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incorrect if, 0d, and -0d optimization #6331

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scabug opened this issue Sep 6, 2012 · 3 comments
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incorrect if, 0d, and -0d optimization #6331

scabug opened this issue Sep 6, 2012 · 3 comments
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@scabug scabug commented Sep 6, 2012

Scala incorrectly optimizes away the if statement (without -optimize) in the following:

scala> val x = if(error("x")) 0d else -0d
x: Double = 0.0

scala> val x = if(error("x")) 1d else 1d
x: Double = 1.0

The main problem is that although 0d == -0d is true, it is wrong to treat them as the same. It seems plausible this kind of constant equality check might be applied elsewhere as well.

Second, the side-effect in the if statement is discarded, although I don't know if there is a scenario where this could possibly matter.

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@scabug scabug commented Sep 6, 2012

Imported From: https://issues.scala-lang.org/browse/SI-6331?orig=1
Reporter: @harrah
Affected Versions: 2.10.0-M7
Other Milestones: 2.10.0

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@scabug scabug commented Sep 7, 2012

@retronym said (edited on Sep 7, 2012 3:11:13 PM UTC):
typedIf lubs the branches of the conditional:

https://github.com/scala/scala/blob/2.10.x/src/compiler/scala/tools/nsc/typechecker/Typers.scala#L4072

adapt replaces <expr>: ConstantType\(x) with x

https://github.com/scala/scala/blob/2.10.x/src/compiler/scala/tools/nsc/typechecker/Typers.scala#L1067

Probable fixes:

  1. call .deconst before lubbing the types of the branches; make sure treatment is consistent in other places lubs are taken. (Or maybe: Do this in lub/glb itself)
  2. Update Const#equals to differentiate +-0d (it already has similar handling for NaN). I think we can use Double.doubleToRawLongBits(this.value) == Double.doubleToRawLongBits(that.value) (and the equivalent for Float) to handle all the floating point esoterica correctly.
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@scabug scabug commented Sep 8, 2012

@retronym said:
scala/scala#1271

My last comment wasn't accurate: deconst was already called by ptOrLub. The real problem was a new fast path that avoids lubbing, which was added by virtpatmat.

@scabug scabug closed this Oct 1, 2012
@scabug scabug added the backend label Apr 7, 2017
@scabug scabug added this to the 2.10.0-M7 milestone Apr 7, 2017
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