Re: Tree SSA If-combine optimization pass in GCC

[Richard Biener <richard dot guenther at gmail dot com>]

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.

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:
   ...

Richard.

> Thanks & Regards
> Ajit