[LLVMdev] TBAA fail on optimization, why?
junkoi2004 at gmail.com
Thu Sep 27 01:04:46 PDT 2012
On Thu, Sep 27, 2012 at 4:01 PM, Duncan Sands <baldrick at free.fr> wrote:
> Hi Jun,
>> ok, below is my code. i run this through "opt" with option: "-basicaa
>> -mem2reg -tbaa"
>> however, the resulted bitcode file is still the same; no optimization is
> that's because you didn't specify any optimization that uses alias analysis!
> LLVM makes a distinction between analyses and transforms. Analyses, like
> are passes which look at your IR but don't modify it: instead they deduce
> things about it (eg which memory accesses alias each other) and provide
> that information to other passes. Transforms are passes that modify the IR,
> i.e. these are the passes that do the actual optimizing. Consider the -dse
> (dead store elimination pass). This is a transform pass, however it needs
> to know which memory accesses alias each other. It doesn't compute this
> instead it requests this information from the alias analysis subsystem,
> queries any alias analysis passes that may have run. By specifying -tbaa
> have only done the analysis part, not the transform part.
thanks, now i got it.
>> meanwhile, the last "store" instruction is easily eliminated with "-O3"
> Try -tbaa -basicaa mem2reg -dse
this indeed works, thanks!!!
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