ssa: panic on static nil dereference

[cpunion]

Summary

• emit explicit runtime nil-deref panics for statically nil pointer loads/stores and nil field/array bases
• add test/go assignment-order coverage for RHS-before-map-insert and multiple assignment map update before later nil store
• remove fixed GOROOT xfails for fixedbugs/issue22881.go and fixedbugs/issue23017.go

Notes

• GOROOT CI is slow/disabled in this repo, so validation here is intentionally targeted.
• The new stable coverage is in test/go/assignment_order_test.go. A full llgo test ./test/go package build is currently blocked by an existing unrelated generic-local-type link error; the new file was run directly under llgo test.

Targeted commands

• Reproduced before fix: go test ./test/goroot -run TestGoRootRunCases/fixedbugs/issue22881.go -count=1 -args -goroot "$(go env GOROOT)" -dirs fixedbugs -case '^fixedbugs/issue22881\.go$' -xfail /tmp/llgo-no-xfail.yaml -run-timeout 30s timed out in llgo run.
• go test ./test/go -count=1
• timeout 90s go run ./cmd/llgo test -run 'TestMapUpdateRHSNilDerefOrder|TestMultipleAssignmentMapUpdateBeforeNilStore' ./test/go/assignment_order_test.go
• go test ./ssa -count=1
• go test ./cl -run TestCompileLargeNilDerefInterfaceGuards -count=1
• go test ./test/goroot -run TestGoRootRunCases/fixedbugs/issue22881.go -count=1 -args -goroot "$(go env GOROOT)" -dirs fixedbugs -case '^fixedbugs/issue22881\.go$' -xfail /tmp/llgo-no-xfail.yaml -run-timeout 30s
• go test ./test/goroot -run TestGoRootRunCases/fixedbugs/issue23017.go -count=1 -args -goroot "$(go env GOROOT)" -dirs fixedbugs -case '^fixedbugs/issue23017\.go$' -xfail /tmp/llgo-no-xfail.yaml -run-timeout 30s
• go test ./test/goroot -run TestGoRootRunCases -count=1 -args -goroot "$(go env GOROOT)" -dirs fixedbugs -case '^fixedbugs/(issue22881|issue23017)\.go$' -run-timeout 30s

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ssa/memory.go	60.00%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[cpunion]

Update on a9ab6096efedd67638e790091e7c05c634ede357.

Root cause:

• The reported failed job llgo (ubuntu-latest, 19, 1.24.2) at run 26268180787 failed during dependency setup, before building/testing this PR. The job log shows wget could not resolve apt.llvm.org and exited while adding the LLVM apt key, so that particular failure is CI infrastructure/DNS, not an order/evaluation regression.
• The actual order/evaluation issue covered by this PR is static nil pointer dereference codegen: constant nil loads/stores/field/index addresses could reach LLVM without LLGo's runtime nil-deref panic path, which could change assignment/map-update ordering behavior or crash incorrectly.

Fix:

• Existing head 642917bc... adds static nil-deref assertions in SSA Load, Store, FieldAddr, and array-pointer IndexAddr, and removes the corresponding goroot xfails.
• I added test/go coverage in a9ab6096e for the goroot issue22881/issue23017 style cases: RHS panic must happen before map insertion, multiple assignment must expose earlier assignments before later panics, and address-taking on the LHS must use pre-assignment values.

Local tests:

• go test ./test/go -run 'Test(MapUpdateRHSNilDerefOrder|MapUpdateAppendRHSOrder|MultipleAssignmentMapUpdateBeforeNilStore|MultipleAssignmentOrderBeforeLaterPanic|MultipleAssignmentAddressUsesPreAssignmentValue)$' ✅
• go run ./cmd/llgo test -run 'Test(MapUpdateRHSNilDerefOrder|MapUpdateAppendRHSOrder|MultipleAssignmentMapUpdateBeforeNilStore|MultipleAssignmentOrderBeforeLaterPanic|MultipleAssignmentAddressUsesPreAssignmentValue)$' ./test/go ✅
• go test ./test/go ✅
• go test ./ssa ✅ (ok, then cached after test-only commit)
• go test ./cl ✅ (ok, then cached after test-only commit)
• git diff --check ✅

CI:

• New head CI has started for a9ab6096e; current checks are queued/in progress. I will keep monitoring and iterate if any real failure appears.

[cpunion]

Update for fixedbugs/issue23017.go coverage on commit 0bff43bf6d485100e48432daa99ec950c07095f7:

• Classification: same root as the assignment order/evaluation/store semantics work. On the pre-fix baseline (642917bc3^) with xfail disabled, fixedbugs/issue23017.go timed out in llgo run; current HEAD passes the focused goroot case.
• Stable coverage: test/go/assignment_order_test.go covers the issue23017 shapes: earlier map/pointer store surviving a later nil/index/nil-map panic, nil-map assignment preventing later stores, and LHS address evaluation using the pre-assignment pointer for field/nested-field stores and swap.
• Xfail handling: fixedbugs/issue23017.go is removed from xfail because the covered goroot case now passes. No additional xfail was removed in this update.
• Not covered/changed here: no GC/liveness/finalizer/goroutine work, and no chan/print/recover/runtime-domain fixes beyond the existing assignment-order nil-deref behavior needed for these cases.

Local validation:

• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot /Users/lijie/sdk/go1.26.0 -dirs fixedbugs -case '^fixedbugs/issue23017\\.go$' -directive-mode runlike passed.
• go test ./test/go -count=1 passed.
• go test ./ssa ./cl -count=1 passed (cl took 586.759s).

CI after push is triggered and currently queued/pending.