8391545: RISC-V: Implement Zibi extension (branch with immediate)

[zangcq]

Add JVM/HotSpot support for the RISC-V Zibi extension (v0.7), which
defines two B-type conditional branches that compare an integer register
against a small immediate constant:

  beqi rs1, cimm, offset   (funct3=0b010): branch if rs1 == constant
  bnei rs1, cimm, offset   (funct3=0b011): branch if rs1 != constant

Both reuse the BRANCH major opcode and the standard B-type immediate
layout; the 5-bit field that would hold rs2 carries cimm instead. The
comparison constant decodes as cimm=0 -> -1 and cimm=1..31 -> 1..31
(comparison with zero keeps using beq/bne with x0).

Spec: https://github.com/riscv/zibi/blob/main/src/unpriv/zibi.adoc

Changes:

• Add the UseZibi flag and ext_Zibi feature detection. Auto-detection
  is left unwired because there is no hwprobe/HWCAP bit yet; enable
  with -XX:+UseZibi.
• Add the beqi/bnei encodings plus is_zibi_cimm/encode_zibi_cimm
  helpers in the assembler.
• Add near/far label and address forms in the MacroAssembler, with the
  far form realized by negating the condition (beqi <-> bnei) over the
  same constant.
• Add the C2 cmpi_branch encoder helper.
• Add immIZibi/immLZibi operands and the CmpI/CmpL near/far branch and
  loop instructs, all gated on UseZibi so codegen falls back to
  materialize-plus-reg-branch when the extension is off.

The existing conditional-branch bind/patch machinery works unchanged
because beqi/bnei share the BRANCH opcode and B-immediate layout.

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Progress

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• Change must be properly reviewed (2 reviews required, with at least 1 Reviewer, 1 Author)

Issue

• JDK-8391545: RISC-V: Implement Zibi extension (branch with immediate) (Enhancement - P4)

Contributors

• Pengcheng Wang <wangpengcheng.pp@bytedance.com>

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[bridgekeeper]

👋 Welcome back zangcq! 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.

[zangcq]

/contributor add Pengcheng Wang wangpengcheng.pp@bytedance.com

[openjdk]

@zangcq
Contributor Pengcheng Wang <wangpengcheng.pp@bytedance.com> successfully added.

[openjdk]

@zangcq The following label will be automatically applied to this pull request:

• hotspot

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[zangcq]

/issue JDK-8391545

[openjdk]

@zangcq The primary solved issue for a PR is set through the PR title. Since the current title does not contain an issue reference, it will now be updated.

[mlbridge]

Webrevs

• 00: Full (889f00b1)

[RealFYang]

But Zibi is still unratified? If true, I would suggest wait a while for SPEC ratification.

[zangcq]

Yes, Zibi is still unratified — currently at v0.7 (experimental). The ISA Manual integration PR (riscv/riscv-isa-manual#3343) was opened on 2026-08-24 and has not yet been merged. Thanks for your suggestion.