Comment by adgjlsfhk1
2 months ago
> On the other hand, detecting integer overflow in software is extremely expensive
this just isn't true. both addition and multiplication can check for overflow in <2 instructions.
2 months ago
> On the other hand, detecting integer overflow in software is extremely expensive
this just isn't true. both addition and multiplication can check for overflow in <2 instructions.
Fewer than two is exactly one instruction. Which?
dammmit I meant <=2. https://godbolt.org/z/4WxeW58Pc sltu or snez for add/multiply respectively.
This result is misleading.
First, the code claims to be returning "unsigned long" from each of these functions, but the value will only ever be 0 or 1 (see [1]). The code is actually throwing away the result and just returning whether overflow occurred. If we take unsigned long *c as another argument to the function, so that we actually keep the result, we end up having to issue an extra instruction for multiplication (see [2]; I'm ignoring the sd instruction since it is simply there to dereference the *c pointer and wouldn't exist if the function got inlined).
Second, this is just unsigned overflow detection. If we do signed overflow detection, now we're up to 5 instructions for add and mul (see [3]). Considering that this is the bigger challenge, it compares quite unfavorably to architectures where this is just 2 instructions: the operation itself and a branch against a condition flag.
[1]: https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins...
[2]: https://godbolt.org/z/7rWWv57nx
[3]: https://godbolt.org/z/PnzKaz4x5
4 replies →
[flagged]
+1 -- misinformation is best corrected quickly. If not, AI will propagate it and many will believe the erroneous information. I guess that would be viral hallucinations.
One can quickly correct misinformation without being rude. It's not hard, and does not lessen the impact of the correction to do so. There's no reason to tolerate the kind of rudeness the parent post exhibits.