[Oberon] Oberon-7 - IF ELSE optimisation

[Chris Burrows]

> -----Original Message-----
> From: Oberon [mailto:oberon-bounces at lists.inf.ethz.ch] On Behalf Of
> Tomas Kral
> Sent: Saturday, 9 March 2019 12:55 AM
> To: Oberon at lists.inf.ethz.ch
> Subject: [Oberon] Oberon-7 - IF ELSE optimisation
> 
> Hi,
> 
> Trying to make this more effective,
> 
> IF (x >= 0) & (y >= 0) THEN
> ELSIF (x < 0) & (y >= 0) THEN
> ELSIF (x >= 0) & (y < 0) THEN
> ELSIF (x < 0) & (y < 0) THEN
> END ;
> 
> Recoded to below, correct?
> Above has better readability.
> 
> IF x >= 0 THEN IF y >=0 ELSE (*y < 0*) THEN END ELSIF (*x < 0*) y >=
> 0 THEN ELSE (*y < 0*) END ;
> 
> Compiler seems optimising loose ELSE IF to ELSIF?
> 

That doesn't compile. I found it easier to understand the refactoring by
including some statements i.e. S0, S1 etc.  e.g.

Original:

  IF (x >= 0) & (y >= 0) THEN S0
  ELSIF (x < 0) & (y >= 0) THEN S1
  ELSIF (x >= 0) & (y < 0) THEN S2
  ELSIF (x < 0) & (y < 0) THEN S3
  END

Step1: 

  IF (x >= 0) & (y >= 0) THEN S0
  ELSIF (x < 0) & (y >= 0) THEN S1
  ELSIF (x >= 0) & (y < 0) THEN S2
  ELSE S3
  END

Step2:

  IF (x >= 0) THEN
    IF (y >= 0) THEN S0 ELSE S2 END
  ELSIF (y >= 0) THEN S1 
  ELSE S3 
  END

Then, possibly, because the negative tests have been eliminated, step3:

  xPositive := x >= 0;
  yPositive := y >= 0;

  IF xPositive THEN
    IF yPositive THEN S0 ELSE S2 END
  ELSIF yPositive THEN S1 
  ELSE S3 
  END

Regards,
Chris

Chris Burrows
CFB Software
http://www.astrobe.com