Tree SSA If-combine optimization pass in GCC

Richard Biener
Tue Feb 17 12:19:00 GMT 2015

On Tue, Feb 17, 2015 at 11:26 AM, Ajit Kumar Agarwal
<> wrote:
> -----Original Message-----
> From: Richard Biener []
> Sent: Tuesday, February 17, 2015 3:42 PM
> To: Ajit Kumar Agarwal
> Cc:; 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 <> 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.

I'd be curious what those cases look like.  Care to file some bugreports
with testcases?

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