[PATCH] Add a -Wcast-align=strict warning

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

Hi,

as you know we have a -Wcast-align warning which works only for
STRICT_ALIGNMENT targets.  But occasionally it would be nice to be
able to switch this warning on even for other targets.

Therefore I would like to add a strict version of this option
which can be invoked with -Wcast-align=strict.  With the only
difference that it does not depend on STRICT_ALIGNMENT.

I used the code from check_effective_target_non_strict_align
in target-supports.exp for the first version of the test case,
where we have this:

return [check_no_compiler_messages non_strict_align assembly {
     char *y;
     typedef char __attribute__ ((__aligned__(__BIGGEST_ALIGNMENT__))) c;
     c *z;
     void foo(void) { z = (c *) y; }
} "-Wcast-align"]

... and to my big surprise it did _not_ work for C++ as-is,
because same_type_p considers differently aligned types identical,
and therefore cp_build_c_cast tries the conversion first via a
const_cast which succeeds, but did not emit the cast-align warning
in this case.

As a work-around I had to check the alignment in build_const_cast_1
as well.


Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
Is it OK for trunk?


Thanks
Bernd.

Attachment: changelog-cast-align.txt
Description: changelog-cast-align.txt

Index: gcc/c/c-typeck.c
===================================================================
--- gcc/c/c-typeck.c	(revision 251617)
+++ gcc/c/c-typeck.c	(working copy)
@@ -5578,7 +5578,7 @@ build_c_cast (location_t loc, tree type, tree expr
 	}
 
       /* Warn about possible alignment problems.  */
-      if (STRICT_ALIGNMENT
+      if ((STRICT_ALIGNMENT || warn_cast_align == 2)
 	  && TREE_CODE (type) == POINTER_TYPE
 	  && TREE_CODE (otype) == POINTER_TYPE
 	  && TREE_CODE (TREE_TYPE (otype)) != VOID_TYPE
Index: gcc/common.opt
===================================================================
--- gcc/common.opt	(revision 251617)
+++ gcc/common.opt	(working copy)
@@ -564,6 +564,10 @@ Wcast-align
 Common Var(warn_cast_align) Warning
 Warn about pointer casts which increase alignment.
 
+Wcast-align=strict
+Common Var(warn_cast_align,2) Warning
+Warn about pointer casts which increase alignment.
+
 Wcpp
 Common Var(warn_cpp) Init(1) Warning
 Warn when a #warning directive is encountered.
Index: gcc/cp/typeck.c
===================================================================
--- gcc/cp/typeck.c	(revision 251617)
+++ gcc/cp/typeck.c	(working copy)
@@ -7265,8 +7265,8 @@ build_reinterpret_cast_1 (tree type, tree expr, bo
 					       complain))
 	return error_mark_node;
       /* Warn about possible alignment problems.  */
-      if (STRICT_ALIGNMENT && warn_cast_align
-          && (complain & tf_warning)
+      if ((STRICT_ALIGNMENT || warn_cast_align == 2)
+	  && (complain & tf_warning)
 	  && !VOID_TYPE_P (type)
 	  && TREE_CODE (TREE_TYPE (intype)) != FUNCTION_TYPE
 	  && COMPLETE_TYPE_P (TREE_TYPE (type))
@@ -7273,7 +7273,7 @@ build_reinterpret_cast_1 (tree type, tree expr, bo
 	  && COMPLETE_TYPE_P (TREE_TYPE (intype))
 	  && TYPE_ALIGN (TREE_TYPE (type)) > TYPE_ALIGN (TREE_TYPE (intype)))
 	warning (OPT_Wcast_align, "cast from %qH to %qI "
-                 "increases required alignment of target type", intype, type);
+		 "increases required alignment of target type", intype, type);
 
       /* We need to strip nops here, because the front end likes to
 	 create (int *)&a for array-to-pointer decay, instead of &a[0].  */
@@ -7447,6 +7447,14 @@ build_const_cast_1 (tree dst_type, tree expr, tsub
 		 the user is making a potentially unsafe cast.  */
 	      check_for_casting_away_constness (src_type, dst_type,
 						CAST_EXPR, complain);
+	      /* ??? comp_ptr_ttypes_const ignores TYPE_ALIGN.  */
+	      if ((STRICT_ALIGNMENT || warn_cast_align == 2)
+		  && (complain & tf_warning)
+		  && TYPE_ALIGN (TREE_TYPE (dst_type))
+		     > TYPE_ALIGN (TREE_TYPE (src_type)))
+		warning (OPT_Wcast_align, "cast from %qH to %qI "
+			 "increases required alignment of target type",
+			 src_type, dst_type);
 	    }
 	  if (reference_type)
 	    {
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 251617)
+++ gcc/doc/invoke.texi	(working copy)
@@ -266,7 +266,8 @@ Objective-C and Objective-C++ Dialects}.
 -Wno-attributes  -Wbool-compare  -Wbool-operation @gol
 -Wno-builtin-declaration-mismatch @gol
 -Wno-builtin-macro-redefined  -Wc90-c99-compat  -Wc99-c11-compat @gol
--Wc++-compat  -Wc++11-compat  -Wc++14-compat  -Wcast-align  -Wcast-qual  @gol
+-Wc++-compat  -Wc++11-compat  -Wc++14-compat  @gol
+-Wcast-align  -Wcast-align=strict  -Wcast-qual  @gol
 -Wchar-subscripts  -Wchkp  -Wcatch-value  -Wcatch-value=@var{n} @gol
 -Wclobbered  -Wcomment  -Wconditionally-supported @gol
 -Wconversion  -Wcoverage-mismatch  -Wno-cpp  -Wdangling-else  -Wdate-time @gol
@@ -5923,6 +5924,12 @@ target is increased.  For example, warn if a @code
 an @code{int *} on machines where integers can only be accessed at
 two- or four-byte boundaries.
 
+@item -Wcast-align=strict
+@opindex Wcast-align=strict
+Warn whenever a pointer is cast such that the required alignment of the
+target is increased.  For example, warn if a @code{char *} is cast to
+an @code{int *} regardless of the target machine.
+
 @item -Wwrite-strings
 @opindex Wwrite-strings
 @opindex Wno-write-strings
Index: gcc/testsuite/c-c++-common/Wcast-align.c
===================================================================
--- gcc/testsuite/c-c++-common/Wcast-align.c	(revision 0)
+++ gcc/testsuite/c-c++-common/Wcast-align.c	(working copy)
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-Wcast-align=strict" } */
+
+typedef char __attribute__ ((__aligned__(__BIGGEST_ALIGNMENT__))) c;
+typedef struct __attribute__ ((__aligned__(__BIGGEST_ALIGNMENT__)))
+{
+  char x;
+} d;
+
+char *x;
+c *y;
+d *z;
+
+void
+foo (void)
+{
+  y = (c *) x;  /* { dg-warning "alignment" } */
+  z = (d *) x;  /* { dg-warning "alignment" } */
+}