[LLVMdev] Optimization of calls to functions without side effects (from Kaleidoscope example)

[Duncan Sands]

Hi Rob,

> I'm using the gvn pass, not sure about basic-aa.

if you are using LLVM from svn then you need to specify the basic-aa analysis,
otherwise gvn won't unify calls to readonly/readnone functions.  This is new
behaviour introduced by Dan; probably the tutorial should be updated.

Ciao,

Duncan.

>
> I've copied the code as-is from http://llvm.org/docs/tutorial/LangImpl4.html#code and added "F->addFnAttr( Attribute::ReadOnly )" after "Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule)".
>
> The passes it sets up are:
>
>    // Set up the optimizer pipeline.  Start with registering info about how the
>    // target lays out data structures.
>    OurFPM.add(new TargetData(*TheExecutionEngine->getTargetData()));
>    // Do simple "peephole" optimizations and bit-twiddling optzns.
>    OurFPM.add(createInstructionCombiningPass());
>    // Reassociate expressions.
>    OurFPM.add(createReassociatePass());
>    // Eliminate Common SubExpressions.
>    OurFPM.add(createGVNPass());
>    // Simplify the control flow graph (deleting unreachable blocks, etc).
>    OurFPM.add(createCFGSimplificationPass());
>
> It does simplify _some_ things. For example:
>
> ready>  def simplifyThis(x) (x*2)+(2*x);
> Read function definition:
> define double @simplifyThis(double %x) readonly {
> entry:
>    %multmp = fmul double %x, 2.000000e+00
>    %addtmp = fadd double %multmp, %multmp
>    ret double %addtmp
> }
>
>
>> -----Original Message-----
>> From: Duncan Sands [mailto:baldrick at free.fr]
>> Sent: Monday, November 15, 2010 2:57 PM
>> To: llvmdev at cs.uiuc.edu
>> Subject: Re: [LLVMdev] Optimization of calls to functions without side
>> effects (from Kaleidoscope example)
>>
>> Hi Rob,
>>
>>> Hmm ... I tried setting that right after Function::Create but I still
>> get the same result (though flagged with "readonly")
>>
>> did you run the gvn pass (preceded by basic-aa)?
>>
>> Ciao,
>>
>> Duncan.
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev