[LLVMdev] Potential SimplifyCFG optimization; hammock to diamond transformation

[Chad Rosier]

On Tue, Aug 6, 2013 at 1:42 PM, Adve, Vikram Sadanand <vadve at illinois.edu>wrote:

>  Message: 6
> Date: Tue, 6 Aug 2013 10:46:19 -0400
> From: Chad Rosier <chad.rosier at gmail.com>
> To: llvmdev <llvmdev at cs.uiuc.edu>
>
> Subject: [LLVMdev] Potential SimplifyCFG optimization; hammock to
> diamond transformation
> Message-ID:
> <CAMo3wbR6x1wBzb17=GrkERV7kvzx2RdpuheFzyxkQEs3BBvKaw at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
>
> All,
> I have some code that looks like the following:
>
> {
>  double a, b, c;
>  for (...) {
>    ...
>    a = lots of FP math;
>    b = lots of FP math;
>    c = lots of FP math;
>    if (cond) {
>      a = 0.0;
>      b = 0.1;
>      c = 0.2;
>    }
>   ...
>  }
> }
>
> Could we not convert the hammock into a diamond and move the initial
> computation of a, b, and c into the else block.  Something like this:
>
>
>  I believe what you're proposing would be accomplished by PRE, since the
> stores to a, b and c are partially redundant, and once those are "placed"
> by PRE, the FP math would then get placed as you want as well.
>


Yes, that makes perfect sense.


>
> {
>  double a, b, c;
>  for (...) {
>    ...
>    if (cond) {
>      a = 0.0;
>      b = 0.1;
>      c = 0.2;
>    } else {
>      a = lots of FP math;
>      b = lots of FP math;
>      c = lots of FP math;
>    }
>   ...
>  }
> }
>
> Does a similar optimization exist?
>
>
>  SSAPRE existed in LLVM but has been removed.  PRE is one of those
> optimizations that are simply more efficient and also sometimes more
> effective as a bitvector dataflow optimization than as an SSA algorithm.
>

Good to know.


>
>  If not, would the SimplifyCFG pass be
> an appropriate home for such an optimization?
>
>
>  If done there, it is likely to only handle special cases like the above.
>  My guess is that it is better done as a code motion pass than a
> SimplifyCFG extension.
>

Thanks for the feedback, Prof. Vikram.  You have been very helpful.

Chad
>
>
>
>      --Vikram Adve
> Professor, Department of Computer Science
> University of Illinois at Urbana-Champaign
> vadve at illinois.edu
> http://llvm.org
>
>
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