[llvm-branch-commits] [clang] [LifetimeSafety] Track per-field origins for record types (PR #195603)

[Zhijie Wang via llvm-branch-commits]

aeft wrote:

> When I compile LLVM with clang built from this PR, it reports 821 new unique warnings. I looked into several and found they are FPs. I will follow up with details, fixes, and reproducer test cases.

​My latest commit fixes 817 FPs out of 821 warnings. Three related fixes:

1. `handleAssignment` now `markAsWritten` the narrowed `UseFact` for `MemberExpr LHS` (previously only `DeclRefExpr LHS`). Tests: `struct_field_in_loop_safe`.
2. `VisitMemberExpr`'s narrow now peels the per-Expr lvalue wrap from UseFact's UsedOrigins, mirroring handleUse's peel-for-non-reference-DRE rule. Tests: `struct_field_read_in_loop_safe`.
3. `VisitMemberExpr`'s narrow now also fires for reference fields (previously gated on `doesDeclHaveStorage`), so `handleAssignment`'s reference-assignment path can find the narrowed UseFact for ME LHS. Tests: `ref_field_dangle_then_rescue`.

The remaining warnings are TPs but are safe in practice (testing commit: 461e473a8f1d1af85f8392b3b529adfca31eadb1):

```
/workspace/llvm-project/llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp:166:17: warning: address of stack memory escapes to a field [-Wlifetime-safety-dangling-field]
/workspace/llvm-project/llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp:198:27: warning: address of stack memory escapes to a field [-Wlifetime-safety-dangling-field]
/workspace/llvm-project/llvm/lib/ProfileData/InstrProfWriter.cpp:524:30: warning: address of stack memory escapes to a field [-Wlifetime-safety-dangling-field]
/workspace/llvm-project/llvm/lib/ProfileData/InstrProfWriter.cpp:526:32: warning: address of stack memory escapes to a field [-Wlifetime-safety-dangling-field]
```

- `InstrProfWriter.cpp:524`, `:526`: the field is reset to `nullptr` before the local goes out of scope.
- `InstructionSelect.cpp:166`, `:198`: fragile but currently safe, the field is left dangling without an explicit reset, working only because no one reads it after the local dies.

cc @usx95 

https://github.com/llvm/llvm-project/pull/195603