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RE: Tree SSA If-combine optimization pass in GCC



-----Original Message-----
From: Richard Biener [mailto:richard.guenther@gmail.com] 
Sent: Tuesday, February 17, 2015 3:42 PM
To: Ajit Kumar Agarwal
Cc: gcc@gcc.gnu.org; Vinod Kathail; Shail Aditya Gupta; Vidhumouli Hunsigida; Nagaraju Mekala
Subject: Re: Tree SSA If-combine optimization pass in GCC

On Tue, Feb 17, 2015 at 9:22 AM, Ajit Kumar Agarwal <ajit.kumar.agarwal@xilinx.com> wrote:
> Hello All:
>
> I can see the IF-combining (If-merging) pass of optimization on tree-ssa form of intermediate representation.
> The IF-combine or merging takes of merging the IF-THEN-ELSE if the 
> condition Expr found be congruent or Similar.
>
> The IF-combine happens if the two IF-THEN-ELSE are contiguous to each other.
> If the IF-THEN-ELSE happens to be not contiguous but are wide apart with there is code in between.
> Does the If-combine takes care of this. This requires to do the 
> head-duplication and Tail-duplication for the Code in between If-THEN-ELSE to bring the IF-THEN-ELSE contiguous to each other.
>
> After the head and tail duplication of the code in between the 
> IF-THEN-ElSE sequence becomes contiguous to each other. Apart from 
> this, Does the tree-ssa-if-combine pass considers the control flow of the body of the IF-THEN-ELSE. Is there any limitation on control flow of the body of the IF-THEN-ELSE.
>
> Can I know the scope of tree-ssa-ifcombine optimizations pass with respect to the above points.
>
> Thoughts Please?

>>if-combine is a simple CFG + condition pattern matcher.  It does not perform head/tail duplication.  Also there is no "control flow" in the bodies, control flow is part of the CFG that is >>matched so I'm not quite getting your last question.

Thanks ! My last question was If there is a control flow likes  loops inside the IF-THEN-ELSE, which could be possible if the Loop unswitching is performed 
and the Loop body is placed inside the IF-THEN-ELSE, then in that case the two IF-THEN-ELSE can be merged if the cond expr matches and the control flow
of the body of If-then-else matches?

There are many cases in SPEC 2006 benchmarks where the IF-combine could be enabled if the if-then-else sequence is made contiguous by performing
the head/tail duplication. 

>>if-combine was designed to accompany IL-only patterns that get partly translated into control flow.  Like

  >>tem1 = name & bit1;
  >>tem2 = name & bit2;
  >>tem3 = tem1 | tem2;
  >>if (tem3)
    ...

>>vs.

  >>tem1 = name & bit1;
  >>if (tem1)
   >>goto x;
  >>else
    >>{
      >>tem2 = name & bit2;
      >>if (tem2)
       >> goto x;
    >>}

>>x:
   >>...
Thanks for the examples. This explains the scope of if-combine optimization pass.

Thanks & Regards
Ajit

Richard.

> Thanks & Regards
> Ajit

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