[LLVMdev] Safe loads

[Chris Lattner]

On Jan 23, 2012, at 4:22 AM, Roman Leshchinskiy wrote:

> Hello,
> 
> For the Glasgow Haskell Compiler's backend, we would like to let LLVM know
> that certain loads are safe to execute speculatively and hence to hoist
> out of loops. At the moment, there doesn't seem to be a mechanism for
> doing so. There seem to be two ways of implementing this: either allow
> arbitrary instructions to be marked as safe and have
> Instruction::isSafeToSpeculativelyExecute return true for those or mark
> memory regions and extend Value::isDereferenceablePointer to return true
> for those. Is either of these a good idea? Or is there some other way to
> do this? FWIW, I quickly prototyped instruction marking using metadata and
> that seemed to work well enough for us.

I think that marking the load with metadata would make sense.  Is it safe to load the pointer *anywhere*, or just "ahead of the loop"?  If it is safe to move it anywhere (e.g. because it is a load from constant memory, the optimizer just doesn't know it) then using metadata makes sense.  If it is a region where it is safe, then we'd need to implement something like this:
http://nondot.org/sabre/LLVMNotes/MemoryUseMarkers.txt

-Chris