================
@@ -15917,12 +15917,15 @@ struct PaddingCalculator {
const uint64_t DeclaredSizeInBits = Field->getBitWidthValue();
- // Handle over-sized bitfields:
- // unsigned char a : 12;
- // In this case, DeclaredSizeInBits is 12, but the actually occupied bit
- // size is 8, while the remaining 4 bits are padding.
+ // Oversized bit-fields (declared width larger than the field type) keep
+ // only the type's width as the value container. The extra declared bits
+ // are padding and follow that container (Itanium C++ ABI ยง2.4, II.1(b)).
+ // getIntWidth may be narrower still (bool, _BitInt); those occupied bits
+ // are the low-order bits of the value container.
+ const uint64_t ValueFieldBits =
+ std::min(DeclaredSizeInBits, Ctx.getTypeSize(Field->getType()));
const uint64_t OccupiedSizeInBits =
- std::min(DeclaredSizeInBits,
+ std::min(ValueFieldBits,
static_cast<uint64_t>(Ctx.getIntWidth(Field->getType())));
----------------
xiongzile wrote:
> The code is wrong in the case of oversized bitfield of the `_BitInt` type.
> See this example:
>
> ```c
> struct S {
> _BitInt(5) a : 6;
> };
>
> void foo(struct S *arg) {
> __builtin_clear_padding(arg);
> }
> ```
>
> The clearing is lowered like this:
>
> ```assembly
> ldrb r1, [r0]
> and r1, r1, #124
> strb r1, [r0]
> ```
>
> which still assumes that the padding is allocated before the occupied bits.
> Remember that in BE the allocation is from MSB to LSB. I'd expect the and to
> be done with `#248` (`1111 1000`).
But CodeGen currently stores the value in the low 5 bits of the 6-bit field,
i.e. `01111100`, not `11111000`
Also GCC doesn't support `_BitInt(5) a: 6`, so I'm not sure which we should
prefer...?
https://github.com/llvm/llvm-project/pull/225494
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