Re: Short Circuit compiler transformations!!

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

On March 15, 2015 9:15:36 AM GMT+01:00, Ajit Kumar Agarwal <ajit.kumar.agarwal@xilinx.com> wrote:
>Hello All:
>
>Short circuit compiler transformation for conditional branches. The
>conditional branches based on the conditional
>Expressions one of the path is always executed thus short circuiting
>the path. Certains values of the conditional
>Expressions makes the conditional expressions always true or false.
>This part of the conditions in extracted out
>In the IF-THEN cond_express with the check of the value and in the else
>case the original expressions is checked.
>
>This makes for a given values of the variables of the conditional
>expressions makes the conditional expression as
>True or false always and the one path is always executed.
>
>For the example given below.
>
>For( x1 = lb1; x1 < = ub1 ; x1++)
>{
>   If ( C1 && C2)
>     S_then
> }
>
>
>Fig (1).
>
>For the input code given in Fig (1) the condition C1 && C2 is always
>false if C1 or C2 is zero(false) thus short circuiting
>The path and only s_else will be executed if C1 or C2 is false.
>
>Thus the input code is transformed as follows.
>
>For( x1 = lb1; x1 < = ub1 ; x1++)
>{
>   If(!C1)
>    Continue
>  else
>    S_then
> }

This looks wrong as you miss to check C2.

Richard.

>This is much simpler code and there could be complex expressions inside
>the FOR Loop that can be extracted out with 
>Different versions of the conditional IF-THEN-ELSE inside the loops
>based on short circuiting executing one of the path always
>Executed can be extracted in IF-THEN and other cases of condition will
>be inside the else conditions.
>
>Again this has to be optimized based on the profile Data of hot
>conditional branches and the above optimizations is performed
>Based on the hotness of the conditional branches.
>
>The short Circuiting optimization also makes the irreducible loops with
>conditional branches as reducible and there is no need for
>Conversion of irreducible loops to reducible loop with any of the
>existing approach like node splitting for the short circuiting
>candidate.
>
>This optimizations is implemented in LLVM and I would like to know if
>this is implemented in GCC. If not implemented I would like
>To propose this short circuit compiler transformation.
>
>Thoughts Please ?
>
>Thanks & Regards
>Ajit