Re: How to eliminate redundant constant move instructions

On Nov 5, 5:26 am, George Neuner <gneun...@comcast.net> wrote:
> On Tue, 1 Nov 2011 19:21:54 -0700 (PDT), amker <can.fin...@gmail.com>
> wrote:


> >Yes, I have noticed this pass. Seems it can solve the problem if I
> >can:
> >1, extend the pass in value numbering way, at least for const values.
> >2, extend the pass in global data analysis way.
>
> Yes. But value numbering doesn't know or care what specific value is
> in the register - it cares only whether the value in the register
> might have been changed between read uses.
>
> Variable value tracking is the related technique used for constant
> propagation. However what you are after is variable equivalence
> testing - I don't think GCC even tries to do this.


Yes, I guess it is the variable value tracking. But I think GCC does
do such work in some passes, at least cse.c. In this pass gcc records
the constant value if a quantity has a known constant value. Then gcc
simplify expressions containing such constant value by calling
fold_rtx.


Unfortunately, cse only works on basis of extended basic block.
Nothing it can do for global cases as reported in mentioned bug.


Also, could you point me some papers of implementation of such
variable value tracking technique? I googled and found nothing
about it.


Thanks

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