PR 150 - warn about division by
+   zero.  */
+
+#define ZERO (4 - 6 + 2)
+
+int main (int argc, char *argv[])
+{
+  int w = argc % ZERO;		/* { dg-warning "division by zero" } */
+  int x = argc / 0;		/* { dg-warning "division by zero" } */
+  int y = argc / ZERO;		/* { dg-warning "division by zero" } */
+
+  double z = 0.0 / 0.0	;	/* { dg-bogus "division by zero" } */
+  w = (ZERO ? y / ZERO : x);	/* { dg-bogus "division by zero" } */
+  x = (ZERO ? argc % ZERO: x);  /* { dg-bogus "division by zero" } */
+
+  return 0;
+}
Index: testsuite/gcc.dg/noncompile/20010524-1.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/testsuite/gcc.dg/noncompile/20010524-1.c,v
retrieving revision 1.2
diff -u -p -r1.2 20010524-1.c
--- 20010524-1.c	2001/05/25 06:34:16	1.2
+++ 20010524-1.c	2001/10/22 19:58:40
@@ -1,2 +1,2 @@
 int i = 7 / 0; /* { dg-error "not constant" } */
-
+	/* { dg-warning "division by zero" "div by zero" { target *-*-* } 1 } */