Question about addr capture

[Duncan Sands]

Hi Shuxin,

On 22/03/13 22:48, Shuxin Yang wrote:
> Hi,
>
>    Arnold just show me a crazy contrived example which I think it is really
> hard, if not impossible, to analyze.

there are many crazy examples, which is why the logic is so conservative.
However when it was first written there was a debate about whether it has
to be so conservative, since language rules can save you.  For example,
suppose you take a C pointer P and decompose it one bit at a time, then
recompose a new pointer Q from those bits, so Q == P.  I'm told that the
C standard says the compiler is allowed to assume that Q and P don't alias.
At the time it was decided not to exploit this kind of thing, but if you
have some important test cases that would benefit from it, it might be worth
reconsidering [*].

Ciao, Duncan.

[*] As usual, the problem with language rules is that every language has
different rules, so it is necessary to set this up in such a way that every
language can get what it wants.  That's probably pretty painful.

> Thank you all for the input.
>
> Shuxin
>
> On 3/22/13 1:45 PM, Duncan Sands wrote:
>> Hi Shuxin,
>>
>> On 22/03/13 18:56, Shuxin Yang wrote:
>>> Hi, list:
>>>
>>>       Would someone shed some light on the following code.  How can an address
>>> be captured via icmp?
>>
>> using icmp you can determine each bit of the value, one by one, and then
>> reconstruct the value from those bits.
>>
>> Ciao, Duncan.
>>
>> PS: I think there are some fairly comprehensive test cases for capture tracking,
>> and they are commented IIRC.  If you disable some logic, like this, you should
>> see a test case fail.  Hopefully it should then become clear from the test case
>> what the issue is.
>>
>>>
>>> Thanks
>>> Shuxin
>>>
>>> cat -n lib/Analysis/CaptureTracking.cpp
>>> ==========================
>>> 157     case Instruction::ICmp:
>>> 158       // Don't count comparisons of a no-alias return value against null as
>>> 159       // captures. This allows us to ignore comparisons of malloc results
>>> 160       // with null, for example.
>>> 161       if (isNoAliasCall(V->stripPointerCasts()))
>>> 162         if (ConstantPointerNull *CPN =
>>> 163 dyn_cast<ConstantPointerNull>(I->getOperand(1)))
>>> 164           if (CPN->getType()->getAddressSpace() == 0)
>>> 165             break;
>>> 166       // Otherwise, be conservative. There are crazy ways to capture
>>> pointers
>>> 167       // using comparisons.
>>> 168       if (Tracker->captured(U))
>>>
>>>
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