Fewer than two is exactly one instruction. Which?

You are delusional.

Read the RISC-V documentation to see that you need at least 3 instructions.

For addition, overflow happens when you add 2 positive numbers and the result is negative, or when you add 2 negative numbers and the result is positive.

After using an instruction to do the addition, how can you detect this complex condition with a single instruction in the impoverished RISC-V ISA?

EDIT: Someone has downvoted this, presumably because I have not been polite enough.

That may be true, but I consider much more impolite the kind of misleading false information that has been written by the poster to whom I have replied. It is difficult to read that kind of b*s*t and be cool about it.

That is not the correct way to test for integer overflow.

The correct sequence of instructions is given in the RISC-V documentation and it needs more instructions.

"Integer overflow" means "overflow in operations with signed integers". It does not mean "overflow in operations with non-negative integers". The latter is normally referred as "carry".

The 2 instructions given above detect carry, not overflow.

Carry is needed for multi-word operations, and these are also painful on RISC-V, but overflow detection is required much more frequently, i.e. it is needed at any arithmetic operation, unless it can be proven by static program analysis that overflow is impossible at that operation.

Because the other commenter wasn’t posting the actual answer, I went to find the documentation about checking for integer overflow and it’s right here https://docs.riscv.org/reference/isa/unpriv/rv32.html#2-1-4-...

And what did I find? Yep that code is right from the manual for unsigned integer overflow.

For signed addition if you know one of the signs (eg it’s a compile time constant) the manual says

  addi t0, t1, +imm
  blt t0, t1, overflow

But the general case for signed addition if you need to check for overflow and don’t have knowledge of the signs

  add t0, t1, t2
  slti t3, t2, 0
  slt t4, t0, t1
  bne t3, t4, overflow

From what I’ve read most native compiled code doesn’t really check for overflows in optimised builds, but this is more of an issue for JavaScript et al where they may detect the overflow and switch the underlying type? I’m definitely no expert on this.

I have no idea or practical experience with anything this low-level, so idk how much following matters, it's just someone from the crowd offering unvarnished impressions:

It's easy to believe you're replying to something that has an element of hyperbole.

It's hard to believe "just do 2x as many instructions" and "ehhh who cares [i.e. your typical C program doesn't check for overflow]", coupled to a seemingly self-conscious repetition of a quip from the television series Chernobyl that is meant to reference sticking your head in the sand, retire the issue from discussion.