llvm dev - Aug 2013 - [LLVMdev] Clarification between <type> and <ty> for alloca instruction

On 15/08/13 16:12, Tim Northover wrote:
>> 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.
Oops yes you're completely right. I didn't think about that through
very well ( I was doing % 255 in my head instead of % 256).

Do you think it would be wise to document the meaning of <ty> for the
alloca instruction or do you think that it is clear enough and that it
is my unfamiliarity with LLVM IR that caused my confusion?

Thanks,
Dan.

> Do you think it would be wise to document the meaning of <ty> for the
> alloca instruction or do you think that it is clear enough and that it
> is my unfamiliarity with LLVM IR that caused my confusion?
Personally I'd say it's clear enough, but others may disagree.

Cheers.

Tim.