[C PATCH] Abate -Wformat-signedness warnings (PR c/65040)

[Marek Polacek <polacek at redhat dot com>]

This patch tries to avoid emitting -Wformat-signedness warnings in cases
where integer promotions are performed on unsigned values such as:
  unsigned short arg = N;
  printf ("%u\n", arg);
here we can know that the promoted value of arg is in range <0, USHRT_MAX>, or
  printf ("%" PRIx8, (uint8_t) val);
where the warning is just irritating.

Fixed by bailing out when we see a value of unsigned type that has been
promoted and the conversion specifier is of unsigned type.

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

2015-02-12  Marek Polacek  <polacek@redhat.com>

	PR c/65040
	* c-format.c (check_format_types): Don't warn about different
	signedness if the original value is in the range of WANTED_TYPE.

	* c-c++-common/pr65040.c: New test.

diff --git gcc/c-family/c-format.c gcc/c-family/c-format.c
index faaca09..2f49b2d 100644
--- gcc/c-family/c-format.c
+++ gcc/c-family/c-format.c
@@ -2456,8 +2456,7 @@ check_format_types (location_t loc, format_wanted_type *types)
       cur_type = TYPE_MAIN_VARIANT (cur_type);
 
       /* Check whether the argument type is a character type.  This leniency
-	 only applies to certain formats, flagged with 'c'.
-      */
+	 only applies to certain formats, flagged with 'c'.  */
       if (types->char_lenient_flag)
 	char_type_flag = (cur_type == char_type_node
 			  || cur_type == signed_char_type_node
@@ -2486,6 +2485,20 @@ check_format_types (location_t loc, format_wanted_type *types)
 	      ? wanted_type == c_common_unsigned_type (cur_type)
 	      : wanted_type == c_common_signed_type (cur_type)))
 	continue;
+      /* Don't warn about differences merely in signedness if we know
+	 that the current type is integer-promoted and its original type
+	 was unsigned such as that it is in the range of WANTED_TYPE.  */
+      if (TREE_CODE (wanted_type) == INTEGER_TYPE
+	  && TREE_CODE (cur_type) == INTEGER_TYPE
+	  && warn_format_signedness
+	  && TYPE_UNSIGNED (wanted_type)
+	  && TREE_CODE (cur_param) == NOP_EXPR)
+	{
+	  tree t = TREE_TYPE (TREE_OPERAND (cur_param, 0));
+	  if (TYPE_UNSIGNED (t)
+	      && cur_type == lang_hooks.types.type_promotes_to (t))
+	    continue;
+	}
       /* Likewise, "signed char", "unsigned char" and "char" are
 	 equivalent but the above test won't consider them equivalent.  */
       if (wanted_type == char_type_node
diff --git gcc/testsuite/c-c++-common/pr65040.c gcc/testsuite/c-c++-common/pr65040.c
index e69de29..80da8f9 100644
--- gcc/testsuite/c-c++-common/pr65040.c
+++ gcc/testsuite/c-c++-common/pr65040.c
@@ -0,0 +1,21 @@
+/* PR c/65040 */
+/* { dg-do compile } */
+/* { dg-options "-Wformat -Wformat-signedness" } */
+
+unsigned char uc;
+signed char sc;
+unsigned short us;
+signed short ss;
+unsigned int u;
+int i;
+
+void
+foo (void)
+{
+  __builtin_printf ("%u\n", uc); /* { dg-bogus "expects argument of type" } */
+  __builtin_printf ("%u\n", sc); /* { dg-warning "expects argument of type" } */
+  __builtin_printf ("%u\n", us); /* { dg-bogus "expects argument of type" } */
+  __builtin_printf ("%u\n", ss); /* { dg-warning "expects argument of type" } */
+  __builtin_printf ("%u\n", u); /* { dg-bogus "expects argument of type" } */
+  __builtin_printf ("%u\n", i); /* { dg-warning "expects argument of type" } */
+}

	Marek