[PATCH]: Implementation of -Wcast-qual
Roman Divacky
rdivacky at vlakno.cz
Thu Nov 20 01:41:29 PST 2014
Patch hopefully addressing all the concerns attached.
On Wed, Nov 19, 2014 at 10:05:37AM -0800, David Blaikie wrote:
> On Tue, Nov 18, 2014 at 2:05 PM, Roman Divacky <rdivacky at vlakno.cz> wrote:
>
> > New patch attached. I've checked the behaviour against gcc5 and we warn on
> > the
> > same cases.
> >
>
> Great! Thanks so much for your patience & diligence. Does GCC mention the
> same types as Clang? (I notice in some of your examples... oh, you don't
> have such an example. What does GCC and Clang do for "const char **" ->
> "char **"? It looks like, judging by the "int**" -> "const int **" warning,
> we might print just the inner type there as well ("cast from const char* to
> char*") - is that what GCC does? Or does it print the full outer types
> always? Is your patch's behavior going to be better for users (probably,
> but I'm not sure).
>
>
> > For the const int **bahc = (const int **)bah; case gcc5 emits a warning
> > with
> > a different warning (to be safe all intermediate pointers in cast from
> > 'int **'
> > to 'volatile int **' must be 'const' qualified [-Wcast-qual]). Do you think
> > it's worth adding similarly worded warning for that case or is the
> > "dropping
> > const" ok?
> >
>
> I think it's OK, but confusing - it'd be good to make it better, but
> possibly as a follow-up patch. Could you include FIXMEs in the test case
> for both the const and volatile cases for this showing what GCC does (or
> otherwise suggesting better wording). I'm assuming in even the volatile
> case, the correct (and GCC5) text is that you need to add const on the
> intermediate pointers?
>
> Actually, now that I look at the test case, it seems a bit worse - it's not
> printing the top-level type in this case (int**->const int**) it's printing
> the nested type (int*->const int*) and saying it drops const, which is even
> more confusing... hrm. So maybe it makes sense to fix this in the current
> patch rather than a follow-up, given how confusing it is.
>
> One thing to do with this current patch, would be to use a %select rather
> than %2 and %3 being directly substituted (having to add that 's' is a bit
> weird). Something like this:
>
> Warning<"cast from %0 to %1 drops %select{const and volatile
> qualifiers|const qualifier|volatile qualifier}2">
>
> int qualifiers = 0;
> if (const && volatile)
> qualifiers = 0;
> else if (const)
> qualifiers = 1;
> else if (volatile)
> qualifiers = 2;
>
> etc.
>
> & doing the better "all intermediate pointers in cast" thing shouldn't be
> too hard, right? Just another warning in the .td file, plus a condition
> before all that to say "if !const && !volatile <do the other warning>" but
> you'll need to get those types right (providing the top level types, not
> the nested types, especially in that instance).
>
>
>
>
> >
> > On Tue, Nov 18, 2014 at 09:27:17AM -0800, David Blaikie wrote:
> > > The last line should have a warning on it (and assuming the
> > > expected-warning comments in the test are correct, it doesn't):
> > >
> > > GCC 4.9:
> > >
> > > warning: cast from type 'int**' to type 'const int**' casts away
> > qualifiers
> > > [-Wcast-qual]
> > > const int **bahc = (const int **)bah;
> > >
> > > On Tue, Nov 18, 2014 at 6:11 AM, Roman Divacky <rdivacky at vlakno.cz>
> > wrote:
> > >
> > > > Scrap the previous patch. It doesnt work when we add qualifiers. The
> > new
> > > > version is fixed.
> > > >
> > > > On Tue, Nov 18, 2014 at 12:49:59PM +0100, Roman Divacky wrote:
> > > > > Perhaps like this?
> > > > >
> > > > > On Fri, Nov 14, 2014 at 01:17:38PM -0800, David Blaikie wrote:
> > > > > > Yep. Though the diagnostic for dropping const and volatile is a
> > bit off
> > > > > > ("drops const volatile qualifier" should probably read "drops
> > const and
> > > > > > volatile qualifiers"? (maybe you'll need a %select for this - you
> > could
> > > > > > probably use a %select for the const and volatile separately too
> > if you
> > > > > > like)). What does GCC do here? Does it warn on dropping volatile at
> > > > all?
> > > > > >
> > > > > > Chatting to Richard over lunch he mentioned an interesting case
> > where
> > > > we
> > > > > > might want to warn:
> > > > > >
> > > > > > int **x;
> > > > > > auto y = (const int **)x;
> > > > > >
> > > > > > which, if it were a static_cast, would warn:
> > > > > >
> > > > > > cast.cpp:2:10: error: static_cast from 'int **' to 'const int
> > **' is
> > > > not
> > > > > > allowed
> > > > > > auto y = static_cast<const int **>(x);
> > > > > > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > > > >
> > > > > > I'm just not sure we'll be able to get a good diagnostic in both
> > these
> > > > > > cases. But as I think/type this out I think:
> > > > > >
> > > > > > We should just use the same machinery that powers static_cast here,
> > > > > > (Richard mentioned there should be a way to test whether a
> > conversion
> > > > is a
> > > > > > "qualification conversion" which is the description of the valid
> > > > implicit
> > > > > > qualification changes, and not the above const-changing cases))
> > and we
> > > > > > should teach the machinery to give us enough information to create
> > good
> > > > > > diagnostics - telling the user where the missing const, volatile,
> > etc,
> > > > is.
> > > > > >
> > > > > > Sorry to go through so many iterations - it didn't occur to me
> > until
> > > > > > Richard mentioned it that there was this more general approach.
> > > > > >
> > > > > > (wonder what GCC does here? - hmm, looks like it gets the "int** ->
> > > > const
> > > > > > int**" right: cast from type ???int**??? to type ???const int**???
> > > > casts away
> > > > > > qualifiers)
> > > > > >
> > > > > > On Fri, Nov 14, 2014 at 12:02 PM, Roman Divacky <
> > rdivacky at vlakno.cz>
> > > > wrote:
> > > > > >
> > > > > > > Like this?
> > > > > > >
> > > > > > > On Fri, Nov 14, 2014 at 11:20:49AM -0800, David Blaikie wrote:
> > > > > > > > I take it this is consistent with the GCC warning - in terms of
> > > > warning
> > > > > > > on
> > > > > > > > the innermost issue, reporting const or volatile - what about
> > > > dropping
> > > > > > > > const and volatile at the same time?
> > > > > > > >
> > > > > > > > Issues with the current code:
> > > > > > > >
> > > > > > > > * DestPtr and SrcPtr don't need to be initialized to null,
> > they'll
> > > > be
> > > > > > > > written to on the first loop iteration as needed - avoiding
> > excess
> > > > > > > > initialization helps tools like MSan find more bugs rather
> > than the
> > > > > > > program
> > > > > > > > silently using unintended default values
> > > > > > > >
> > > > > > > > * InnerMostDestType and InnerMostSrcType will be dangling
> > pointers
> > > > after
> > > > > > > > the while loop (so accessing them in the proceeding 'if' is UB)
> > > > > > > >
> > > > > > > > * you don't need to check both InnerMostDestType and
> > > > InnerMostSrcType in
> > > > > > > > the following if - it's invariant that if one is non-null (you
> > can
> > > > use
> > > > > > > > QualType values rather than QualType* to address the previous
> > bug,
> > > > and
> > > > > > > use
> > > > > > > > QualTypes "isNull()" member function here) so is the other
> > > > > > > >
> > > > > > > > On Fri, Nov 14, 2014 at 11:07 AM, Roman Divacky <
> > > > rdivacky at vlakno.cz>
> > > > > > > wrote:
> > > > > > > >
> > > > > > > > > Actually, try this patch. It includes check for volatile as
> > well.
> > > > > > > > >
> > > > > > > > > On Wed, Nov 12, 2014 at 12:39:20PM -0800, David Blaikie
> > wrote:
> > > > > > > > > > [+Richard for oversight]
> > > > > > > > > >
> > > > > > > > > > char **y1 = (char **)ptrptr; // expected-warning {{cast
> > from
> > > > 'const
> > > > > > > char
> > > > > > > > > > *const *' to 'char **' drops const qualifier}}
> > expected-warning
> > > > > > > {{cast
> > > > > > > > > from
> > > > > > > > > > 'const char *const' to 'char *' drops const qualifier}}
> > > > > > > > > >
> > > > > > > > > > I think if we're going to warn on multiple layers (I'm not
> > sure
> > > > > > > that's
> > > > > > > > > > ideal - is that consistent with GCC's warning? Does GCC
> > warn on
> > > > > > > > > mismatched
> > > > > > > > > > types too - "const T*" -> "U*"? - do we warn there too, or
> > > > only when
> > > > > > > > > > there's a valid implicit conversion like the void*
> > example?)
> > > > then we
> > > > > > > > > should
> > > > > > > > > > probably drop the top level const, "const char *const" ->
> > > > "char*" -
> > > > > > > the
> > > > > > > > > top
> > > > > > > > > > level const on the first type is confusing/misleading, it's
> > > > only
> > > > > > > relevant
> > > > > > > > > > to show "const char*" and "char*".
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > On Wed, Nov 12, 2014 at 10:41 AM, Roman Divacky <
> > > > rdivacky at vlakno.cz>
> > > > > > > > > wrote:
> > > > > > > > > >
> > > > > > > > > > > I expanded the testcase and fixed the grammar in the
> > actual
> > > > > > > warning.
> > > > > > > > > > >
> > > > > > > > > > > New patch attached.
> > > > > > > > > > >
> > > > > > > > > > > On Tue, Nov 11, 2014 at 05:03:33PM -0800, David Blaikie
> > > > wrote:
> > > > > > > > > > > > (it's a bit easier if you include the test in the same
> > > > patch
> > > > > > > file -
> > > > > > > > > also
> > > > > > > > > > > > you can use Phabricator if you like - some reviewers
> > > > perefer it)
> > > > > > > > > > > >
> > > > > > > > > > > > Since you've got the loop there for seeing through
> > multiple
> > > > > > > levels of
> > > > > > > > > > > > pointer, should you have a test case that exercises
> > that
> > > > on a > 1
> > > > > > > > > level
> > > > > > > > > > > of
> > > > > > > > > > > > depth? Demonstrate that we warn on both levels (if
> > that's
> > > > the
> > > > > > > right
> > > > > > > > > thing
> > > > > > > > > > > > to do?)?
> > > > > > > > > > > >
> > > > > > > > > > > > Optionally (probably in a separate follow-up patch) you
> > > > could
> > > > > > > add a
> > > > > > > > > note
> > > > > > > > > > > > with a fixit to include the missing consts.
> > > > > > > > > > > >
> > > > > > > > > > > > On Tue, Nov 11, 2014 at 10:58 AM, Roman Divacky <
> > > > > > > rdivacky at vlakno.cz>
> > > > > > > > > > > wrote:
> > > > > > > > > > > >
> > > > > > > > > > > > > Hi,
> > > > > > > > > > > > >
> > > > > > > > > > > > > I implemented -Wcast-qual. The patch is actually
> > quite
> > > > short
> > > > > > > > > (attached
> > > > > > > > > > > + a
> > > > > > > > > > > > > test
> > > > > > > > > > > > > case).
> > > > > > > > > > > > >
> > > > > > > > > > > > > This fixes #13772 and also note that -Wcast-qual is
> > used
> > > > in
> > > > > > > llvm
> > > > > > > > > build
> > > > > > > > > > > > > itself.
> > > > > > > > > > > > >
> > > > > > > > > > > > > Is this ok to be commited? Thanks
> > > > > > > > > > > > >
> > > > > > > > > > > > > Roman
> > > > > > > > > > > > >
> > > > > > > > > > > > >
> > > > > > > > > > > > > _______________________________________________
> > > > > > > > > > > > > cfe-commits mailing list
> > > > > > > > > > > > > cfe-commits at cs.uiuc.edu
> > > > > > > > > > > > >
> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> > > > > > > > > > > > >
> > > > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > >
> > > > > > >
> > > >
> > > > > Index: include/clang/Basic/DiagnosticGroups.td
> > > > > ===================================================================
> > > > > --- include/clang/Basic/DiagnosticGroups.td (revision 222228)
> > > > > +++ include/clang/Basic/DiagnosticGroups.td (working copy)
> > > > > @@ -60,7 +60,7 @@
> > > > > def KeywordCompat : DiagGroup<"keyword-compat">;
> > > > > def GNUCaseRange : DiagGroup<"gnu-case-range">;
> > > > > def CastAlign : DiagGroup<"cast-align">;
> > > > > -def : DiagGroup<"cast-qual">;
> > > > > +def CastQual : DiagGroup<"cast-qual">;
> > > > > def : DiagGroup<"char-align">;
> > > > > def Comment : DiagGroup<"comment">;
> > > > > def GNUComplexInteger : DiagGroup<"gnu-complex-integer">;
> > > > > Index: include/clang/Basic/DiagnosticSemaKinds.td
> > > > > ===================================================================
> > > > > --- include/clang/Basic/DiagnosticSemaKinds.td (revision
> > 222228)
> > > > > +++ include/clang/Basic/DiagnosticSemaKinds.td (working copy)
> > > > > @@ -6104,6 +6104,8 @@
> > > > > def warn_zero_size_struct_union_in_extern_c :
> > Warning<"%select{|empty
> > > > }0"
> > > > > "%select{struct|union}1 has size 0 in C, %select{size 1|non-zero
> > > > size}2 in C++">,
> > > > > InGroup<ExternCCompat>;
> > > > > +def warn_cast_qual : Warning<"cast from %0 to %1 drops %2
> > qualifier%3">,
> > > > > + InGroup<CastQual>, DefaultIgnore;
> > > > > } // End of general sema category.
> > > > >
> > > > > // inline asm.
> > > > > Index: lib/Sema/SemaCast.cpp
> > > > > ===================================================================
> > > > > --- lib/Sema/SemaCast.cpp (revision 222228)
> > > > > +++ lib/Sema/SemaCast.cpp (working copy)
> > > > > @@ -143,7 +143,10 @@
> > > > > }
> > > > >
> > > > > static bool CastsAwayConstness(Sema &Self, QualType SrcType,
> > QualType
> > > > DestType,
> > > > > - bool CheckCVR, bool
> > CheckObjCLifetime);
> > > > > + bool CheckCVR, bool
> > CheckObjCLifetime,
> > > > > + QualType *TheOffendingSrcType =
> > nullptr,
> > > > > + QualType *TheOffendingDestType =
> > nullptr,
> > > > > + Qualifiers *CastAwayQualifiers =
> > > > nullptr);
> > > > >
> > > > > // The Try functions attempt a specific way of casting. If they
> > > > succeed, they
> > > > > // return TC_Success. If their way of casting is not appropriate for
> > > > the given
> > > > > @@ -462,7 +465,10 @@
> > > > > /// \param CheckObjCLifetime Whether to check Objective-C lifetime
> > > > qualifiers.
> > > > > static bool
> > > > > CastsAwayConstness(Sema &Self, QualType SrcType, QualType DestType,
> > > > > - bool CheckCVR, bool CheckObjCLifetime) {
> > > > > + bool CheckCVR, bool CheckObjCLifetime,
> > > > > + QualType *TheOffendingSrcType,
> > > > > + QualType *TheOffendingDestType,
> > > > > + Qualifiers *CastAwayQualifiers) {
> > > > > // If the only checking we care about is for Objective-C lifetime
> > > > qualifiers,
> > > > > // and we're not in ARC mode, there's nothing to check.
> > > > > if (!CheckCVR && CheckObjCLifetime &&
> > > > > @@ -487,6 +493,8 @@
> > > > > // Find the qualifiers. We only care about cvr-qualifiers for the
> > > > > // purpose of this check, because other qualifiers (address
> > spaces,
> > > > > // Objective-C GC, etc.) are part of the type's identity.
> > > > > + QualType PrevUnwrappedSrcType = UnwrappedSrcType;
> > > > > + QualType PrevUnwrappedDestType = UnwrappedDestType;
> > > > > while (UnwrapDissimilarPointerTypes(UnwrappedSrcType,
> > > > UnwrappedDestType)) {
> > > > > // Determine the relevant qualifiers at this level.
> > > > > Qualifiers SrcQuals, DestQuals;
> > > > > @@ -497,6 +505,13 @@
> > > > > if (CheckCVR) {
> > > > >
> > RetainedSrcQuals.setCVRQualifiers(SrcQuals.getCVRQualifiers());
> > > > >
> > RetainedDestQuals.setCVRQualifiers(DestQuals.getCVRQualifiers());
> > > > > +
> > > > > + if (RetainedSrcQuals != RetainedDestQuals &&
> > TheOffendingSrcType
> > > > &&
> > > > > + TheOffendingDestType && CastAwayQualifiers) {
> > > > > + *TheOffendingSrcType = PrevUnwrappedSrcType;
> > > > > + *TheOffendingDestType = PrevUnwrappedDestType;
> > > > > + *CastAwayQualifiers = RetainedSrcQuals - RetainedDestQuals;
> > > > > + }
> > > > > }
> > > > >
> > > > > if (CheckObjCLifetime &&
> > > > > @@ -505,6 +520,9 @@
> > > > >
> > > > > cv1.push_back(RetainedSrcQuals);
> > > > > cv2.push_back(RetainedDestQuals);
> > > > > +
> > > > > + PrevUnwrappedSrcType = UnwrappedSrcType;
> > > > > + PrevUnwrappedDestType = UnwrappedDestType;
> > > > > }
> > > > > if (cv1.empty())
> > > > > return false;
> > > > > @@ -2371,6 +2389,28 @@
> > > > >
> > > > > if (Kind == CK_BitCast)
> > > > > checkCastAlign();
> > > > > +
> > > > > + // -Wcast-qual
> > > > > + QualType TheOffendingSrcType, TheOffendingDestType;
> > > > > + Qualifiers CastAwayQualifiers;
> > > > > + if (CastsAwayConstness(Self, SrcType, DestType, true, false,
> > > > > + &TheOffendingSrcType, &TheOffendingDestType,
> > > > > + &CastAwayQualifiers)) {
> > > > > + const char *qualifiers;
> > > > > + const char *suffix = "";
> > > > > + if (CastAwayQualifiers.hasConst() &&
> > > > CastAwayQualifiers.hasVolatile()) {
> > > > > + qualifiers = "const and volatile";
> > > > > + suffix = "s";
> > > > > + } else if (CastAwayQualifiers.hasConst())
> > > > > + qualifiers = "const";
> > > > > + else if (CastAwayQualifiers.hasVolatile())
> > > > > + qualifiers = "volatile";
> > > > > + else {
> > > > > + llvm_unreachable("Impossible qualifier");
> > > > > + }
> > > > > + Self.Diag(SrcExpr.get()->getLocStart(), diag::warn_cast_qual) <<
> > > > > + TheOffendingSrcType << TheOffendingDestType << qualifiers <<
> > > > suffix;
> > > > > + }
> > > > > }
> > > > >
> > > > > ExprResult Sema::BuildCStyleCastExpr(SourceLocation LPLoc,
> > > > > Index: test/Sema/warn-cast-qual.c
> > > > > ===================================================================
> > > > > --- test/Sema/warn-cast-qual.c (revision 0)
> > > > > +++ test/Sema/warn-cast-qual.c (working copy)
> > > > > @@ -0,0 +1,19 @@
> > > > > +// RUN: %clang_cc1 -triple x86_64-unknown-unknown -fsyntax-only
> > > > -Wcast-qual -verify %s
> > > > > +
> > > > > +#include <stdint.h>
> > > > > +
> > > > > +void foo() {
> > > > > + const char * const ptr = 0;
> > > > > + const char * const *ptrptr = 0;
> > > > > + char *y = (char *)ptr; // expected-warning {{cast from 'const
> > > > char *' to 'char *' drops const qualifier}}
> > > > > + char **y1 = (char **)ptrptr; // expected-warning {{cast from
> > > > 'const char *const' to 'char *' drops const qualifier}}
> > > > > + const char **y2 = (const char **)ptrptr; // expected-warning
> > {{cast
> > > > from 'const char *const *' to 'const char **' drops const qualifier}}
> > > > > +
> > > > > + char *z = (char *)(uintptr_t)(const void *)ptr; // no warning
> > > > > + char *z1 = (char *)(const void *)ptr; // expected-warning
> > {{cast
> > > > from 'const void *' to 'char *' drops const qualifier}}
> > > > > +
> > > > > + volatile char *vol = 0;
> > > > > + char *vol2 = (char *)vol; // expected-warning {{cast from
> > 'volatile
> > > > char *' to 'char *' drops volatile qualifier}}
> > > > > + const volatile char *volc = 0;
> > > > > + char *volc2 = (char *)volc; // expected-warning {{cast from 'const
> > > > volatile char *' to 'char *' drops const and volatile qualifiers}}
> > > > > +}
> > > >
> > > >
> >
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