Integrated: 8356246: C2: Compilation fails with "assert(bol->is_Bool()) failed: unexpected if shape" in StringConcat::eliminate_unneeded_control

[Daniel Skantz]

On Tue, 27 May 2025 07:56:59 GMT, Daniel Skantz <dskantz at openjdk.org> wrote:

> This pull request contains a fix for JDK-8356246.
> 
> During stacked concatenations, a pair of `StringBuilder.append().toString()` links SB and SB2 could have a diamond if structure `(Region -> <IfTrue, IfFalse> -> If)` created by String.valueOf that depends on the return value of SB1, which is going away (replaced by top() in `eliminate_call` in stringopts).
> 
> JDK-8271341 added folding of the region of the diamond-if to stringopts to avoid the case where a live part of the graph becomes unreachable as this top() propagates through the graph too quickly.
> 
> JDK-8291775 was a follow-up fix and instead used a constant test as input to the diamond If, as a case was discovered where the If was processed before the Region leading to a broken graph.
> 
> The code in JDK-8271341 assumes that the input to the If is a boolean, not a constant. If two diamond if-region structures in the same StringBuilder candidate share the same test, the second iteration in `eliminate_unneeded_control` will fail with an unexpected input. The proposed solution is to skip over the second iteration as the test has already been replaced by a constant -- both structures will be simplified independently during IGVN.
> 
> Testing:
> T1-4.

This pull request has now been integrated.

Changeset: 6f9e1175
Author:    Daniel Skantz <dskantz at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/6f9e1175a983c735c1beed755ec5b14b476858d7
Stats:     62 lines in 2 files changed: 62 ins; 0 del; 0 mod

8356246: C2: Compilation fails with "assert(bol->is_Bool()) failed: unexpected if shape" in StringConcat::eliminate_unneeded_control

Reviewed-by: rcastanedalo, kvn

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PR: https://git.openjdk.org/jdk/pull/25461