[PATCH] Fix -Wconversion (PR c++/34198)

[Jakub Jelinek <jakub at redhat dot com>]

Hi!

The new -Wconversion and -Wsign-conversion implementation in 4.3 warns even
when the C resp. C++ FE decides to shorten some binary operation (e.g.
bitwise ops) or even for say
unsigned char a;
...
unsigned char b = (int) a;
where the implicit narrowing conversion just undoes previous widening
conversion.  The following patch fixes that by looking through any NOP_EXPRs
in EXPR seeing if the warning would be pointless.

I have tried to use get_unwidened as well, but that results in some warnings
which are desirable to be omitted.  Say for:
signed char sc;
...
signed char i = (int) (unsigned short int) sc;
when using expr = get_unwidened (expr, expr) instead of get_narrower
it doesn't warn, eventhough the implicit conversion from (int) (ushort) sc
to (signed char) really can change the value.  Consider sc = -1, here
(int) (unsigned short int) sc
is 0xffff, while (signed char) 0xffff is -1.  get_unwidened does that,
because the outermost widening conversion is skipped over and then
there are just conversions with equal precision.  With get_narrower
it does the right thing.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2007-11-23  Jakub Jelinek  <jakub@redhat.com>

	PR c++/34198
	* c-common.c (conversion_warning): For INTEGER_TYPE to
	INTEGER_TYPE conversions call get_narrower on expr to avoid
	spurious warnings from binop shortening or when the implicit
	conversion can't change the value.

	* gcc.dg/Wconversion-5.c: New test.
	* g++.dg/Wconversion3.C: New test.

--- gcc/c-common.c.jj	2007-11-20 11:31:10.000000000 +0100
+++ gcc/c-common.c	2007-11-23 09:34:50.000000000 +0100
@@ -1280,6 +1280,14 @@ conversion_warning (tree type, tree expr
       else if (TREE_CODE (TREE_TYPE (expr)) == INTEGER_TYPE
                && TREE_CODE (type) == INTEGER_TYPE)
         {
+	  /* Don't warn about unsigned char y = 0xff, x = (int) y;  */
+	  int uns;
+	  tree orig_expr = expr;
+	  expr = get_narrower (expr, &uns);
+
+	  if (expr == orig_expr)
+	    uns = TYPE_UNSIGNED (TREE_TYPE (expr));
+
           /* Warn for integer types converted to smaller integer types.  */
           if (formal_prec < TYPE_PRECISION (TREE_TYPE (expr))) 
 	    give_warning = true;
@@ -1287,14 +1295,31 @@ conversion_warning (tree type, tree expr
 	  /* When they are the same width but different signedness,
 	     then the value may change.  */
 	  else if ((formal_prec == TYPE_PRECISION (TREE_TYPE (expr))
-		    && TYPE_UNSIGNED (TREE_TYPE (expr)) != TYPE_UNSIGNED (type))
+		    && uns != TYPE_UNSIGNED (type))
 		   /* Even when converted to a bigger type, if the type is
 		      unsigned but expr is signed, then negative values
 		      will be changed.  */
-		   || (TYPE_UNSIGNED (type) && !TYPE_UNSIGNED (TREE_TYPE (expr))))
-	    warning (OPT_Wsign_conversion,
-		     "conversion to %qT from %qT may change the sign of the result",
-		     type, TREE_TYPE (expr));
+		   || (TYPE_UNSIGNED (type) && !uns))
+	    {
+	      if (uns != TYPE_UNSIGNED (TREE_TYPE (expr)))
+		{
+		  /* For signed char s1, s2 = (int) (unsigned char) s1;
+		     get_narrower returns s1, but uns = 1.  Find the
+		     narrowest type with uns == TYPE_UNSIGNED (type).  */
+		  tree unsexpr = orig_expr;
+
+		  while (TREE_CODE (unsexpr) == NOP_EXPR
+			 && unsexpr != expr
+			 && TYPE_UNSIGNED (TREE_TYPE (TREE_OPERAND (unsexpr,
+								    0)))
+			    == uns)
+		    unsexpr = TREE_OPERAND (unsexpr, 0);
+		  expr = unsexpr;
+		}
+	      warning (OPT_Wsign_conversion,
+		       "conversion to %qT from %qT may change the sign of the result",
+		       type, TREE_TYPE (expr));
+	    }
         }
 
       /* Warn for integer types converted to real types if and only if
--- gcc/testsuite/gcc.dg/Wconversion-5.c.jj	2007-11-23 07:45:45.000000000 +0100
+++ gcc/testsuite/gcc.dg/Wconversion-5.c	2007-11-23 08:58:58.000000000 +0100
@@ -0,0 +1,35 @@
+/* PR c++/34198 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -Wconversion" } */
+
+signed char sc;
+unsigned char uc;
+short int ss;
+unsigned short int us;
+int si;
+unsigned int ui;
+
+void test1 (void)
+{
+  int a = uc & 0xff;
+  int b = sc & 0x7f;
+  int c = 0xff & uc;
+  int d = 0x7f & sc;
+  int e = uc & sc;
+  unsigned char f = (int) uc;
+  signed char g = (int) sc;
+  unsigned char h = (unsigned int) (short int) uc;
+  signed char i = (int) (unsigned short int) sc;	/* { dg-warning "may alter its value" } */
+  unsigned char j = (unsigned int) (short int) us;	/* { dg-warning "may alter its value" } */
+  signed char k = (int) (unsigned short int) ss;	/* { dg-warning "may alter its value" } */
+}
+
+void test2 (void)
+{
+  signed char a = (unsigned char) sc;		/* { dg-warning "may change the sign" } */
+  unsigned char b = (signed char) uc;		/* { dg-warning "may change the sign" } */
+  signed char c = (int) (unsigned char) sc;	/* { dg-warning "may change the sign" } */
+  unsigned char d = (int) (signed char) uc;	/* { dg-warning "may change the sign" } */
+  int e = (unsigned int) si;			/* { dg-warning "may change the sign" } */
+  unsigned int f = (int) ui;			/* { dg-warning "may change the sign" } */
+}
--- gcc/testsuite/g++.dg/warn/Wconversion3.C.jj	2007-11-23 07:52:16.000000000 +0100
+++ gcc/testsuite/g++.dg/warn/Wconversion3.C	2007-11-23 09:00:23.000000000 +0100
@@ -0,0 +1,35 @@
+// PR c++/34198
+// { dg-do compile }
+// { dg-options "-O2 -Wconversion -Wsign-conversion" }
+
+signed char sc;
+unsigned char uc;
+short int ss;
+unsigned short int us;
+int si;
+unsigned int ui;
+
+void test1 (void)
+{
+  int a = uc & 0xff;
+  int b = sc & 0x7f;
+  int c = 0xff & uc;
+  int d = 0x7f & sc;
+  int e = uc & sc;
+  unsigned char f = (int) uc;
+  signed char g = (int) sc;
+  unsigned char h = (unsigned int) (short int) uc;
+  signed char i = (int) (unsigned short int) sc;	// { dg-warning "may alter its value" }
+  unsigned char j = (unsigned int) (short int) us;	// { dg-warning "may alter its value" }
+  signed char k = (int) (unsigned short int) ss;	// { dg-warning "may alter its value" }
+}
+
+void test2 (void)
+{
+  signed char a = (unsigned char) sc;		// { dg-warning "may change the sign" }
+  unsigned char b = (signed char) uc;		// { dg-warning "may change the sign" }
+  signed char c = (int) (unsigned char) sc;	// { dg-warning "may change the sign" }
+  unsigned char d = (int) (signed char) uc;	// { dg-warning "may change the sign" }
+  int e = (unsigned int) si;			// { dg-warning "may change the sign" }
+  unsigned int f = (int) ui;			// { dg-warning "may change the sign" }
+}

	Jakub