[LLVMdev] Activating MIPS Code Emitter.

[Jafar J]

Yes your absolutely right, the Opcode and the Operands in each machine instruction are sufficient to generate the final binary representation of the MachineInstruction but not exactly. If you take a look at the format of each MIPS instruction, you’ll see that there are some fixed bits for each instruction which are not available inside the machine instruction object –From what I saw so far-. Furthermore, how will I know how to map each operand to its right place i.e. rt, rd, rs, immediate, offset, etc.., without knowing the semantic of the instruction and the physical numbering of each register, to finally generate the complete 32-bit encoding.

MachineInstr::getOpcode() for a register operand returns an enum value that doesn’t represent the actual physical numbering of the register, to know what register this returned value represents I should refer to MipsGenRegisterInfo.td, that’s where I think the mapping happens.

However, inside the MipsCodeEmitter class there is a method that returns the binary encoding of the machine instruction (uint64_t MipsCodeEmitter::getBinaryCodeForInstr(const MachineInstr &MI)). That’s why I thought about activating the code emitter during post-ra would solve my problem if that’s even possible.

Thanks,
    - Jafar J.

From: Jim Grosbach 
Sent: Thursday, May 30, 2013 8:49 PM
To: Jafar J 
Cc: llvmdev at cs.uiuc.edu ; Mailing List 
Subject: Re: [LLVMdev] Activating MIPS Code Emitter.



Thanks, that helps. The code emitter is definitely not the way you want to go about solving this problem, though. Are the instruction opcode (MachineInstr::getOpcode()) and the operand values not sufficient? All the information present in the encoding should be inferable from those, as that’s where the encoding comes from. 

-Jim




On May 30, 2013, at 10:12 AM, Jafar J <pluck90 at hotmail.com> wrote:


  I need to represent each instruction with its (32-bit) binary encoding, and I reached to a conclusion that I could get the encoding through the MipsCodeEmitter. What I’m trying to do exactly is write a scheduler which tries to minimize the switching activity between the scheduled instructions in each basic block. One way to do that is by representing each instruction with its complete binary encoding, which will be available after the register allocation.
  Thanks for the reply!
  -Jafar J

  From: Jim Grosbach
  Sent: Thursday, May 30, 2013 7:55 PM
  To: Jafar J
  Cc: llvmdev at cs.uiuc.edu ; Mailing List
  Subject: Re: [LLVMdev] Activating MIPS Code Emitter.

  What are you actually trying to do? The code emitters have nothing to do with the post-RA scheduler. 

  -Jim

  On May 30, 2013, at 6:23 AM, Jafar J <pluck90 at hotmail.com> wrote:


    Hello,
     
        Is it possible to activate the MIPS code emitter during Post-RA scheduler. I tried including both MipsCodeEmitter.cpp and JITCodeEmitter.h to PostRASchedulerList.cpp, but when I rebuild the compiler I get an error that says “/lib/Target/Mips/MCTargetDesc/MipsMCTargetDesc.h fatal error: MipsGenRegisterInfo.inc file not found”. I’m assuming that the MipsGenRegisterInfo.inc is not yet generated when I’m trying to include it in the PostRAScheduler. Is this the way to activate the mipsCodeEmitter during PostRA Scheduler or am I missing something here.

    Thanks,
        Jafar J.
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