[llvm-commits] [llvm] r117070 - /llvm/trunk/lib/Analysis/TypeBasedAliasAnalysis.cpp

[John McCall]

On Oct 22, 2010, at 7:05 AM, Duncan Sands wrote:

> Hi Dan,
> 
>>  // If the third field is present, it's an integer which if equal to 1
>> -// indicates that the type is "constant" (meaning
>> -// pointsToConstantMemory should return true; see
>> +// indicates that the type is "constant" (meaning pointsToConstantMemory
>> +// should return true; see
>>  // http://llvm.org/docs/AliasAnalysis.html#OtherItfs).
> 
> is this "constant" field actually useful?  How are you imagining that it will
> be used?  Even in Ada, which has a pretty strong type system and notions of
> constantness, the fact that you have a view of an object via a constant type
> doesn't mean that the object can't be altered, it just means it can't be
> altered via this view.  In C, this is something like saying: suppose the
> address of an int is held in a "const int*", then you can't alter the int via
> this pointer; but you can always alter it by putting the address of the int in
> a "int *" and using that instead.  So the fact that you have a "const int*"
> doesn't mean you that it is valid for pointsToConstantMemory to return true.

Both C and C++ say that modifying an object defined with a const qualifier
has undefined results, so while you can't use this for pointer-to-const, you
could use it for const variables after they've been initialized.

John.