Re: [PATCH] Add a -Wcast-align=strict warning
On Wed, 13 Sep 2017, Bernd Edlinger wrote: > So you suggest to use min_align_of_type instead of TYPE_ALIGN. > > That would also make sense for the traditional -Wcast-align on > strict-alignment targets, right? Yes, and yes (though I'm not sure if any strict-alignment targets have this peculiarity). -- Joseph S. Myers jos...@codesourcery.com
Re: [PATCH] Add a -Wcast-align=strict warning
On 09/13/17 22:03, Joseph Myers wrote:
> On Wed, 13 Sep 2017, Bernd Edlinger wrote:
>
>> On 09/13/17 19:06, Joseph Myers wrote:
>>> What does this warning do in cases where a type has different alignments
>>> inside and outside structs? I'm thinking of something like
>>>
>>> struct s { long long x; } *p;
>>> /* ... */
>>> (long long *)p
>>>
>>> on 32-bit x86 - where long long's preferred alignment is 8 bytes, but in
>>> structures it's 4 bytes. (Likewise for double in place of long long.) I
>>> think a warning for a (long long *)p cast might be surprising in that
>>> case.
>>>
>>
>> Well, yes this does get a warning. But doesn't that cast then violate
>> the underlying alignment requirement of long long* ?
>
> That's the difference between preferred alignment (__alignof__) and
> alignment required in all contexts (C11 _Alignof). The above seems valid,
> just like it's valid to take the address of a long long struct element.
> That is, the alignment for the target of a pointer to long long is really
> 4 bytes here, even though the alignment for a standalone long long object
> is 8 bytes. And there's a case for the warning to look at the required
> alignment in all contexts, not TYPE_ALIGN.
>
So you suggest to use min_align_of_type instead of TYPE_ALIGN.
That would also make sense for the traditional -Wcast-align on
strict-alignment targets, right?
Thanks,
Bernd.
Re: [PATCH] Add a -Wcast-align=strict warning
On Wed, 13 Sep 2017, Bernd Edlinger wrote:
> On 09/13/17 19:06, Joseph Myers wrote:
> > What does this warning do in cases where a type has different alignments
> > inside and outside structs? I'm thinking of something like
> >
> > struct s { long long x; } *p;
> > /* ... */
> > (long long *)p
> >
> > on 32-bit x86 - where long long's preferred alignment is 8 bytes, but in
> > structures it's 4 bytes. (Likewise for double in place of long long.) I
> > think a warning for a (long long *)p cast might be surprising in that
> > case.
> >
>
> Well, yes this does get a warning. But doesn't that cast then violate
> the underlying alignment requirement of long long* ?
That's the difference between preferred alignment (__alignof__) and
alignment required in all contexts (C11 _Alignof). The above seems valid,
just like it's valid to take the address of a long long struct element.
That is, the alignment for the target of a pointer to long long is really
4 bytes here, even though the alignment for a standalone long long object
is 8 bytes. And there's a case for the warning to look at the required
alignment in all contexts, not TYPE_ALIGN.
--
Joseph S. Myers
jos...@codesourcery.com
Re: [PATCH] Add a -Wcast-align=strict warning
On 09/13/17 19:06, Joseph Myers wrote:
> What does this warning do in cases where a type has different alignments
> inside and outside structs? I'm thinking of something like
>
> struct s { long long x; } *p;
> /* ... */
> (long long *)p
>
> on 32-bit x86 - where long long's preferred alignment is 8 bytes, but in
> structures it's 4 bytes. (Likewise for double in place of long long.) I
> think a warning for a (long long *)p cast might be surprising in that
> case.
>
Well, yes this does get a warning. But doesn't that cast then violate
the underlying alignment requirement of long long* ?
Of course there is probably a reason why -Wcast-align is not enabled by
default, and likewise this warning emits a fair amount of false
positives, but nevertheless I think it is often worth looking at the
places where this warning flags a possible alignment issue.
However, neither -Wcast-align nor -Wcast-align=strict are enabled unless
explicitly requested.
Bernd.
Re: [PATCH] Add a -Wcast-align=strict warning
What does this warning do in cases where a type has different alignments
inside and outside structs? I'm thinking of something like
struct s { long long x; } *p;
/* ... */
(long long *)p
on 32-bit x86 - where long long's preferred alignment is 8 bytes, but in
structures it's 4 bytes. (Likewise for double in place of long long.) I
think a warning for a (long long *)p cast might be surprising in that
case.
--
Joseph S. Myers
jos...@codesourcery.com
[PING] [PATCH] Add a -Wcast-align=strict warning
Ping...
On 09/04/17 10:07, Bernd Edlinger wrote:
> 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.
>
[PATCH] Add a -Wcast-align=strict warning
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.
gcc:
2017-09-03 Bernd Edlinger
* common.opt (Wcast-align=strict): New warning option.
* doc/invoke.texi: Document -Wcast-align=strict.
c:
2017-09-03 Bernd Edlinger
* c-typeck.c (build_c_cast): Implement -Wcast-align=strict.
cp:
2017-09-03 Bernd Edlinger
* typeck.c (build_reinterpret_cast_1,
build_const_cast_1): Implement -Wcast-align=strict.
testsuite:
2017-09-03 Bernd Edlinger
* c-c++-common/Wcast-align.c: New test.
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 *) for array-to-pointer decay, instead of [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
