makeAddress and large frame sizes

[Doug Simon]

How does C1 and C2 on SPARC handle this?

On Apr 29, 2014, at 4:49 PM, D.Sturm <D.Sturm42 at gmail.com> wrote:

> Hi,
> This is a bit complicated, but I hope I can make the problem
> comprehensible.
> 
> The Aarch64 ISA only allows a 9-bit signed unscaled offset added to a
> register for memory accesses. Consequently for stacks that are larger than
> 2^8 byte we have a problem when using AbstractAssembler.makeAddress to
> access something on the stack with an offset larger than 256 bytes. (SPARC
> seems to have the same problem and seems to just ignore the problem -
> 13-bit signed offsets work fine for all JTT tests I guess)
> 
> I can solve that problem when loading values from the stack since I can use
> the result register as a scratch register (and don't
> use CompilationResultBuilder.asAddress but compute the stack offset myself).
> 
> But when storing values *into* the stack I really need to allocate a
> temporary register if the offset is too large. Always allocating a scratch
> register even if I only need it in case of a large stack and worse even for
> reg->reg moves since I don't know whether the register allocator will
> actually use a register for the destination and not a stackslot when
> generating LIR instructions seems like a really unfortunate solution.
> 
> I was thinking I could specify a scratch register for all moves and then
> in beforeRegisterAllocation() remove the load if I can guarantee that the
> stack won't be too large (hard since I don't know how many registers the
> register allocator will have to spill? some heuristic seems necessary).
> 
> Sounds reasonable or any problems with that approach? Or maybe some
> completely different solution?
> 
> 
> --Daniel