[codedokode]

> So you are criticising RISC-V not compared to its actual x86 and Arm competition -- where overflow checking is also not free

Do you suggest we should carry on bad design decisions made in the past? x86 is an exhibition of bad choices and I don't think we need to copy them.

> and is seldom used

I believe it is not like this. I think that in most cases you need non-wrapping addition, for example, if you are calculating totals for a customer's order, counting number of visits for a website, or calculating loads in a concrete beam.

Actually wrapping addition is the one that is seldomly used, in niche areas like cryptography. So it surprises me that the kind of addition that is used more often (non-wrapping) requires more instructions than exotic wrapping addition. What were CPU designers thinking I fail to understand.

[brucehoult]

You can't solve all the world's problems in one step. RISC-V solves a number of important problems, while making it as easy as practical to run existing software quickly on it.

If you want to have checked arithmetic, RISC-V's openness allows you to make a custom extension, implement hardware for it (FPGA is cheap and easy), implement software support and demonstrate the claimed benefits, and encourage others to also implement your extension, or standardise it.

It is simply not possible to do this in the x86 or Arm worlds. And that is one of the problems RISC-V solves -- a meta problem, not some one individual pet problem, but potentially all of them.

[ghusbands]

I agree that wrapping is a bad default, but I can provide some rationale.

If you do wrapped addition without flags, you have one self-contained instruction that even covers signed and unsigned integers. If you want other behaviour, you then have to specialize for signed or unsigned, specialize for the choice of wrap/trap/flag, and make those traps and flags work nicely with whatever other traps or flags you might have.

So, yeah, if you want the simplest possible thing, driven by some decision other than the best outcomes for software in general, then you would choose wrapping addition without flags or traps.