[cfe-dev] Clang builtins for C++20 STL features

[Casey Carter via cfe-dev]

On Fri, Nov 30, 2018 at 12:35 PM James Y Knight via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Thu, Nov 29, 2018, 9:53 PM Richard Smith via cfe-dev <
> cfe-dev at lists.llvm.org wrote:
>
>> * P0528R3 Atomic Compare-And-Exchange With Padding Bits
>>> We need compiler magic here, in some form. Billy O'Neal wrote to the
>>> C1XX team: "To implement the new atomic_ref as well as the change to
>>> compare the value representation of atomics only, the library needs a way
>>> to zero out the padding in arbitrary T, which we can't discover with
>>> library tech alone. We would like an intrinsic that accepts a
>>> trivially-copyable T and produces a copy with the padding zeroed, or takes
>>> a T* and zeros the padding inside that T, or similar."
>>>
>>
>> I think this should be done in-place in memory; producing a copy has the
>> problem that you're passing around a value of type T, and that might permit
>> the padding bits to become undefined again.
>>
>> void __builtin_clear_padding(T *ptr)
>> Effects: Set to zero all bits that are padding bits in the representation
>> of every value of type T.
>>
>
> Is the intent here that every value stored into a std::atomic (e.g. via
> store, exchange, or compare_exchange) would be passed through
> __builtin_clear_padding first, before being stored into the atomic object?
> And presumably the same would need to occur implicitly for C-style `_Atomic
> T`?
>

Yes; see P0582R3 "The Curious Case of Padding Bits" (
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0528r3.html) for
details. The intent is to make compare-exchange work for types with padding
bits by zeroing those padding bits before storing a value in an atomic or
comparing with the value stored in an atomic.
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