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

Paul Shortis pauls at dataworx.com.au
Wed Jul 25 04:14:37 PDT 2012

On 25/07/12 18:21, Richard Osborne wrote:

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).

LLVM Developers mailing list LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev Thanks Richard,

You're correct, they are similar. In the XCoreInstrInfo.td patterns what I'm struggling with is why this ....

def BL_lu10 : _FLU10< (outs), (ins calltarget:$target, variable_ops), "bl $target", [(XCoreBranchLink immU20:$target)]>;

def : Pat<(XCoreBranchLink tglobaladdr:$addr), (BL_lu10 tglobaladdr:$addr)>; def : Pat<(XCoreBranchLink texternalsym:$addr), (BL_lu10 texternalsym:$addr)>;

is necessary. Are the Pat<> s just 'casting' tglobaladdr:$addr and texternalsym:$addr to an immU20 to force a match ?

I'm guessing similar Pat<> 's aren't required for the BL_u10/immU10 cases because they match without any assistance ?

Is the XCoreBranchLink enough of a match hint that an address wrapper isn't required to clarify the pattern match for these call instructions ?

Cheers, Paul

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