RFR: 8306922: IR verification fails because IR dump is chopped up [v2]

[Emanuel Peter]

> Some IR tests were failing because the logs were messed up (badly interleaved), due to multiple threads writing at the same time.
> 
> The issue was in `Compile::print_ideal_ir`. While we did lock the `tty`, this lock was broken because the node dumping sometimes calls into the VM, which can trigger Safepoints, which in turn can explicitly break the locks, with `ttyLocker::break_tty_lock_for_safepoint`.
> 
> What good is a lock if it can be broken? Not much. Though it is only broken if there is a Safepoint, so if we can avoid safepointing while holding the lock we should be safe. To verify that, we can write:
> 
> 
>   NoSafepointVerifier nsv;
>   ttyLocker ttyl;
> 
> 
> The verifier triggered immediately, as expected.
> And I now gather the whole dump into a separate `stringStream` first, so that all the Safepoints can happen before I even acquire the lock. Then, I acquire the log and just print the `stringStream` to `tty / xtty`, without triggering any Safepoint.
> 
> We still need to acquire a lock, else other threads may be printing also, and we get a bad interleaving in the locks.
> 
> I had to enable `dump_bfs` to print to an arbitrary stream, instead of just `tty`.
> 
> I un-problemlisted `compiler/c2/irTests/TestVectorConditionalMove.java`, and fixed a CompileCommand issue for it.
> 
> **Question**
> 
> Maybe we should in general go through all uses of `ttyLocker`, and add a `NoSafepointVerifier` to ensure no such lock is broken? Or maybe we just build the verifier into the ttyLocker? Should I file an RFE?
> 
> **Testing**
> 
> I ran `TestVectorConditionalMove.java` 1000x on master, just with the CompileCommand issue fixed. It triggered an IR issue 5 times (hit rate `0.005`). With the fix, it never triggers (`0.995^1000 = 0.007`). I also ran up to tier6 and stress-testing.

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  Apply suggestions from code review
  
  Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14591/files
  - new: https://git.openjdk.org/jdk/pull/14591/files/b61a0767..1cf4d545

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14591&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14591&range=00-01

  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14591.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14591/head:pull/14591

PR: https://git.openjdk.org/jdk/pull/14591