[llvm-dev] Confused on how to do a machinefunction pass

Fri Nov 6 07:56:40 PST 2015

Hi
So I m trying to write my first backend pass. I created a file in
target\x86\wawanalyzer.cpp based on hexagonHardwareLoops.cpp file. Now it's
just a simple file that prints hello to the output. I have attached the
file to this post.

Then I did the following:

1. add "FunctionPass *createwawAnalyzer();" line to x86.h
2. add file name to CMakelist.txt.
3. This is going to run after postRAscheduler and before code emission so I
changed the x86TrgetMachine.cpp as follow:
void X86PassConfig::addPreEmitPass() {
/////////////add mypass here///////
  addPass(createwawAnalyzer());
//////////////////////////////////
  if (getOptLevel() != CodeGenOpt::None)
    addPass(createExecutionDependencyFixPass(&X86::VR128RegClass));

  if (UseVZeroUpper)
    addPass(createX86IssueVZeroUpperPass());

  if (getOptLevel() != CodeGenOpt::None) {
    addPass(createX86PadShortFunctions());
    addPass(createX86FixupLEAs());
  }
then I run make on mypass and remake  llvm3.7\lib and llvm3.7\tool\llc.
but when I run llc -help it doesn't show my pass. Did I miss some steps or
did something wrong? What's the correct way to run the pass with llc
command?

I appreciate any help,
Regards
Fami

On Wed, Nov 4, 2015 at 1:02 PM, fateme Hoseini <hoseini.f at gmail.com> wrote:

> Thank you so much.
> That helped alot.
>
> Fami
>
> On Wed, Nov 4, 2015 at 9:40 AM, John Criswell <jtcriswel at gmail.com> wrote:
>
>> On 11/3/15 7:54 PM, fateme Hoseini wrote:
>>
>> Dear John,
>> Thank you so much for your help. I looked at those documents. Could you
>> kindly answer the following questions:
>>
>> Does it mean that I have to make my own backend target in order to write
>> a machine pass even if I want to run a simple machinefunction pass?  for
>> example,if I want my pass to get MIPS instructions as an input, I have to
>> copy all the files from mips target and add a file to it which implements
>> my pass. and there should be a way to connect the pass to other MIPS
>> backend files?
>>
>>
>> No, you do not need to create a new backend.  All you need to do is to
>> add your pass to the list of passes that are run when the MIPS code
>> generator is used.
>>
>> In LLVM 3.3, there was a file in the X86 backend that had code to
>> schedule all the MachineFunctionPass'es  when the X86 code generator was
>> used.  That was in lib/Target/X86/X86TargetMachine.cpp.  You can probably
>> find a similar file for LLVM 3.7 for the MIPS backend.
>>
>> So, to summarize, you'll add your source file to the MIPS backend, add a
>> line somewhere to run your pass when the MIPS code generator is used, and
>> then recompile llvm/lib and llvm/tools/llc.
>>
>>
>>
>> As a result, is the way that I described in my first post completely
>> wrong because I cannot run a stand alone machine function pass like a
>> frontend function pass?
>>
>>
>> As far as I know, there is no way to load a MachineFunctionPass and run
>> it using llc, opt, or clang.  I believe you need to compile it into the
>> MIPS backend code.
>>
>>
>> Also another question, I'm a PhD student and I'm completely new to the
>> llvm backend process. I want to estimate my project time. How long
>> approximately do you think it will take to be get familiar with backend and
>> be able to write machinefunction pass? for further steps I have to
>> implement a register allocation algorithm.
>>
>>
>> It's difficult to estimate (especially since I haven't implemented
>> anything as complicated as a register allocator).  Your profiling pass may
>> take anywhere from 1 week to 1 month depending on how complicated it is.  I
>> think a register allocator would take anywhere from 1 month up to 6 months
>> depending on how robust you need it to be, but since I've never built one,
>> I can't say for certain.
>>
>> Regards,
>>
>> John Criswell
>>
>>
>>
>> Regards,
>> Fami
>>
>>
>> On Tue, Nov 3, 2015 at 3:52 PM, John Criswell <jtcriswel at gmail.com>
>> wrote:
>>
>>> Dear Fami,
>>>
>>> A MachineFunctionPass is run by the code generator, so you can only use
>>> it in tools like llc, clang, and libLTO.  The opt program only manipulates
>>> LLVM IR and therefore does not run MachineFunctionPass'es.
>>>
>>> Regarding documentation, have you looked at the following?
>>>
>>> http://llvm.org/docs/WritingAnLLVMBackend.html
>>> http://llvm.org/docs/CodeGenerator.html
>>> http://llvm.org/docs/MIRLangRef.html
>>>
>>> Regards,
>>>
>>> John Criswell
>>>
>>>
>>> On 11/3/15 3:47 PM, fateme Hoseini via llvm-dev wrote:
>>>
>>> Hi everyone,
>>> I am a LLVM newbie. I need to write a machinefunction pass for my
>>> project. This should be an analyzer of the machine code to do some
>>> profiling. I have written a couple of function passes for front end, but it
>>> seems they are completely different. I searched through forum and llvm
>>> documents, but couldn't find anything useful or questions were unanswered.
>>> Can anyone reference me to any material on how to do that or help me with
>>> my roblem.
>>>
>>> I have created a folder in: lib/Transform and put my pass in it.
>>> I am writing a simple pass like this:
>>>
>>> using namespace llvm;
>>>
>>> namespace {
>>>     struct analyzer : public MachineFunctionPass {
>>> static char ID;
>>> analyzer() : MachineFunctionPass(ID) {}
>>> virtual bool runOnMachineFunction(MachineFunction &MF) {
>>> errs() << "hello " ;
>>> return false;
>>> }
>>>   };
>>> }
>>> char analyzer::ID = 0;
>>> static RegisterPass<analyzer> X("analyzer", "WAW analyzer");
>>>
>>> I make it successfully. But when I load it for my test code I get this
>>> error:
>>>
>>> Pass 'WAW analyzer' is not initialized.
>>> Verify if there is a pass dependency cycle.
>>> Required Passes:
>>> opt: /llvm/llvm-3.7/lib/IR/LegacyPassManager.cpp:635: void
>>> llvm::PMTopLevelManager::schedulePass(llvm::Pass*): Assertion `PI &&
>>> "Expected required passes to be initialized"' failed.
>>> .....
>>>
>>> I don't know how to solve it? Do we have to run machinefunction passes
>>> with OPT command? Is this correct to put machinefunction pass in an
>>> external folder or do we have to change some llvm built-in files?
>>>
>>> Regards,
>>> Fami
>>>
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>>
>>> --
>>> John Criswell
>>> Assistant Professor
>>> Department of Computer Science, University of Rochesterhttp://www.cs.rochester.edu/u/criswell
>>>
>>>
>>
>>
>> --
>> John Criswell
>> Assistant Professor
>> Department of Computer Science, University of Rochesterhttp://www.cs.rochester.edu/u/criswell
>>
>>
>
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