[PATCH] D41228: [ObjC] Enable __strong pointers in structs under ARC

[John McCall via Phabricator via cfe-commits]

rjmccall added inline comments.


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Comment at: include/clang/AST/Type.h:1121
+  /// after it is moved, as opposed to a truely destructive move in which the
+  /// source object is placed in an uninitialized state.
+  PrimitiveCopyKind isNonTrivialToPrimitiveDestructiveMove() const;
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ahatanak wrote:
> rjmccall wrote:
> > "truly"
> > 
> > Hmm.  Now that I'm thinking more about it, I'm not sure there's any point in tracking non-triviality of a C++-style destructive move separately from the non-triviality of a copy.  It's hard to imagine that there would ever be a non-C++ type that primitively has non-trivial copies but trivial C++-style moves or vice-versa.  Type-based destructors imply that the type represents some kind of resource, and a C++-style move will always be non-trivial for resource types because ownership of the resource needs to be given up by the old location.  Otherwise, a type might be non-trivial to copy but not destroy because there's something special about how it's stored (like volatility), but it's hard to imagine what could possibly cause it to be non-trivial to destroy but not copy.
> > 
> > If we were tracking non-triviality of an *unsafe* destructive move, one that leaves the source in an uninitialized state, that's quite different.
> > 
> > I think there are three reasonable options here:
> > 
> > - Ignore the argument I just made about the types that we're *likely* to care about modeling and generalize your tracking to also distinguish construction from assignment.  In such an environment, I think you can absolutely make an argument that it's still interesting to track C++-style moves separately from copies.
> > 
> > - Drop the tracking of destructive moves completely.  If you want to keep the method around, find, but it can just call `isNonTrivialToPrimitiveCopy()`.
> > 
> > - Change the tracking of *destructive* moves to instead track *deinitializing* moves.  The implementation would stop considering `__strong` types to be non-trivial to move.
> > 
> > But as things stand today, I do not see any point in separately tracking triviality of C++-style destructive moves.
> The second option seems most reasonable to me. We can always make changes if someone comes up with a type that requires tracking destructive moves separately.
Well, we already have a type that would have a trivial deinitializing move: ARC `__strong` pointers.  What we don't have is anything in IRGen that currently would take advantage of that fact.  Anyway, I agree that we can wait to start tracking this until we add such code to the compiler.


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Comment at: lib/CodeGen/CGNonTrivialStruct.cpp:193
+
+    TrivialFieldIsVolatile |= FT.isVolatileQualified();
+    if (Start == End)
----------------
ahatanak wrote:
> rjmccall wrote:
> > ahatanak wrote:
> > > rjmccall wrote:
> > > > I feel like maybe volatile fields should be individually copied instead of being aggregated into a multi-field memcpy.  This is a more natural interpretation of the C volatile rules than we currently do.  In fact, arguably we should really add a PrimitiveCopyKind enumerator for volatile fields (that are otherwise trivially-copyable) and force all copies of structs with volatile fields into this path precisely so that we can make a point of copying the volatile fields this way.  (Obviously that part is not something that's your responsibility to do.)
> > > > 
> > > > To get that right with bit-fields, you'll need to propagate the actual FieldDecl down.  On the plus side, that should let you use EmitLValueForField to do the field projection in the common case.
> > > I added method visitVolatileTrivial that copies volatile fields individually. Please see test case test_copy_constructor_Bitfield1 in test/CodeGenObjC/strong-in-c-struct.m.
> > Okay, great!  I like the name.
> > 
> > Does this mean we're now copying all structs that contain volatile fields with one of these helper functions?  If so, please add a C test case testing just that.  Also, you should retitle this review and stress that we're changing how *all* non-trivial types are copied, and that that includes both volatile and ARC-qualified fields.
> No, the current patch doesn't copy volatile fields of a struct individually unless the struct is a non-trivial type (which means its primitive copy kind is PCK_Struct). I'll look into today how I can force structs with volatile fields that are not non-trivial to be copied using the helper functions.
> 
> It seems like we would need a boolean flag in RecordDecl that tracks the presence of volatile fields in the struct or one of its subobjects. I assume we want to copy volatile fields individually in C++ too, in which case the flag needs to be set in both C and C++ mode. Is that right?
Oh, I see, there's specific logic to not flag the struct trivial if a field is merely VolatileTrivial.  Okay, that works for me; we can think about the right thing to do in a follow-up commit.  It probably needs to be raised as an RFC on cfe-dev.

The C++ standard has explicit language about triviality that says that volatile fields don't make defaulted copy-constructors non-trivial.  Whatever we do here can't break that.  But that doesn't necessarily imply that we should continue to use a simple memcpy to copy the struct; we could still use the C logic to copy "trivial" C++ structs with volatile members.

If we do try to apply this rule to C++, you will need to propagate these bits appropriately for C++ types, including from bases.


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Comment at: lib/Sema/SemaDecl.cpp:15424
+      if (FT.isNonTrivialToPrimitiveCopy() != QualType::PCK_Trivial &&
+          FT.isNonTrivialToPrimitiveCopy() != QualType::PCK_VolatileTrivial)
+        Record->setNonTrivialToPrimitiveCopy();
----------------
Please compute isNonTrivialToPrimitiveCopy once here, it's not trivial.

...arguably we should figure out a way to do all these type checks once and then set the bits from that.  We can do that as a separate patch, though.


https://reviews.llvm.org/D41228