RFR: 8257513: C2: assert((constant_addr - _masm.code()->consts()->start()) == con.offset()) [v2]

[Christian Hagedorn]

> Running the testcase with `-XX:+StressCodeBuffers` initializes the code buffer with a small size:
> https://github.com/openjdk/jdk/blob/0c8cc2cde47bed3d5c3edc203869068a0676812b/src/hotspot/share/opto/output.cpp#L1339-L1340
> This leads to more frequent expansions in `ConstantTable::emit()` when calling the `_masm.*_constant()` methods. In the `T_VOID` case, we additionally fill the jump-table with dummy words:
> https://github.com/openjdk/jdk/blob/0c8cc2cde47bed3d5c3edc203869068a0676812b/src/hotspot/share/opto/constantTable.cpp#L146-L150
> 
> When there is an expansion during this loop, then `constant_addr` is still pointing to an outdated address before expanding the code buffer and relocating the data. This lets the assertion fail when subtracting the new address `_masm.code()->consts()->start()` after the expansion.
> 
> I extended this assertion for `T_VOID` to take an expansion into account. 
> 
> There is a second issue that is currently not handled correctly. The `_masm.*_constant()` methods could return `NULL` when we failed to allocate new space in an expansion which, however, rarely happens. But when running with `-XX:+StressCodeBuffers`, `NULL` is much more likely to be returned because we intentionally free a blob from time to time:
> https://github.com/openjdk/jdk/blob/0c8cc2cde47bed3d5c3edc203869068a0676812b/src/hotspot/share/asm/codeBuffer.cpp#L846-L854
> 
> I therefore suggest to also change the associated assertions in `ConstantTable::emit()` into bailout code to handle this case (as also done in the `LIR_Assembler` using the `AbstractAssembler::*_constant()` methods).
> 
> Thanks,
> Christian

Christian Hagedorn has updated the pull request incrementally with one additional commit since the last revision:

  Add @vm.debug == true to test

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1803/files
  - new: https://git.openjdk.java.net/jdk/pull/1803/files/da22ba7e..c55885d8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1803&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1803&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1803.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1803/head:pull/1803

PR: https://git.openjdk.java.net/jdk/pull/1803