[LLVMdev] Instantiating Target-Specifc ASM Parser

[Stefan Hepp]

Hi,

We are using the AsmParser to analyse inline asm instructions in our 
backend, i.e. to get the actual size in bytes of an instruction or to 
find all calls,.. I assume this might be similar to what you want to do.
I let AsmPrinter do all the hard work of parsing inline asm and 
instantiating everything. This might be a bit of an overkill though if 
you do not need inline-asm parsing (it replaces placeholders with proper 
register names, ...). For your case it might be sufficient to have a 
look at the code in AsmParser::EmitInlineAsm(StringRef Str, ..). You 
will need to have a proper context and MCStreamer object though..

Here is what I did to make this happen:

1) Extract a header for the MCNullStreamer class to include/llvm/MC to 
be able to make subclasses. I will attach the modified files for your 
convenience.

2) Make the EmitInlineAsm functions in AsmPrinter public. Move 
construction and deletion of the Mangler in AsmPrinter.cpp into the 
constructor/destructor of AsmPrinter. In AsmPrinter::EmitInlineAsm 
(AsmPrinterInlineAsm.cpp:93), add an 'if (MMI) { .. }' to skip the 
DiagnosticHandler stuff if MMI is not set.

3) Create a subclass of MCNullStreamer to visit all parsed instructions 
in the inline asm code, like so:

class InstrAnalyzer : public MCNullStreamer {
   const MCInstrInfo &MII;
   unsigned size;
public:
   InstrAnalyzer(MCContext &ctx)
     : MCNullStreamer(ctx), MII(ctx.getInstrInfo()), size(0)
     { }
   virtual void EmitInstruction(const MCInst &Inst) {
     const MCInstrDesc &MID = MII.get(Inst.getOpcode());
     size += MID.getSize();
   }
};

4) Create a new AsmPrinter and use it to parse and emit asm, e.g. like 
so (our backend is called Patmos, so do not wonder about this..):

unsigned int PatmosInstrInfo::getInstrSize(const MachineInstr *MI) const {
   if (MI->isInlineAsm()) {
     // PTM is the TargetMachine
     // TODO is there a way to get the current context?
     MCContext Ctx(*PTM.getMCAsmInfo(),
                   *PTM.getRegisterInfo(), *PTM.getInstrInfo(), 0);

     // PIA is deleted by AsmPrinter
     PatmosInstrAnalyzer *PIA = new InstrAnalyzer(Ctx);

     // PTM.getTargetLowering()->getObjFileLowering() might not yet be
     // initialized, so we create a new section object for this temp context
     const MCSection* TS = Ctx.getELFSection(".text",
                                             ELF::SHT_PROGBITS, 0,
                                             SectionKind::getText());
     PIA->SwitchSection(TS);

     PatmosAsmPrinter PAP(PTM, *PIA);
     PAP.EmitInlineAsm(MI);

     return PIA->getSize();
   }
   else if (MI->isBundle()) {
     // handle bundles..
     unsigned size = ...;
     return size;
   }
   else {
     // trust the desc..
     return MI->getDesc().getSize();
   }
}

I tested this with LLVM 3.2, and it should work with LLVM 3.3 (though I 
am not finished testing it..).

Cheers,
  Stefan Hepp


On 07/16/2013 02:27 PM, Dunn, Kyle wrote:
> Hello,
>
> I am working on backend development and would like to utilize my
> target's MCAsmParser inside of an MCInst-level class implementation. I
> noticed that the AsmParser is registered with the target registry
> however I am having no luck grepping for a "template" of how to
> instantiate it and have yet to find specific documentation on how it is
> done. Any ideas or help is greatly appreciated!
>
>
> Cheers,
> Kyle Dunn
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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