Re: [RFC] sparse SSA construction

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Hi Chris,

On 16 August 2017 at 13:50, Christopher Li <sparse@xxxxxxxxxxx> wrote:
> On Wed, Aug 16, 2017 at 8:39 AM, Dibyendu Majumdar
> <mobile@xxxxxxxxxxxxxxx> wrote:
>> It is an advantage and a disadvantage in the sense that it makes SSA
>> construction not reusable - or have I misunderstood? Looking at the
>
> Not sure I understand the question.

I meant that it would nice to save a function like SSA(linear input)
-> SSA output - because then you can rerun the SSA conversion several
times during optimization.

>
>> implementation I felt that it could be run as a second phase - is that
>> assumption wrong?
>
> That assumption is correct. But not having the advantage the paper
> claim any more.
>

I think there is still an advantage as the implementation is bottom
up, uses memoization / dynamic programming techniques, and hence might
actually be quite efficient. I think Luc is better placed to comment
on this.

>>
>> I am just trying to understand the issue as it seems the paper claims
>> it creates pruned SSA for all programs - not just reducible control
>> flow. Have you contacted the authors regarding the issues you are
>
> Sorry I might mislead you. I said the paper solution for irreducable
> graph is *not* simple *nor* efficient any more. The paper do suggest
> a way to do it. But I consider that way complex (in execution time sense).
> I haven't take a closer look at the code yet.
>
>> having with gotos? Might be worth doing so.
>
> My reading of the paper is yes. Having "goto" will interrupt that
> CFG less method. It can also lead to irreducible graph. Which means
> extra step to clean it up. That extra step is expensive in the O() terms.
>

But Luc's implementation handles gotos fine without the extra phase -
hence please please try out the implementation before arriving at
conclusions.

Regards
Dibyendu
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