> On the other hand, detecting integer overflow in software is extremely expensive, increasing both the program size and the execution time considerably,

Most languages don't care about integer overflow. Your typical C program will happily wrap around.

If I really want to detect overflow, I can do this:

    add t0, a0, a1
    blt t0, a0, overflow

Which is one more instruction, which is not great, not terrible.

▲ sitharus 4 hours ago | parent | next [-]

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.

	▲	sitharus 2 hours ago \| parent [-]
		A bit more reading shows there's a three instruction general case version for 32-bit additions on the 64-bit RISC-V ISA. I'm not familiar with RISC-V assembly and they didn't provide an example, but I _think_ it's as easy as this since 64-bit add wouldn't match the 32-bit overflowed add. `add t0, t1, t2 addw t3, t1, t2 bne t0, t3, overflow`

▲ adrian_b 5 hours ago | parent | prev | next [-]

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.

▲ refulgentis 5 hours ago | parent | prev [-]

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.

	▲	adrian_b 5 hours ago \| parent [-]
		There was no hyperbole in what I have said. The sequence of instructions given above is incorrect, it does not detect integer overflow (i.e. signed integer overflow). It detects carry, which is something else. The correct sequence, which can be found in the official RISC-V documentation, requires more instructions. Not checking for overflow in C programs is a serious mistake. All decent C compilers have compilation options for enabling checking for overflow. Such options should always be used, with the exception of the functions that have been analyzed carefully by the programmer and the conclusion has been that integer overflow cannot happen. For example with operations involving counters or indices, overflow cannot normally happen, so in such places overflow checking may be disabled.