Integrated: 8322636: [JVMCI] HotSpotSpeculationLog can be inconsistent across a single compile

[David Leopoldseder]

On Fri, 22 Dec 2023 09:55:16 GMT, David Leopoldseder <davleopo at openjdk.org> wrote:

> This PR fixes a subtle inconsistency in `HotSpotSpeculationLog` .
> 
> Normal uses of `HotSpotSpeculationLog` work by using a `SpeculationReason` and asking the speculation log via `maySpeculate` if the speculation can be performed, i.e., if it failed before for the given method.  An example for this can be seen in Graal https://github.com/oracle/graal/blob/master/compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/nodes/loop/CountedLoopInfo.java#L591C15-L591C15 
> The implicit assumption is that the speculation log, `HotSpotSpeculationLog` in particular collects failed speculations at the beginning of a compile and then stays consistent during the compile. Why is that? - Because if there are new failed speculations added to the failed speculations during the compile - the compiler would speculate again on those in an inconsistent way. E.g. at the beginning of a compile a certain speculation has not failed yet and the compiler thinks it can do optimization xyz using a speculation  - later during the compilation process it consults the speculation log but gets a different answer. All those inconsistent speculations that already failed will anyway later fail code installation in jvmci (they will throw a bailout during `HotSpotCodeCacheProvider#installCode` https://github.com/openjdk/jdk/blob/master/src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/HotSpotSpeculationLog.java#L192 ). Thus, we should at least return a consistent result during
  a compile.
> The problem for consistency here, that also makes troubles on the graal side, is that `maySpeculate` itself can collect failed speculations if there have not been any previously, i.e., `failedSpeculations == null`.
> In order to make the speculation log consistent across an entire JVMCI compile this PR removes the collection of failed speculations in `maySpeculate`.

This pull request has now been integrated.

Changeset: 35a1b77d
Author:    David Leopoldseder <davleopo at openjdk.org>
Committer: Tom Rodriguez <never at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/35a1b77da541e4df3c4d1bab0825ea39e653808c
Stats:     9 lines in 1 file changed: 6 ins; 3 del; 0 mod

8322636: [JVMCI] HotSpotSpeculationLog can be inconsistent across a single compile

Reviewed-by: dnsimon, never

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PR: https://git.openjdk.org/jdk/pull/17183