[LLVMdev] memory lifetime and invariance

[Chris Lattner]

On May 20, 2009, at 9:25 AM, Andrew Lenharth wrote:

> On Mon, May 18, 2009 at 3:02 PM, Chris Lattner <clattner at apple.com>  
> wrote:
>> FYI, I wrote up some thoughts on this here:
>> http://nondot.org/sabre/LLVMNotes/MemoryUseMarkers.txt
>>
>> The intention is to allow front-ends to express things like "I know
>> this memory is a constant in this region" and to allow the optimizer/
>> codegen to perform more aggressive stack slot merging.
>
> It seems no more prone to abuse to reserve an address space for const
> memory.  Within a const region, all pointers to the const data are in
> the const address space.  So rather than starting with a intrinsic,
> you start with a ptrcast.  This would also make checking trivial
> stores to const regions easy in the verifier.

I don't see how this helps.  Fundamentally, you have something like  
this:

   store x -> ptr
   <many loads and stores>
   load ptr
...

The trick is to know that ptr does not alias the loads and stores.   
The further issue is that you don't know when the load is generated  
whether it is in the const section or not, so you couldn't bitcast its  
pointer operand even if you knew that.  A silly C++ example is:

class foo{
   int ivar;
   foo() ...

   int bar() { return ivar; }
};

If "bar" is called from the constructor, then the load is from a  
potentially varying value, if it is called from code after the  
constructor, it is known const (assuming the instance was declared  
const).

-Chris