With C, it is _BitInt(256) if the compiler supports it. The upper limit of _BitInt is implementation-defined though, so 256 is not guaranteed to be supported. Eg clang on RV64 only supports upto 128, but does support 256 on x64_64. gcc seems to not support _BitInt on RV64 at all, but does support 256 on x86_64.

With C++ the existence of such "extended integer" types is implementation defined. clang at least supports the same _BitInt construct for C++ too. gcc seems to not support it.

So, for the 256 case on x86_64, both clang and gcc seem to only generate the simple adc ripple version: https://gcc.godbolt.org/z/nxoEda3q5 https://gcc.godbolt.org/z/bYf4bor3f