8371967: Add Visual Studio 2026 to build toolchain for Windows

[antonvoznia]

Clean backport. Only the VS 2026 references in doc/building.md were changed manually because of a different text format.

Problem this solves:
The build system currently rejects newer Visual Studio versions that we increasingly need to use:

1. Microsoft has stopped offering VS 2022 as the default download. New developer machines and build agents provisioned today install VS 2026 by default. Forcing developers/build infra to manually procure VS 2022 installers creates friction.
2. Compiler and toolchain fixes increasingly land in VS 2026 first. VS 2022 backports are delayed or skipped. As JDK 21 is an LTS release, it with current tooling becomes more important over time.

Risk assessment:
Low risk. The change is additive: it appends VS 2026 to the VALID_VS_VERSIONS list, so existing VS 2019/2022 builds are unaffected. I have validated the backport locally by configuring and building JDK 21 on Windows x64 with VS 2026 and running tier-1 tests (no regression).

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8371967 needs maintainer approval
• JDK-8373593 needs maintainer approval

Issues

• JDK-8371967: Add Visual Studio 2026 to build toolchain for Windows (Bug - P4 - Requested)
• JDK-8373593: Support latest VS2026 MSC_VER in abstract_vm_version.cpp (Enhancement - P4 - Requested)

Reviewers

• Paul Hohensee (@phohensee - Reviewer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk21u-dev.git pull/2892/head:pull/2892
$ git checkout pull/2892

Update a local copy of the PR:
$ git checkout pull/2892
$ git pull https://git.openjdk.org/jdk21u-dev.git pull/2892/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 2892

View PR using the GUI difftool:
$ git pr show -t 2892

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk21u-dev/pull/2892.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back antonvoznia! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

This backport pull request has now been updated with issue from the original commit.

[mlbridge]

Webrevs

• 02: Full - Incremental (adbb5d3a)
• 01: Full - Incremental (1273ae49)
• 00: Full (1c74950d)

[antonvoznia]

/issue JDK-8373593

[openjdk]

@antonvoznia
Adding additional issue to issue list: 8373593: Support latest VS2026 MSC_VER in abstract_vm_version.cpp.

[phohensee]

Looks like the only reason the bot wanted a review was because the PR combines two JBS issues.

[openjdk]

⚠️ @antonvoznia This change is now ready for you to apply for maintainer approval. This can be done directly in each associated issue or by using the /approval command.

[antonvoznia]

/approval JDK-8371967 request jdk21u-dev . Clean backport, only VS 2026 references in doc/building.md were adjusted manually due to a different text format.

Motivation: VS 2026 is now Microsoft's default download, and toolchain fixes land there first while VS 2022 backports lag. Keeping JDK 21 (LTS) buildable with current tooling matters more over time.

Risk: Low. Additive change, VS 2026 appended to VALID_VS_VERSIONS, VS 2019/2022 builds unaffected. Validated locally on Windows x64: configure, build, tier-1 tests, no regressions.

[openjdk]

@antonvoznia
JDK-8371967: The approval request has been created successfully.

[antonvoznia]

/approval JDK-8373593 request jdk21u-dev, additional clean backport of fix that should be integrated with JDK-8371967: Add Visual Studio 2026 to build toolchain for Windows

[openjdk]

@antonvoznia
JDK-8373593: The approval request has been created successfully.