On 2022-01-07 at 00:39:59, Jessica Clarke wrote: > On 7 Jan 2022, at 00:31, brian m. carlson <sandals@xxxxxxxxxxxxxxxxxxxx> wrote: > > If you want to get really language-lawyer-y about it, you can actually > > argue that this is a compliant implementation of the C standard. > > Integer types are permitted to have padding bits, and some combinations > > of padding bits are allowed to be trap representations. Technically, in > > our representation, the metadata bits are padding bits, because they do > > not contribute to the precision like value bits. It is therefore the > > case that the *value* of a uintptr_t still fits into a uintmax_t, but > > the latter has no padding bits, and casting the latter to the former > > yields a trap representation when further cast back to a pointer. This > > may not the intent of the spec, and not how anyone thinks of it because > > CHERI is the first implementation that pushes the boundary here, but > > it’s technically legal under that interpretation. You may disagree with > > the interpretation, and I don’t like to use it most of the time because > > it’s complicated and involves yet more ill-defined parts of the spec > > (e.g. it says arithmetic operations on valid values (they mean objects, > > I assume, as the value only includes value bits, but the input could be > > a trap representation on some implementations) never generate a trap > > representation other than as part of an exceptional condition such as > > an overflow, but nowhere defines what counts as an arithmetic > > operation). > > > So, no, C does not actually require what you say. It requires that void > * -> uintptr_t -> void * give you a valid pointer. It requires that > uintptr_t -> uintmax_t preserves the *value* of the uintptr_t, which we > do, because the value is formed from only the value bits which > contribute to the precision, which is 64 bits in this case, and > uintmax_t is still 64-bit. It requires that uintmax_t -> uintptr_t, > since uintptr_t’s precision is the same as uintmax_t’s, be always > valid, which is is. But it does not require that that uintptr_t has the > same representation as the original uintptr_t, which it does not for > us. And therefore it does not require that casting that uintptr_t back > to a void * yields a valid pointer. So if you want to really dig into > the details of the standard, we are technically compliant, even if some > might argue it’s not in the spirit of the standard. Sure, implementations are allowed to have padding bits. They're also allowed, at the moment, to use signed-magnitude or ones' complement integers, have CHAR_BIT greater than 8, have sizeof(char) == sizeof(short), not implement any of the customary sizes of intN_t or uintN_t, not provide uintptr_t, and use middle-endian numbers. However, if your ABI is only compliant in the face of those features (especially when it could have been written in a way which would have been compliant without the use of those features), it's intentionally hostile to real-world developers, and I don't think we should support it[0]. I'd be willing to revisit this if your ABI were defined in a reasonable, sane way, where sizeof(uintmax_t) >= sizeof(uintptr_t), without padding bits, where the alignment of pointers from malloc is suitable for all types, and where the alignment of a type is no greater than sizeof(type). I'm not opposed to a small amount of finagling for this case, but I am very much opposed to defining your C ABI in an intentionally difficult way. 128-bit integers in 64-bit Linux were not originally part of the C ABIs and if the ABI is ill defined now, that's a historical accident. But this is a new ABI for a new architecture and it could have been defined in a responsible way, but wasn't. As an aside, I was actually going to point out that you could propose a nice Rust or Go ABI with the status quo, but if your C ABI requires padding bits, then you're probably going to have a hard time doing so, since I don't believe those languages support padding bits and they need to support the C ABI. [0] For the record, I care strongly about portability, and I would not accept a runtime having any of the qualities I mentioned in the first paragraph. -- brian m. carlson (he/him or they/them) Toronto, Ontario, CA
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