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

[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When there is an expansion during this loop, then `constant_addr` is still pointing to an outdated address before expanding the code buffer and relocating the data.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

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Commit messages:
 - 8257513: C2: assert((constant_addr - _masm.code()->consts()->start()) == con.offset())

Changes: https://git.openjdk.java.net/jdk/pull/1803/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1803&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257513
  Stats: 90 lines in 4 files changed: 81 ins; 0 del; 9 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