[LLVMdev] Clarification between and for alloca instruction

Tim Northover
Thu, 15 Aug 2013 15:12:13 -0000

> I've obviously being playing with C++ too long because my
> instinct immediately told me that dynamically sized arrays on the
> stack are't allowed but apparently that's fine for C99 (g++ also
seems
> fine with this is you don't specify -pedantic)

It's all fun and games until someone decides to evaluate sizeof(arr). ;-)

I think a more limited form is coming to C++11, by the way.

> Interesting thanks for clarifying. Now you've raised another question.
> I'm interested to know how you know that this is how overflow behaves
> for this particular instruction. I can imagine several different
> behaviours (I'm assuming NumElements is unsigned but that doesn't
seem
> to be specified)
>
> - wrap around overflow. In which case 257 = 255 + 2 so result would be 2

This is what happens (wraparound/truncation/whatever). But I think 256
-> 0, 257 -> 1.

Cheers.

Tim.