Conditional optimization: true || x == true, false && y == false, etc.

[scabug]

There are four key optimizations that Mondrian's optimizer could make but doesn't.

Mondrian's optimizer is smart enough to turn if (true) a else b into a. To my delight, it is smart enough to do this even if true is inside a final val and not the literal true. This zeroth-order optimization is very helpful.

However, Mondrian's optimizer misses several key first-order optimizations that are quite useful.

• true || x == true
• false && y == false
• true && x == x
• false || y == y
• x && true == x
• y || false == y

Unfortunately, you have to be careful with the two remaining cases unless you can prove that x & y are side-effect free.

• x || true == {x; true}
• y && false == {y; false}

This is something that is pretty easy to encounter in performance-critical cases where you don't really want to have to rely on the JIT optimizer to be smart enough.

  class {
    final val extremeLogs = false
    for (tightLoop) {
      if (extremeLogs && logger.isTraceEnabled) { ... }
      ...
    }
  }

One would hope that the optimizer would actually be smart enough to recurse through boolean expressions and apply these transformations from the outside in. There are also some additional safe transformations like x && true && y == x && y.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6105?orig=1
Reporter: Sarah Gerweck (gerweck)
Affected Versions: 2.9.2

[scabug]

@magarciaEPFL said:
I'll think about it.

[scabug]

@magarciaEPFL said:
This is supported by the experimental optimizer that eventually will replace the current one.

Description: https://groups.google.com/d/topic/scala-internals/iz5PiIMTPs0/discussion

Sources at branch: https://github.com/magarciaEPFL/scala/tree/GenBCodeOpt

[SethTisue]

@lrytz did this happen?

[lrytz]

works in 2.12

[lrytz]

(i didn't intend to close, was on the wrong ticket)

these are not yet implemented

• true && x ==> x
• false || y ==> y
• x && true ==> x
• y || false ==> y

[NthPortal]

the first two are not necessarily true for Float and Double, as NaN != NaN

[lrytz]

I think you misunderstood what is meant, my comment was not very clear. there's no comparison involved. I updated my comment above.

[da-tubi]

See https://github.com/da-tubi/scala-optimizer/blob/master/ShortCircuitFold.scala

object ShortCircuitFold {
  def main(args: Array[String]): Unit = {
    val x = ((1 to 100).sum == 5050)
    val y = ((1 to 100).sum == 5051)
    val c1 = true || x
    val c2 = false && y 
    val c3 = true && x 
    val c4 = false || y
    val c5 = x && true
    val c6 = y || false 
    println(c1)
    println(c2)
    println(c3)
    println(c4)
    println(c5)
    println(c6)
  }
}

true || x ==> x

val c1 = true || x

// Default
      66: iconst_1
      67: ifne          74
      70: iload_2
      71: ifeq          78
      74: iconst_1
      75: goto          79
      78: iconst_0
      79: istore        4

// -opt:l:method
      66: iconst_1
      67: istore        4

// -opt:unreachable-code,simplify-jumps,compact-locals,copy-propagation
      66: iconst_1
      67: istore        4

// -opt:unreachable-code,simplify-jumps,compact-locals
      66: iconst_1
      67: pop
      68: iconst_1
      69: istore        4

// -opt:unreachable-code,simplify-jumps
      66: iconst_1
      67: pop
      68: iconst_1
      69: istore        4

// -opt:simplify-jumps
      66: iconst_1
      67: pop
      68: goto          75
      71: nop
      72: nop
      73: nop
      74: athrow
      75: iconst_1
      76: goto          80
      79: athrow
      80: istore        4

false && y ==> y

val c2 = false && y 

// Default
// -opt:l:method
      69: iconst_0
      70: istore        5

true && x ==> x

    val c3 = true && x 

      72: iload_2
      73: ifeq          80
      76: iconst_1
      77: goto          81
      80: iconst_0
      81: istore        6

[da-tubi]

related: scala/scala-dev#29

[da-tubi]

If x or y are constants, it will be folded during the typechecker phase, see scala/scala#9427