[LLVMdev] How to analyze where the address comes from?

[Bruce Hoult]

Random thought: what if both operands of an add come from pointers? e.g. in
a naive coding of binary search as p = (p1+p2)/2

Is that simply illegal in LLVM? Or course it is in most languages including
C without a lot of casts, though once you compile it that's what the
machine instructions will be. And silently (but usually harmlessly)
overflow if you're in the top half of the address space...

So of course you *should* write it as p = p1 + (p2-p1)/2, which type
checks, doesn't need casts, doesn't overflow, and produces an add of a
pointer and an integer. But does all code a compiler encounters actually do
this?

On Thu, Nov 27, 2014 at 2:50 PM, Ruiling Song <ruiling.song83 at gmail.com>
wrote:

>
> To find the source of the pointer for the LoadInst, you'll need to climb
> up the def-use chain.  In the case of an add instruction, you will have to
> search back through both operands to figure out which one originates from a
> pointer.  You will also have to handle phi-nodes, so you'll probably need a
> list of processed phi-nodes to ensure that you don't iterate indefinitely.
>
> I tried the above idea, but I find it is easy find out whether a operand
> comes from a pointer. But for the other operand, which comes from a
> integer, it is hard to determine it does not come from a pointer, as
> integer may come from various kinds of instructions, the stop-condition to
> prevent further search is not obvious. As for the two operands of 'Add', i
> don't know which comes from pointer, obviously I have to go through both of
> them. I am not sure whether I understand your idea fully.
>
>
>>
>> The only other way to do it is find all the definitions that you consider
>> to be pointer "origins" (e.g., function arguments, the results of load
>> instructions, etc.) and iterate through their uses until you find the load
>> instruction that uses the pointer (in this case, %153).  In other words,
>> instead of starting at a use and searching for the definition, you start at
>> all possible definitions and look for the use.  If you're searching for a
>> lot of pointers, this may end up being more efficient as you won't be
>> traversing the same definitions over and over again.
>>
>> In short, you're attacking the problem in the right way, and I don't
>> think there's really any better way of doing it.
>>
>> Regards,
>>
>> John Criswell
>>
>>
>>
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