[LLVMdev] Potential SimplifyCFG optimization; hammock to diamond transformation

[Chad Rosier]

The specific code I'm looking at does not have any function calls.  The
"lots of FP math" is just a series of FP addition, subtraction and division.

On Tue, Aug 6, 2013 at 11:03 AM, David Tweed <david.tweed at arm.com> wrote:

> Hi,****
>
> ** **
>
> I imagine this optimization is for FP math stuff that doesn't involve
> functions which might set errno (or when we've specified we don't care
> about that)?****
>
> ** **
>
> Cheers,
> Dave****
>
> ** **
>
> *From:* llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] *On
> Behalf Of *Chad Rosier
> *Sent:* 06 August 2013 15:46
> *To:* llvmdev
> *Subject:* [LLVMdev] Potential SimplifyCFG optimization; hammock to
> diamond transformation****
>
> ** **
>
> 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:
>
> {
>   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?  If not, would the SimplifyCFG pass be
> an appropriate home for such an optimization?
>
>  Chad****
>
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