[LLVMdev] Memory Subsystem Representation

[David A. Greene]

Chris Lattner <clattner at apple.com> writes:

> On May 3, 2011, at 12:01 PM, David A. Greene wrote:
>
>> "Rotem, Nadav" <nadav.rotem at intel.com> writes:
>> 
>>> Can you describe which passes may benefit from this information ?  My
>>> intuition is that until there are a number of passes which require
>>> this information, there are other ways to provide this
>>> information. One way would be to use Metadata.
>> 
>> We have Cray-specific passes that use this information.  Some of the
>> stuff Polly is doing almost certainly would benefit.
>

> This sounds like an interesting addition, but we don't just
> speculatively add analysis passes to LLVM.

Well, we do have such an analysis pass.  I simply can't make it public.
My plan was to provide the information we use today and leave
enhancements to others as they are needed.

> Doing so leads to overdesign and lack of purpose.  I'd suggest
> designing and implementing a specific loop optimization pass, hard
> coding it to a specific target, then generalize it by adding a way to
> get target parameters.  This is how LSR was built, for example, which
> drove the address mode selection stuff.

Unfortunately, I don't have the bandwidth to design a whole new loop
pass.  If someone has the time and interest I would be very happy to
work with those people.

>> Metadata seems a very clunky way to do this.  It is so target-specific
>> that it would render IR files completely target-dependent.  These are
>> rather complex structures we're talking about.  Encoding it in metadata
>> would be inconvenient.
>

> I agree that metadata isn't the right way to go, but this argument
> doesn't fly at all with me.  The whole point of these passes are to do
> target-specific optimizations.

Yes, but one may want to take a single IR file and target multiple
machines from it.

> The right way to expose this sort of thing is with the new
> TargetRegistry interfaces.  That said, speculatively adding target
> hooks isn't the right way to go, the client should come first.

Ok, I will look at TargetRegistry.  And again, it isn't speculative,
it's just not public.

                              -Dave