Comment by IshKebab
1 day ago
> On the other hand, even RVA23 is quite poor at signed overflow checking.
On the other hand it avoids integer flags which is nice. I doubt it makes a measurable performance impact either way on modern OoO CPUs. There's going to be no data dependence on the extra instructions needed to calculate overflow except for the branch, which will be predicted not-taken, so the other instructions after it will basically always run speculatively in parallel with the overflow-checking instructions.
It's nice for a C simulator to avoid condition codes. It's not so nice if you want consistent overflow checks (e.g., for automatically overflowing from fixnums to bignums).
Even with XNOR (which isn't even part of RVA23, if I recall correctly), the sequence for doing an overflow check is quite messy. On AArch64 and x86-64, it's just the operation followed by a conditional jump: https://godbolt.org/z/968Eb1dh1
Non-flag based overflow checks are still pretty cheap. The overflow check is only 1 extra instruction for unsigned (both add and multiply), and 3/4 extra for signed overflow (see https://godbolt.org/z/nq1nb5Whr for details). It's also worth noting that in many cases, the overflow checks will be removable or simplify-able by the compiler entirely (e.g. if you're adding 1 or know the sign of one of the operands etc). As such, the extra couple instructions are likely worthwhile if it makes designing a wider core easier. Signed overflow instructions would be reasonable to add, but it's not like modern high performance cores are bottlenecked by scalar instructions that don't touch memory anyway.