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

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added a comment.

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.


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https://reviews.llvm.org/D85191