[LLVMdev] Small Data Adressing support

[David Wiberg]

Hi all,

I'm trying figure out how to implement support for Small Data
Addressing (SDA) for an out-of-tree PowerPC target. The short
description of SDA is that small globals are gathered in the same
memory area and addressed using a base register + an offset. From what
I've seen the Hexagon target already support this but as that target
doesn't seem to support writing object files it doesn't take me all
the way.

My plan has been:
1. Use custom lowering for GlobalAddress and insert a target specific
ADDR_SDA node if the object pointed to is suitable for SDA. The target
specific node gets a TargetGlobalAddress node as leaf.
2. Use a pattern to match the graph "set->load->ADDR_SDA" and create a
load instruction with fixed base register from this pattern. The
pattern is similar to the one I use for the normal load instruction.

When I tried to implement this I hit an assert during tablegenning
which claims that "set" is an unknown node.

If I compare this approach with Hexagon it instead matches against
"atomic_load_32". I tried this too but then I don't have any
destination register available and can't use the load instruction.

If someone has a good picture of how to implement this I would really
appreciate some advice. I suspect other targets have similar
addressing modes and if so it would be nice to be pointed towards the
implementation.

Thanks in advance!

- David