RFR: 8330157: C2: Add a stress flag for bailouts

[Tobias Hartmann]

On Tue, 11 Jun 2024 07:14:20 GMT, Daniel Skantz <duke at openjdk.org> wrote:

> This patch adds a diagnostic/stress flag for C2 bailouts. It can be used to support testing of existing bailouts to prevent issues like [JDK-8318445](https://bugs.openjdk.org/browse/JDK-8318445), and can test for issues only seen at runtime such as [JDK-8326376](https://bugs.openjdk.org/browse/JDK-8326376). It can also be useful if we want to add more bailouts ([JDK-8318900](https://bugs.openjdk.org/browse/JDK-8318900)).
> 
> We check two invariants.
> a) Bailouts should be successful starting from any given `failing()` check.
> b) The VM should not record a bailout when one is pending (in which case we have continued to optimize for too long).
> 
> a), b) are checked by randomly starting a bailout at calls to `failing()` with a user-given probability.
> 
> The added flag should not have any effect in debug mode.
> 
> Testing:
> 
> T1-5, with flag and without it. We want to check that this does not cause any test failures without the flag set, and no unexpected failures with it. Tests failing because of timeout or because an error is printed to output when compilation fails can be expected in some cases.

src/hotspot/share/opto/c2_globals.hpp line 71:

> 69:                                                                             \
> 70:   product(bool, StressBailout, false, DIAGNOSTIC,                           \
> 71:          "Perform bailouts randomly at C2 failing() checks")                \

Suggestion:

          "Perform bailouts randomly at C2 failing() checks")               \

src/hotspot/share/opto/chaitin.cpp line 558:

> 556:     uint new_max_lrg_id = Split(_lrg_map.max_lrg_id(), &split_arena);  // Split spilling LRG everywhere
> 557:     if (C->failing()){
> 558:         return;

Suggestion:

      return;

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

PR Review Comment: https://git.openjdk.org/jdk/pull/19646#discussion_r1637580983
PR Review Comment: https://git.openjdk.org/jdk/pull/19646#discussion_r1637581340