Add warning options -W[no-]compare-distinct-pointer-types

GCC emits pedwarns unconditionally when comparing pointers of
different types, for example:

  int xdp_context (struct xdp_md *xdp)
    {
        void *data = (void *)(long)xdp->data;
        __u32 *metadata = (void *)(long)xdp->data_meta;
        __u32 ret;

        if (metadata + 1 > data)
          return 0;
        return 1;
   }

  /home/jemarch/foo.c: In function ‘xdp_context’:
  /home/jemarch/foo.c:15:20: warning: comparison of distinct pointer types lacks a cast
         15 |   if (metadata + 1 > data)
                 |                    ^

LLVM supports an option -W[no-]compare-distinct-pointer-types that can
be used in order to enable or disable the emission of such warnings.
It is enabled by default.

This patch adds the same options to GCC.

Documentation and testsuite updated included.
Regtested in x86_64-linu-gnu.
No regressions observed.

gcc/ChangeLog:

	PR c/106537
	* doc/invoke.texi (Option Summary): Mention
	-Wcompare-distinct-pointer-types under `Warning Options'.
	(Warning Options): Document -Wcompare-distinct-pointer-types.

gcc/c-family/ChangeLog:

	PR c/106537
	* c.opt (Wcompare-distinct-pointer-types): New option.

gcc/c/ChangeLog:

	PR c/106537
	* c-typeck.cc (build_binary_op): Warning on comparing distinct
	pointer types only when -Wcompare-distinct-pointer-types.

gcc/testsuite/ChangeLog:

	PR c/106537
	* gcc.c-torture/compile/pr106537-1.c: New test.
	* gcc.c-torture/compile/pr106537-2.c: Likewise.
	* gcc.c-torture/compile/pr106537-3.c: Likewise.

6 files changed, 111 insertions, 3 deletions

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index c7b567b..2242524 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -1935,6 +1935,10 @@ Winvalid-imported-macros
 C++ ObjC++ Var(warn_imported_macros) Warning
 Warn about macros that have conflicting header units definitions.
 
+Wcompare-distinct-pointer-types
+C ObjC Var(warn_compare_distinct_pointer_types) Warning Init(1)
+Warn if pointers of distinct types are compared without a cast.
+
 flang-info-include-translate
 C++ Var(note_include_translate_yes)
 Note #include directives translated to import declarations.
diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index 6f2fff5..e6ddf37 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -12772,7 +12772,7 @@ build_binary_op (location_t location, enum tree_code code,
 	  else
 	    /* Avoid warning about the volatile ObjC EH puts on decls.  */
 	    if (!objc_ok)
-	      pedwarn (location, 0,
+	      pedwarn (location, OPT_Wcompare_distinct_pointer_types,
 		       "comparison of distinct pointer types lacks a cast");
 
 	  if (result_type == NULL_TREE)
@@ -12912,8 +12912,8 @@ build_binary_op (location_t location, enum tree_code code,
 	      int qual = ENCODE_QUAL_ADDR_SPACE (as_common);
 	      result_type = build_pointer_type
 			      (build_qualified_type (void_type_node, qual));
-	      pedwarn (location, 0,
-		       "comparison of distinct pointer types lacks a cast");
+              pedwarn (location, OPT_Wcompare_distinct_pointer_types,
+                       "comparison of distinct pointer types lacks a cast");
 	    }
 	}
       else if (code0 == POINTER_TYPE && null_pointer_constant_p (orig_op1))
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 3380ed8..01aa9ef 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -345,6 +345,7 @@ Objective-C and Objective-C++ Dialects}.
 -Wcast-align  -Wcast-align=strict  -Wcast-function-type  -Wcast-qual
 -Wchar-subscripts
 -Wclobbered  -Wcomment
+-Wcompare-distinct-pointer-types
 -Wno-complain-wrong-lang
 -Wconversion  -Wno-coverage-mismatch  -Wno-cpp
 -Wdangling-else  -Wdangling-pointer  -Wdangling-pointer=@var{n}
@@ -9106,6 +9107,11 @@ The latter front end diagnoses
 @samp{f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran},
 which may be disabled with @option{-Wno-complain-wrong-lang}.
 
+@opindex Wcompare-distinct-pointer-types
+@item -Wcompare-distinct-pointer-types @r{(C and Objective-C only)}
+Warn if pointers of distinct types are compared without a cast.  This
+warning is enabled by default.
+
 @opindex Wconversion
 @opindex Wno-conversion
 @item -Wconversion
diff --git a/gcc/testsuite/gcc.c-torture/compile/pr106537-1.c b/gcc/testsuite/gcc.c-torture/compile/pr106537-1.c
new file mode 100644
index 0000000..3f3b065
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr106537-1.c
@@ -0,0 +1,34 @@
+/* { dg-do compile }
+   { dg-options "-O0" }
+   This testcase checks that warn_compare_distinct_pointer_types is enabled by
+   default.  */
+
+typedef int __u32;
+
+struct xdp_md
+{
+  char *data;
+  char *data_meta;
+};
+
+int xdp_context (struct xdp_md *xdp)
+{
+  void *data = (void *)(long)xdp->data;
+  __u32 *metadata = (void *)(long)xdp->data_meta;
+  __u32 ret;
+
+  if (metadata + 1 > data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 1;
+  if (metadata + 1 >= data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 2;
+  if (metadata + 1 < data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 3;
+  if (metadata + 1 <= data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 4;
+  if (metadata + 1 == data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 5;
+  if (metadata + 1 != data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 5;
+
+  return 1;
+}
diff --git a/gcc/testsuite/gcc.c-torture/compile/pr106537-2.c b/gcc/testsuite/gcc.c-torture/compile/pr106537-2.c
new file mode 100644
index 0000000..6876adf
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr106537-2.c
@@ -0,0 +1,32 @@
+/* { dg-do compile } */
+/* { dg-options "-Wcompare-distinct-pointer-types" } */
+
+typedef int __u32;
+
+struct xdp_md
+{
+  char *data;
+  char *data_meta;
+};
+
+int xdp_context (struct xdp_md *xdp)
+{
+  void *data = (void *)(long)xdp->data;
+  __u32 *metadata = (void *)(long)xdp->data_meta;
+  __u32 ret;
+
+  if (metadata + 1 > data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 1;
+  if (metadata + 1 >= data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 2;
+  if (metadata + 1 < data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 3;
+  if (metadata + 1 <= data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 4;
+  if (metadata + 1 == data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 5;
+  if (metadata + 1 != data) /* { dg-warning "comparison of distinct pointer types" } */
+    return 5;
+
+  return 1;
+}
diff --git a/gcc/testsuite/gcc.c-torture/compile/pr106537-3.c b/gcc/testsuite/gcc.c-torture/compile/pr106537-3.c
new file mode 100644
index 0000000..73c9b25
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr106537-3.c
@@ -0,0 +1,32 @@
+/* { dg-do compile } */
+/* { dg-options "-O0 -Wno-compare-distinct-pointer-types" } */
+
+typedef int __u32;
+
+struct xdp_md
+{
+  char *data;
+  char *data_meta;
+};
+
+int xdp_context (struct xdp_md *xdp)
+{
+  void *data = (void *)(long)xdp->data;
+  __u32 *metadata = (void *)(long)xdp->data_meta;
+  __u32 ret;
+
+  if (metadata + 1 > data) /* There shouldn't be a warning here.  */
+    return 1;
+  if (metadata + 1 >= data) /* There shouldn't be a warning here. */
+    return 2;
+  if (metadata + 1 < data) /* There shouldn't be a warning here.  */
+    return 3;
+  if (metadata + 1 <= data) /* There shouldn't be a warning here.  */
+    return 4;
+  if (metadata + 1 == data) /* There shouldn't be a warning here.  */
+    return 5;
+  if (metadata + 1 != data) /* There shouldn't be a warning here.  */
+    return 5;
+
+  return 1;
+}