[LLVMdev] General strategy to optimize LLVM IR

[Andrew Trick]

On Jul 16, 2013, at 11:07 AM, David Blaikie <dblaikie at gmail.com> wrote:

> On Tue, Jul 16, 2013 at 8:16 AM, Stéphane Letz <letz at grame.fr> wrote:
>> Hi,
>> 
>> Our DSL emit sub-optimal LLVM IR that we optimize later on (LLVM IR ==> LLVM IR) before dynamically compiling it with the JIT. We would like to simply follow what clang/clang++ does when compiling with -O1/-O2/-O3 options. Our strategy up to now what to look at the opt.cpp code and take part of it in order to implement our optimization code.
>> 
>> It appears to be rather difficult to follow evolution of the LLVM IR optimization strategies. With LLVM 3.3 our optimization code does not produce code as fast as the one produced with clang -03 anymore. Moreover the new vectorizations passes are still not working.
>> 
>> It there a recommended way to add -O1/-O2/-O3 kind of optimizations on LLVM IR code? Any code to look at beside the opt.cpp tool?
> 
> I'm not /entirely/ sure what you're asking. It sounds like you're
> asking "what passes should my compiler's -O1/2/3 flag's correspond to"
> and one answer to that is to look at Clang (I think Clang's is
> different from opt/llc's, maybe).

PassManagerBuilder decides what passes to run. Unfortunately, the clang driver uses a back door to set a bunch of flags that configure PassManagerBuilder See EmitAssemblyHelper::CreatePasses. I find this extremely difficult to follow and don’t know of any way to derive an equivalent “opt” command line. Good luck.

-Andy

> 
>> 
>> Thanks.
>> 
>> Stéphane Letz
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