majnemer added a comment. In D114425#3217478 <https://reviews.llvm.org/D114425#3217478>, @Quuxplusone wrote:
> In D114425#3216802 <https://reviews.llvm.org/D114425#3216802>, @majnemer > wrote: > >> OOC, how hard would it be to generalize this builtin a little? It is nice >> that we have builtins like `__builtin_add_overflow` which do the right thing >> regardless of their input. >> >> It seems like it would be nice if we started to expose more intrinsics which >> did the right thing regardless of operand width; another bonus is that it >> composes well with language features like `_BitInt`. > > IMHO such builtins are nice iff the programmer can be 100% sure that the > compiler will interpret them the same way as a human reader. > `__builtin_add_overflow` is easy because its first two arguments are > "mathematical integers" (where integer promotion doesn't matter) and its > third argument is a pointer (where integer promotion can't happen). So you > can really throw any combination of types at it, and it'll do "the right > thing" https://godbolt.org/z/sa7b894oa (although I admit I was surprised that > this worked). > For a hypothetical `__builtin_bswap`, you would probably need a similar > pointer-based interface like > > short s16 = 0xFEDC; > __builtin_bswap(&s16); // hypothetically > assert(s16 == 0xDCFE); > > assert(__builtin_bswap16(s16) == 0x0000DCFE); > assert(__builtin_bswap32(s16) == 0xDCFEFFFF); // the problem to solve: s16 > eagerly promotes to int, which changes the result > > The downside is that the pointer-based interface is less ergonomic than > today's value-based signatures, and probably worse codegen at `-O0` (because > the programmer has to materialize the operand into a named variable, and then > the compiler won't remove that variable because you might want to debug it). > The upside (as you said) is that a generic builtin could work with `_ExtInt` > types and so on. I think one way of side stepping that problem would be to say that `__builtin_bswap` only works with unsigned types. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D114425/new/ https://reviews.llvm.org/D114425 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
