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SIMD exceptions are fine with masking

Author: Hubert Lamontagne

Date: 2016-08-20 18:49

Agner wrote:

Perhaps the choice of method depends on the purpose of overflow detection:

If the purpose is to abort the application in case of overflow for security reasons then method (3) might be the easiest solution.

This is why the add instruction triggers a fault on... IBM Z-system I *THINK*. MIPS has a separate specialized instruction for this as well. It's the same reason for 0-divide exceptions, it's used in applications where aborting failing applications is a good thing in general (like server software talking to a database where the earlier you shut it down, the less damage you inflict...). This is not too popular among end-user facing applications like video games and stuff like photoshop, because in that case an application shutdown is already the worst thing that can happen.

Agner wrote:

If the purpose is to recover from an overflow condition by re-calculating with bigger integers then method (3) might also be acceptable if overflows are rare.

If I'm not mistaken, that's how LISP did it. The catch with this one is that it requires dynamic typing to work. If you're running a language with dynamic typing, the chances that you're going to be able to run any vector code at all are low. Even for scalar code, dynamic typed languages typically have some other much worse bottlenecks than integer computation (multiple levels of indirection to get to data in Python, for instance). For something like C++ code, IMHO it's just way easier and faster to supersize all your integers to 64bits from the start than to deal with any sort of size change.

Agner wrote:

If the purpose is to detect the carry output of unsigned addition then we may perhaps prefer method (4).

This happens in one very precise case: BIGNUM computation. How important is BIGNUM for you? My impression on this is that BIGNUM falls kinda low on priority lists for architectures like ARM. If you design your architecture for BIGNUM, you can probably make it much more efficient for this kind of stuff than a generic architecture like MIPS. I guess it comes down to what you're targetting.

Agner wrote:

If the purpose is some kind of advanced compensation for signed overflow then I don't know what
method is best.

My experience on that is that if you're running into signed overflow problems, the easiest way is simply to switch to floating point or supersize your integers to 64bits.

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