RFR; JDK-8214031: Assertion error in value break statement with conditional operator in switch expression (original) (raw)

Jan Lahoda jan.lahoda at oracle.com
Tue Nov 27 20:21:56 UTC 2018

Hi,

javac does not handle switch expressions that return booleans correctly. According to the spec, javac should track definite [un]assignment when true/false for switch expressions, but fails to do so. Moreover, for more complex code, like:

int v = ...; int x; boolean b = switch (v) { case 0: x = 12; break true; case 1: x = 13; break true; default: break false; } && x == 12;

The current switch expression desugaring is not powerful enough to generate valid bytecode for this input. The issue is that the current desugaring will translate the switch expression into code like: ( boolean $result; switch (v) { case 0: x = 12; $result = true; break; case 1: x = 13; $result = true; break; default: $result = false; break; } $result //"return" expression ) && x == 12

But by assigning the true/false result to a variable, we are loosing track of which variables are assigned (when true/false) and which are not. The bytecode needs to ensure the "x == 12" is jumped over when the switch expression's value is false, and that, I believe, is not possible when desugaring in Lower.

So, the proposed fix is to avoid desugaring in Lower, letting switch expressions go to Gen and produce bytecode for them, including handling them in Gen.genCond for true/false switch expressions. This is mostly modelled after the conditional expression handling. Some generalizations in Lower are required as well, to handle String and enum switch expressions. Handling of definite assignment in Flow is fixed as well.

JBS: https://bugs.openjdk.java.net/browse/JDK-8214031 Webrev: http://cr.openjdk.java.net/~jlahoda/8214031/webrev.00/

Any feedback is welcome!

Thanks, Jan

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