[14] RFR(S): 8234617: C1: Incorrect result of field load due to missing narrowing conversion

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8234617
http://cr.openjdk.java.net/~thartmann/8234617/webrev.00/

Writing an (integer) value to a boolean, byte, char or short field includes an implicit narrowing
conversion [1]. With -XX:+EliminateFieldAccess (default), C1 tries to omit field loads by caching
and reusing the last written value. The problem is that this value is not necessarily converted to
the field type and we end up using an incorrect value.

For example, for the field store/load in testShort, C1 emits:
  [...]
  0x00007f0fc582bd6c:   mov    %dx,0x12(%rsi)
  0x00007f0fc582bd70:   mov    %rdx,%rax
  [...]

The field load has been eliminated and the non-converted integer value (%rdx) is returned.

The fix is to emit an explicit conversion to get the correct field value after the write:
  [...]
  0x00007ff07982bd6c:   mov    %dx,0x12(%rsi)
  0x00007ff07982bd70:   movswl %dx,%edx
  0x00007ff07982bd73:   mov    %rdx,%rax
  [...]

Thanks,
Tobias

[1] https://docs.oracle.com/javase/specs/jvms/se13/html/jvms-6.html#jvms-6.5.putfield