[PATCH] D125802: Fix std::has_unique_object_representations for _BitInt types with padding bits

[Erich Keane via Phabricator via cfe-commits]

erichkeane added a comment.

This likely also needs a Release Note.



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Comment at: clang/lib/AST/ASTContext.cpp:2697
   int64_t FieldSizeInBits =
       Context.toBits(Context.getTypeSizeInChars(Field->getType()));
   if (Field->isBitField()) {
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MitalAshok wrote:
> erichkeane wrote:
> > This answer ends up being wrong in the case of _BitInt, consider _BitInt(7).  Its field-size would be 7, yet this would result in 8.  I'm not sure of the fallout of this though.
> The field size should be 8 for _BitInt(7), since it takes 8 bits in the struct (including the padding bit). It is filtered out later with "hasUniqueObjectRepresentations" on line 2709 if it wasn't in a bit field
I see, thanks for the explanation.


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Comment at: clang/test/SemaCXX/has_unique_object_reps_bitint.cpp:7
+static_assert(__has_unique_object_representations(_BitInt(sizeof(int) * 8u)));
+static_assert(sizeof(_BitInt(24)) != 4 || !__has_unique_object_representations(_BitInt(24)));
+
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MitalAshok wrote:
> erichkeane wrote:
> > Whats going on here?  I don't particularly get the condition.
> sizeof(_BitInt(24)) == sizeof(int32_t) (4) since we align to the next larger builtin integer type. But if that weren't the case for some reason (i.e., that changes in the future, I don't know if _BitInt is ABI stable), this wouldn't start failing for unrelated reasons
I see!  Can you clarify that with a comment here?




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  https://reviews.llvm.org/D125802/new/

https://reviews.llvm.org/D125802