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

Wed Nov 4 10:02:37 PST 2015

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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