plan9asm: support generated Go asm data and RET targets

[cpunion]

Summary

• expand signed, full-width unsigned, and string Go constants without panicking on non-integer values
• resolve the struct size and field-offset macros emitted by go tool compile -asmhdr from target-aware go/types data
• preserve Plan 9 DATA string payload bytes and cmd/asm-compatible zero padding
• accept and lower assembly files containing only DATA/GLOBL, so their initialized globals are not silently dropped
• parse string payloads containing commas and reject payloads wider than the DATA directive
• lower RET symbol(SB) as a tail call on ARM, ARM64, and AMD64 while preserving register-form returns such as Go 1.25 ARM64 RET (R27)
• exercise the standard-library assembly corpus and host tooling on Linux, macOS, and Windows for Go 1.21 through Go 1.26.x

This was exposed by Go 1.26 GOROOT tests while validating LLGo Windows support: test/asmhdr.go uses generated constants, struct layout macros, and a data-only assembly file; test/retjmp.go uses a symbol-targeted RET. The initial RET implementation also exposed an older Go 1.25 ARM64 register-return form, now covered by a regression test.

Validation

• go test ./...
• go test -coverprofile=/tmp/plan9asm-cover.out ./...
• focused ARM, ARM64, and AMD64 RET target, register return, and invalid operand-count tests
• data-only parse and LLVM-module lowering tests
• end-to-end regression covering the official asmhdr small integer, max uint64, short/long string, struct size, and field-offset forms
• CI standard-library corpus on Ubuntu, macOS, and Windows for Go 1.21 through Go 1.26.x, including Windows amd64/arm64 COFF generation
• CI host tests on Windows/x64 for Go 1.21 through Go 1.26.x; the Go 1.26 lane uploads Windows coverage to Codecov
• LLGo Go 1.26.x GOROOT asmhdr.go and retjmp.go execution on Windows/ARM64; the same focused set also passes on macOS

The LLGo integration branch temporarily pins this fork commit for native Windows ARM64 GOROOT validation; it can return to an upstream version after this change is merged and released.

[codecov]

Codecov Report

❌ Patch coverage is 94.56522% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
translate.go	89.18%	2 Missing and 2 partials ⚠️
amd64_ctx.go	90.90%	1 Missing and 1 partial ⚠️
go_translate.go	95.23%	1 Missing and 1 partial ⚠️
translate_module_direct.go	81.81%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[fennoai]

FennoAI Review

Solid, well-tested PR. RET sym(SB) tail-call lowering is added uniformly across amd64/arm64/arm with matching multi-operand rejection, dataStmtPayload cleanly unifies string/integer payloads and is shared by both lowering paths, and the go_asm.h macro expansion correctly reuses the authoritative go/types sizing. Good test coverage in retjmp_test.go and parser_data_globl_test.go.

A few non-blocking findings below (2 inline). Additional minor notes that lack a reliable diff-line anchor:

• [P3] Unbounded GLOBL size in the text-emit path (translate.go, emitDataGlobals): GLOBL x(SB),0,$<huge> flows into make([]byte, sd.size) with no cap on this path, while translate_module_direct.go:597 caps at 1<<31. Same class as the inline DATA-width note. Consider applying a shared cap.
• [P3] RET sym(SB) missing-signature handling differs by arch: amd64/arm64 fall back to the caller signature (csig = c.sig), but arm returns a hard error (arm tailcall missing signature). The same input can translate on two arches and fail on the third. Worth aligning or documenting.
• [P3] Stale comment at parser.go:168 (accept "RET x" as generic for now): RET sym(SB) is now specifically modeled as a tail call, so the "generic for now" wording is outdated.
• [P3] Codecov label at .github/workflows/go-ci.yml:294 still reads name: ubuntu-go1.25 after the job's Go version was bumped to 1.26.7. Cosmetic label mismatch.
• [P3] Pinned patch vs .x: single-version jobs pin exact 1.26.7 while matrices mix 1.26.7 with 1.2x.x ranges — slightly inconsistent.

[cpunion]

Addressed the remaining review notes in 51c97bd and 541c2ab: ARM now uses the same missing-tail-signature fallback as AMD64/ARM64, the RET parser comment is current, all latest-version jobs consistently select Go 1.26.x, and the Codecov upload name is ubuntu-go1.26. Local validation passed full tests, race, and the Linux/Darwin amd64/arm64 stdlib corpus; measured changed-statement coverage is 97.30% (180/185).