[LLVMdev] Purpose of MSP430Wrapper

[Richard Osborne]

On 25 Jul 2012, at 04:49, Paul Shortis wrote:

> Hello,
> 
> I'm considering creating an LLVM backend for a 16 bit processor and 
> modelling it around the (experimental) MSP430 back end.
> 
> When reviewing MSP430InstrInfo.td I see
> 
> def MSP430Wrapper : SDNode<"MSP430ISD::Wrapper", SDT_MSP430Wrapper>;
> 
> and can see in MSP430ISelLowering.cpp that
> 
> ISD::GlobalAddress:
> ISD::BlockAddress:
> ISD::ExternalSymbol
> 
> all get lowered to MSP430ISD::Wrapper(address space) plus the wrapped 
> address node.
> 
> What has me mystified is that in some of the patterns in 
> MSP430InstrInfo.td take the form
> 
> def : Pat<(i16 (MSP430Wrapper tglobaladdr:$dst)), (MOV16ri 
> tglobaladdr:$dst)>;
> 
> and others ...
> 
> def : Pat<(MSP430call (i16 tglobaladdr:$dst)), (CALLi tglobaladdr:$dst)>;
> 
> In other words, some patterns rely on MSP430Wrapper being part of the 
> pattern then extract the wrapped info anyway, and some others just 
> directly match tglobaladdr and friends.
> 
> I have noticed that many other backends use the same idiom but so far I 
> can't see what it offers to the matching process.
> 
> Can someone please explain what I'm not seeing ?
> 
> Thanks, Paul.

I'm not sure about the MSP430 backend but I can talk about the XCore backend which uses a similar idiom. The XCore has 3 ways of accessing a global: using PC relative addressing, via an immediate offset from the thread's DP (data pointer) register or via an immediate offset from the thread's CP (constant pool) register. When lowering a global address to a target global address XCoreISelLowering wraps the tglobaladdress node with one of three wrappers (PCRelativeWrapper, DPRelativeWrapper, CPRelativeWrapper) based on properties of the global (is it a function, is it constant, etc) .

The wrapper is used in patterns in XCoreInstrInfo.td to ensure the right instruction is selected to access the global (for example to get the address of the global into a register the backend uses the LDAP instruction if the PCRelativeWrapper is present, LDAWCP if the CPRelativeWrapper is present and LDAWDP if the DPRelativeWrapper is present).