[PATCH] D85191: [AST] Get field size in chars rather than bits in RecordLayoutBuilder.

[Bjorn Pettersson via Phabricator via cfe-commits]

bjope added a comment.

In D85191#2196863 <https://reviews.llvm.org/D85191#2196863>, @rsmith wrote:

> In D85191#2195923 <https://reviews.llvm.org/D85191#2195923>, @ebevhan wrote:
>
>> In D85191#2193645 <https://reviews.llvm.org/D85191#2193645>, @rsmith wrote:
>>
>>>> This is not ideal, since it makes the calculations char size dependent, and breaks for sizes that are not a multiple of the char size.
>>>
>>> How can we have a non-bitfield member whose size is not a multiple of the size of a char?
>>
>> Downstream, we have fixed-point types that are 24 bits large, but where the char size is 16. The type then takes up 2 chars, where 8 of the bits are padding. The only way in Clang to express that the width of the bit representation of a type should be smaller than the number of chars it takes up in memory -- and consequently, produce an `i24` in IR -- is to return a non-charsize multiple from getTypeInfo.
>
> This violates the C and C++ language rules, which require the size of every type to be a multiple of the size of char. A type with 24 value bits and 8 padding bits should report a type size of 32 bits, just like `bool` reports a size of `CHAR_BIT` bits despite having only 1 value bit, and x86_64 `long double` reports a type size of 128 bits despite having only 80 value bits.

I don't see that it breaks the language rules. The sizeof result for the 24 bit type should be 2 in the target described by @ebevhan  (two 16-bit bytes). But I imagine that without this patch it is reported as 24/16=1, right?

So isn't the problem that toCharUnitsFromBits is rounding down when given a bitsize that isn't a multiple of CHAR_BIT? Would it perhaps make sense to let it round up instead?


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