RFR: 8323657: Compilation of snippet results in VerifyError at runtime with --release 9 (and above)

[Jan Lahoda]

Consider code like:

public class Test {
    public static void main(String... args) {
        System.err.println((test() ? null : null) + "");
    }
    private static boolean test() {
        System.err.println("test called!");
        return true;
    }
}


This should print:

test called!
null


but it prints:

$ java /tmp/Test.java
null


The correct semantics can be achieved by using `--source 8`:

$ java --source 8 /tmp/Test.java
test called!
null


Or `-XDstringConcat=inline` when compiling.

The reason is that when `StringConcat.IndyConstants` converts String concatenation parameters, it will short-circuit any expressions whose type is BOT (i.e. basically the `null` type). But, in this case, the expression that has the BOT type is a complex expression with a side-effect, so short-circuiting it leads to a broken semantics.

My proposal is to "short-circuit" only `null` literals, and generate other expressions normally. There may be some corner cases where we'd generate the expression unnecessarily (if it does not have any side-effect), but looking for side-effects seems unnecessarily complex and fragile given I'd expect non-trivial expressions with the BOT type are very rare. (The only case which comes to mind is exactly the above one - a conditional expressions, whose both result values are `null`.)

Please note that when the expression is incorrectly short-circuited, it may lead to various other effects - the original bug report shows a case where the resulting code does not pass the verification.

-------------

Commit messages:
 - 8323657: Compilation of snippet results in VerifyError at runtime with --release 9 (and above)

Changes: https://git.openjdk.org/jdk/pull/17483/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17483&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323657
  Stats: 65 lines in 2 files changed: 64 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17483.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17483/head:pull/17483

PR: https://git.openjdk.org/jdk/pull/17483