Actually, those functions are intrinsics in the Go compiler on many platforms, so the compiler generates a single ADD / ADC opcode. The source code that you see is a fallback for backends that have not implemented them as intrinsics yet.
Edit: OK, I have much worse time generating "adc" instructions, and only gcc sees the opportunity to emit "setc" where appropriate. I see the issue now.
That's true, but I'm specifically addressing the compactness and readability concern.
With respect to efficiency and particularly timing, Go and C seems to be in the same boat here in that depending on your compiler and platform it may either compile to a constant time chain of add/adc equivalent instructions or something sub-optimal and branchy.
