[LLVMdev] Specifying Additional Compilation Passes to lli

[Evan Cheng]

On Sep 16, 2008, at 12:17 PM, Thomas B. Jablin wrote:

>
> ----- "Evan Cheng" <evan.cheng at apple.com> wrote:
>
>> On Sep 16, 2008, at 8:44 AM, Thomas B. Jablin wrote:
>>
>>> Evan,
>>> So, if I understand you correctly, the design you have in mind is
>>> to: create a PassManager, pass it to the JIT on construction, and
>>> modify runJITOnFunction to run the second PassManager on the
>>> Function being jit'd before running the codegen PassManager.
>> Thanks.
>>
>> Optimiztions should be done before JIT, right? Why not run the
>> optimizations (using the second PM) on the function that's scheduled
>>
>> for JIT before? Perhaps I am not understanding what you are trying to
>>
>> do.
>>
>> Evan
>
> Evan,
>    My goal is to support dynamic optimizations. Typically, a dynamic  
> optimization instruments code at runtime, then when enough profiling  
> information has been gathered the code is recompiled without the  
> instrumentation and with additional optimizations, if appropriate.  
> Sophisticated JITs, like Hotspot, will sometimes compile the same  
> function or code region multiple times in response to profiling  
> information or to specialize code for varying inputs.

Dynamic optimizations are being done by several systems which utilize  
LLVM JIT. I believe they are use a separate pass manager to control  
what optimization passes are run at runtime. I'd advise you to do the  
same. If I understand your requirement correctly, you'd like the JIT  
to run additional optimization passes when a function is lazily  
compiled, right? That will require overriding the default  
JITCompilerFn in JITEmitter.cpp.

You'll also have to roll your own lli replacement. Fortunately that's  
pretty straight forward.

Evan

>
>    Consider the following example function:
>
> int power(int (*func)(int), int x, int n) {
>  int i;
>  for (i = 0; i < n; i++) {
>    x = power(x);
>  }
>  return x;
> }
>
>    Hotspot can replace the indirect function call with an inlined  
> copy of the called function. This transformation is impossible  
> statically, since function pointed to by the first argument is  
> unknown at compile time. In order to employ similar optimizations in  
> LLVM, passes should be specifiable from the lli command-line.   
> Furthermore, passes need to be able to interrogate the execution  
> engine to find the Function corresponding to a pointer to a given  
> function stub. (I have implemented this as well, and will submit it  
> in my next patch.) Finally, the passes will need utility methods for  
> examining the stack frame of the method being called, and for  
> requested that functions be re-examined at each invocation. In the  
> distant future, OSR (On-Stack Replacement) within the LLVM JIT would  
> be nice too.
> Tom
>
>>
>>>
>>> Tom
>>>
>>> ----- Original Message -----
>>> From: "Evan Cheng" <evan.cheng at apple.com>
>>> To: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
>>> Sent: Tuesday, September 16, 2008 2:38:30 AM GMT -05:00 US/Canada
>>> Eastern
>>> Subject: Re: [LLVMdev] Specifying Additional Compilation Passes to
>> lli
>>>
>>>
>>> On Sep 15, 2008, at 9:56 AM, Thomas B. Jablin wrote:
>>>
>>>> Evan,
>>>> My overall goal is to support dynamic optimization in LLVM.  In
>>>> order to do so, I must gather profiling information at runtime,
>> then
>>>> recompile the profiled functions. Currently, I'm just adding and
>>>> removing calls into my profiler in a custom pass.  What is the
>>>> advantage of giving the JIT a second profile manager over my
>> current
>>>> implementation? Thanks.
>>>> Tom
>>>
>>> It's just a cleaner design. There are well defined pass manager,
>>> executionengine, and JIT api's. We don't want to unnecessarily
>> extend
>>> them. JITState PM controls the codegen passes. If you want to add
>> LLVM
>>> level optimization passes, you can simple use a separate PM to run
>>
>>> them.
>>>
>>> Evan
>>>
>>>>
>>>>
>>>> ----- Original Message -----
>>>> From: "Evan Cheng" <evan.cheng at apple.com>
>>>> To: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
>>>> Sent: Monday, September 15, 2008 12:46:16 PM GMT -05:00 US/Canada
>>>> Eastern
>>>> Subject: Re: [LLVMdev] Specifying Additional Compilation Passes to
>>
>>>> lli
>>>>
>>>> Hi Tom,
>>>>
>>>> I don't think this is the right approach. May I ask you what
>> passes
>>>> you are trying to add? If you just want to run a number of llvm
>> level
>>>> optimization passes, the right approach is to add your own pass
>>>> manager instead.
>>>>
>>>> Evan
>>>>
>>>> On Sep 11, 2008, at 10:30 AM, Thomas B. Jablin wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> I'm interested in specifying some additional passes to the JIT
>> via
>>>>> the command-line. The enclosed patch allows lli to take compiler
>>>>> passes as command-line arguments in the same way opt does. This
>> is
>>>>> my first submission, so any comments, criticisms, or observations
>>>>> would be very welcome. Thanks.
>>>>>
>>>>> Tom
>>>>> Jablin
>>>>> <
>>>>> PassArgumentsForLLI
>>>>> .diff>_______________________________________________
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>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>
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