Integrated: 8357105: C2: compilation fails with "assert(false) failed: empty program detected during loop optimization"

[Daniel Skantz]

On Thu, 22 May 2025 15:19:08 GMT, Daniel Skantz <dskantz at openjdk.org> wrote:

> This pull request contains a fix for JDK-8357105.
> 
> The problem is performing stacked string concatenation optimization between a pair of StringBuilder.append().toString()-links SB1 and SB2, where the parameter of an append call in SB2 has a complex dependency on the result of SB1, which in turn is replaced by top() during stringopts -- similar to JDK-8271341, which had a diamond if-structure using the result of SB1, while in this case the use is an unstable If. In the attached regression test, a live part of the graph gets optimized away during later phases and ultimately the whole graph vanishes.
> 
> The proposed solution is to simply exclude this specific case. This bug has existed for a long time and stacked concats is a niche optimization.
> 
> Testing:
> Tier1-4.
> 
> Extra testing:
> Ran Tier1-4 with an instrumented build and observed that we do not disable stacked concatenation in any previously known case after the fix.

This pull request has now been integrated.

Changeset: a300c356
Author:    Daniel Skantz <dskantz at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/a300c356555019a42c19bf0c16184f6dee4ad96e
Stats:     66 lines in 2 files changed: 64 ins; 0 del; 2 mod

8357105: C2: compilation fails with "assert(false) failed: empty program detected during loop optimization"

Reviewed-by: syan, rcastanedalo

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PR: https://git.openjdk.org/jdk/pull/25395