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

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

This does the division by zero warning for the C front end only, does
not warn for the unevaluated part of various expressions, and adds a
command line option.

The C++ front end requires more work, since it has appears to have no
concept of skip_evaluation, so I won't do that in this patch.

Bootstrapping x86 Linux.  OK to commit?

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.
 	* 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 17:38:17
@@ -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 17:38:19
@@ -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 17:38:37
@@ -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 17:38:51
@@ -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 (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 (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: 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 10:38:51 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)
+#define DZERO (2.0 - 2.0)
+
+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 17:38:51
@@ -1,2 +1,2 @@
 int i = 7 / 0; /* { dg-error "not constant" } */
-
+	/* { dg-warning "division by zero" "div by zero" { target *-*-* } 1 } */
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 17:39:23
@@ -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
@@ -1773,6 +1773,12 @@ machines.
 @opindex Wcomment
 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
+
+@item -Wdiv-by-zero
+@opindex Wdiv-by-zero
+Warn about compile-time integer division by zero.  Floating point division
+by zero is not warned about, as it can be a legitimate way of obtaining
+infinities and NaNs.
 
 @item -Wformat
 @opindex Wformat