goroot: reduce xfail backlog across go1.24-go1.26

[cpunion]

Summary

This PR continues the goroot xfail cleanup on top of #1759.

It keeps the current scope limited to:

• Go 1.24 ~ 1.26
• ubuntu + macos
• cases that do not depend on debug/trace/finalizer/liveness

Included

• runtime/SSA fixes for channel/select, nil/interface loads, zero-sized allocations, string slicing, and integer div/rem edge cases
• removal of stale xfails once the corresponding cases pass on both darwin/arm64 and linux/amd64
• narrowed goroot matrix and refreshed xfail inventory carried from the earlier runner work
• test/goroot runner support for an opt-in -directive-mode=runlike path covering additional run-like directives without changing the default CI scope yet

Current XFail Inventory

Current test/goroot/xfail.yaml inventory in this branch:

• 409 xfails
• 18 host skips
• 4 timeout overrides
• 431 total entries

Current Runner Scope

test/goroot now has three discovery modes:

• legacy
• ci (current CI behavior)
• runlike (broader local/experimental expansion)

All modes use the same default directories:

• ., ken, chan, interface, internal/runtime/sys, syntax, dwarf, fixedbugs, codegen, abi, typeparam, typeparam/mdempsky, arenas
• cases must still match build.Context.MatchFile

legacy mode

legacy is the original narrow subset:

• only directive run
• only when the directive has no arguments

Bare // run counts:

• linux/amd64: Go 1.24.11 = 975, Go 1.25.0 = 983, Go 1.26.0 = 996
• linux/arm64: Go 1.24.11 = 973, Go 1.25.0 = 981, Go 1.26.0 = 994
• darwin/arm64: Go 1.24.11 = 972, Go 1.25.0 = 980, Go 1.26.0 = 993

ci mode

ci is the current PR's widened CI surface:

• run
• run <args>
• runoutput
• rundir
• runindir
• buildrun

Current run-family counts with -directive-mode=ci:

• linux/amd64: Go 1.24.11 = 1141, Go 1.25.0 = 1149, Go 1.26.0 = 1162
• linux/arm64: Go 1.24.11 = 1139, Go 1.25.0 = 1147, Go 1.26.0 = 1159
• darwin/arm64: Go 1.24.11 = 1138, Go 1.25.0 = 1146, Go 1.26.0 = 1158

runlike mode

runlike remains available for broader local expansion and additionally includes:

• buildrundir

Current run-like counts with -directive-mode=runlike:

• linux/amd64: Go 1.24.11 = 1145, Go 1.25.0 = 1153, Go 1.26.0 = 1166
• linux/arm64: Go 1.24.11 = 1141, Go 1.25.0 = 1149, Go 1.26.0 = 1161
• darwin/arm64: Go 1.24.11 = 1140, Go 1.25.0 = 1148, Go 1.26.0 = 1160

Remaining mixed errorcheckandrundir cases are still counted separately because they need an errorcheck-aware harness rather than a pure run/output comparator:

• linux/amd64: 5 on Go 1.24.11 / 1.25.0 / 1.26.0
• linux/arm64: 5 on Go 1.24.11 / 1.25.0 / 1.26.0
• darwin/arm64: 5 on Go 1.24.11 / 1.25.0 / 1.26.0

Intentionally Excluded For Now

• debug
• trace
• finalizer
• liveness
• errorcheckandrundir in the runner mode used by CI (needs an errorcheck-compatible execution path)
• buildrundir in CI mode for now

Known buildrundir blockers found during local expansion:

• retjmp.go: translated asm return-jump semantics still diverge (panic: f2 not called)
• asmhdr.go: plan9asm constant expansion still rejects string consts (panic: "test" not an Int)

Validation

Each stale-xfail removal in this branch was rechecked on both:

• darwin/arm64
• linux/amd64

Recent regression fix additionally verified with:

• LLGO_BUILD_CACHE=0 go run ./cmd/llgo test ./test/std/archive/tar
• ./dev/docker.sh amd64 ... ./dev/llgo.sh test -timeout=20m github.com/goplus/llgo/test/std/archive/tar

Current runner expansion verified locally with:

• go test ./internal/build -count=1
• go test ./test/goroot -count=1
• /Users/lijie/sdk/go1.26.0/bin/go test ./test/goroot -run '^TestGoRootRunCases$' -count=1 -timeout 60m -args -goroot /Users/lijie/sdk/go1.26.0 -go /Users/lijie/sdk/go1.26.0/bin/go -directive-mode ci -case '^(args\.go|strength\.go|alias3\.go|fixedbugs/issue29612\.go|fixedbugs/issue46234\.go)$'
• go test ./internal/build -run 'TestPkgSFiles(QueryForTempDirPackage|ListsAsmForTempDirPackage)|TestPlan9Asm' -count=1

Known follow-up from the broader local runlike path:

• buildrundir is not yet safe to enable in CI because of the two blockers above.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.19%. Comparing base (6b1e842) to head (0ed0610).

Additional details and impacted files

@@                      Coverage Diff                       @@
##           feat/go126-source-patch-ci    #1773      +/-   ##
==============================================================
- Coverage                       93.16%   92.19%   -0.98%     
==============================================================
  Files                              48       49       +1     
  Lines                           13616    13745     +129     
==============================================================
- Hits                            12686    12672      -14     
- Misses                            742      871     +129     
- Partials                          188      202      +14     

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