[LLVMdev] Purpose of MSP430Wrapper (original) (raw)

Richard Osborne richard at xmos.com
Wed Jul 25 01:21:54 PDT 2012

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", SDTMSP430Wrapper>; 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).

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