[llvm-dev] Confused on how to do a machinefunction pass
John Criswell via llvm-dev
llvm-dev at lists.llvm.org
Wed Nov 4 06:40:28 PST 2015
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
> <mailto: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
>>
>>
>>
>> _______________________________________________
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>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
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>
>
> --
> John Criswell
> Assistant Professor
> Department of Computer Science, University of Rochester
> http://www.cs.rochester.edu/u/criswell
>
>
--
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell
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