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

JF Bastien via cfe-dev cfe-dev at lists.llvm.org
Thu Dec 13 11:45:31 PST 2018


> On Dec 11, 2018, at 5:37 PM, Arthur O'Dwyer <arthur.j.odwyer at gmail.com> wrote:
> 
> FWIW, I've always thought that the simplest solution is just to expose something like "std::is_trivially_equality_comparable_v<T>" (which would be true whenever comparing objects of type T for equality could be done as-if by memcpy), and then constrain "std::atomic<T>::compare_exchange_foo" to be provided iff "std::is_trivially_equality_comparable_v<T>".  P0528 was already accepted and merged by the time I started talking about this, though.

You should read the first version of the paper, which suggested exactly this. You should then read the EWG discussion, which led to the currently adopted solution.


> "std::is_trivially_equality_comparable_v<T>" would be similar, but IIRC not 100% identical, to the existing "std::has_unique_object_representations_v<T>".  As of C++2a, it is detectable by the compiler even for class types (as long as the programmer =default's their type's operator==).
> 
> Anyway, I don't think programmers have any business trying to compare-and-swap structs with padding bits. (Or floats.)
> 
> –Arthur
> 
> 
> On Tue, Dec 11, 2018 at 6:51 PM James Y Knight via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
> After considering this a bit, I'm not sure the __builtin_clear_padding proposal is workable. Worse, I'm not sure that the standards proposal itself is workable, either.
> 
> 1. Regarding general implementability of P0528:
> 
> Given a call to
>   bool atomic_compare_exchange_strong(std::atomic<T>* obj, T* expected, T desired)
> we need to first, compare OBJ to EXPECTED -- ignoring padding. Then:
> a. if they're equal, store DESIRED into the bytes pointed to by OBJ.
> b. if they're not equal, copy the bytes pointed to by OBJ into the bytes pointed to by EXPECTED.
> 
> The problem is: how to do a comparison ignoring padding? We cannot reasonably modify the comparison operation itself (it's either a hardware instruction, or else a library routine taking only a size). The obvious solution is to clear the padding on both sides of the comparison (thus __builtin_clear_padding).
> 
> We can clear the padding in EXPECTED, although we'll need to make a copy first, as we're not allowed to store into it unless the compare-exchange fails. But, we cannot modify OBJ except as part of the "exchange", when the comparison has already succeeded. 
> 
> If we can ensure that the existing value in OBJ has always had its padding cleared, we're okay. We can be fully in-control of all stores that occur into a std::atomic, and, I believe that's true for C11 _Atomic types as well. (it's unclear to me whether using memcpy to non-atomically overwrite a C11 "_Atomic struct Foo" is allowable, but I'll assume that it is not.)
> 
> However, it seems infeasible to make this guarantee for std::atomic_ref, as that can be created pointing to any existing object. Said existing object may have its padding in a non-zero state. Of course, <http://wg21.link/P0528r3 <http://wg21.link/P0528r3>> doesn't mention atomic_ref, but the atomic_ref paper, <http://wg21.link/P0019r8 <http://wg21.link/P0019r8>> mentions this concern, and simply states "We require that the resolution of padding bits follow [P0528r2]."
> 
> It's also infeasible to ensure for the GCC __atomic_compare_exchange builtin, as it also operates on arbitrary existing objects. (Of course, this is irrelevant to the C++ standard.)
> 
> I believe a workable alternative would be to generate an extra load of OBJ into a temporary memory, copy only the non-padding bytes of EXPECTED on top of that, and then do the compare_exchange operation of OBJ with the temporary. Finally, copy the temporary back into EXPECTED upon a fail result. While that seems like it could work, requiring an extra load before compare-exchange operations is rather poor.

I suggest you synchronize with your company’s C++ representatives, who were present at the EWG discussion which led to this change. They were strongly in favor of this direction, and AFAIK it was understood that there would be a cost for structs with padding bits.


> Which leads into...
> 
> 2. Do we really need to make this change?
> 
> The initial paper gives a sample program in http://wg21.link/P0528r0 <http://wg21.link/P0528r0> asserting that it might infinite loop. That would be quite unfortunate.
> 
> Repeating the example here, it's basically:
> ```
>   T desired = ...;
>   T expected;
>   while (
>     !atomic->compare_exchange_strong(
>       expected,
>       desired // Padding bits added and removed here
>   ));
> ```
> 
> However, that padding bits in DESIRED may be modified is not super problematic. As long as padding bits in EXPECTED (passed by reference) are not spuriously-modified, we should be fine.
> 
> That is, in practice, I believe the loop should execute at most twice, not infinitely. Upon the first failure, all the bits (including padding) pointed to by ATOMIC are copied into the object pointed to by EXPECTED (a reference parameter). At the next iteration of the loop, the call will compare all the bits, including the padding, which was copied as-is from OBJ. As nothing writes to EXPECTED between the two calls, the padding bits will then remain unchanged, and therefore the comparison will succeed on the second iteration.
> 
> However, Richard Smith informs me that while the above is true in Clang, apparently in theory it is broken -- nothing in the C++ standard prohibits padding bits of an object from be discarded or modified arbitrarily, at any time! That is, according to the spec (but not Clang), the below function "foo" could arbitrarily return non-zero, randomly! (I used "my_mem*" simply for demonstrating that even if memcpy/memcmp calls themselves are simply function calls and not specially-handled, the problem remains.). One could imagine this occurring in a non-pathologically-evil compiler, if it were to copy EXPECTED into registers and then back into memory between the two calls -- and while doing so, only copies the non-padding parts of the object, for efficiency.
> 
> ```
> struct Foo { char x; /* padding exists here */ long y; };
> 
> int foo(char *mem) {
>   Foo expected;
>   my_memcpy((char*)&expected, mem, sizeof(Foo));
>   return my_memcmp((char*)&expected, mem, sizeof(Foo));
> }
> ```
> 
> The C standard does appear to prohibit this behavior (C11 6.2.6.1/6 <http://6.2.6.1/6>), specifying that the padding bytes get set to unspecified values only upon writes to the object.
> 
> In summary: in C++, it would appear that the motivating example can in theory infinite-loop, but in Clang, and the C standard, it'd seem that it cannot.
> 
> Changing C++ to require the same semantics for padding stability as C could be a reasonable alternative resolution to the original issue, especially if no implementations actually make use of this allowance today. You may still have a single spurious failure, but it would never infinite-loop.
> 
> 
> 3. Finally, if the decision is to go forward with P0528 anyways, and if padding bits *can* arbitrarily change, then `__builtin_clear_padding(T* ptr)` is not a theoretically sound interface, because the padding bits could change again, after the call to clear them returns.
> 
> We'd need to either implement the support within the atomic builtins themselves (which, as noted, cannot be done within the existing GCC builtins), or define some alternative interface.
> 
> 
> On Fri, Nov 30, 2018 at 4:13 PM JF Bastien <jfbastien at apple.com <mailto:jfbastien at apple.com>> wrote:
> 
> 
>> On Nov 30, 2018, at 1:10 PM, James Y Knight <jyknight at google.com <mailto:jyknight at google.com>> wrote:
>> 
>> 
>> 
>> On Fri, Nov 30, 2018 at 4:06 PM JF Bastien <jfbastien at apple.com <mailto:jfbastien at apple.com>> wrote:
>> 
>> 
>>> On Nov 30, 2018, at 12:39 PM, Casey Carter via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>>> 
>>> On Fri, Nov 30, 2018 at 12:35 PM James Y Knight via cfe-dev <cfe-dev at lists.llvm.org <mailto: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 <mailto: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 <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.
>> 
>> I think you rather want to zero out padding bits in the atomic, not in the value, and you want to do it after storing into the atomic (unless you’re doing zeroing, and then copying the non-padding elements one at a time, in which case you could just memset instead of having a builtin).
>> 
>> Errr...certainly you can't do additional stores into an atomic after storing a new value into it, that would kinda ruin the "atomic" part.
> 
> Haha yeah you’re totally right! Ignore me, no idea what I was thinking.
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