The handoff, with every number in it measured on this tree
The baseline rows, the twelve-program DWARF sweep re-run against the code that was actually committed rather than a superseded version of it, the lldb transcript, the unwind out of flan_bounds_error, and the compile unit's range checked against nm rather than asserted.
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@ -43,7 +43,7 @@ which is the same thing this backend already does for instructions.
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**`.cfi` is the opposite finding and it is good news.** GAS's CFI machinery computes its advances from frag
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positions and not from `md_assemble`, so `.cfi_def_cfa_offset` and friends interleaved with `.byte` come out
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*exactly* right — measured, `readelf --debug-dump=frames` on a `.byte`-only function gives the correct
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advances. Nothing here emits them yet (see "What was not reached"), but the next lane should know it can.
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advances. So this lane emits them, in a `--debug` build — see below.
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## What landed
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@ -86,6 +86,7 @@ In `lib/x86.ml`:
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| `emit_fn ?dw` | opens the subprogram, seeds it with a row at the function symbol on the `defn`'s line, closes it with a label one past the last byte |
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| `emit_dwarf` | `.debug_abbrev` (two abbreviations), `.debug_info` (one CU, one `DW_TAG_subprogram` per function), `.debug_line` (header, directory table, file table, program) |
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| `program ~debug` | a numbered `.file` at the top, `.Ldwtext` / `.Ldwtext_end` around the text, and the sections at the end |
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| `cfi_after_push` / `cfi_after_mov` / `cfi_after_leave`, above `emit_fn` | the whole frame model in five directives, at the three sites that share a prologue: `emit_fn`, `emit_main`, `emit_globals_init` |
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Two decisions worth knowing about because they are not obvious:
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@ -157,19 +158,64 @@ project's own source-level debugging case runs under **lldb**, which does not ha
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debug.flan:19` and `break flan.tick` both work everywhere. Matching `emit.ml` was chosen over diverging from
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it; if this is worth fixing it should be fixed in both places at once.
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**lldb**, which is the debugger this project's own source-level case runs under, does better still — it
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resolves `flan.tick` by name, shows the source inline and gives a column:
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```
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$ lldb -b -o "breakpoint set --name flan.tick" -o run -o bt ./dbg
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* thread #1, stop reason = breakpoint 1.1
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frame #0: 0x0000000000400647 dbg`flan.tick at debug.flan:18:3
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17 (defn tick [c (Ptr Cell) n i32] i32
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-> 18 (let [bump (+ n 1)]
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^
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19 (set (.heat c) (+ (.heat c) 1.5))
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(lldb) bt
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* frame #0: 0x0000000000400647 dbg`flan.tick at debug.flan:18:3
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frame #1: 0x00000000004007b5 dbg`flan.main at debug.flan:25:28
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frame #2: 0x0000000000400bea dbg`main + 41
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frame #3: 0x00007ffff7cb9575 libc.so.6`__libc_start_call_main + 117
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```
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That transcript is now a test rather than a transcript: `test/test_acceptance.ml`'s lldb block has an `--x86`
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arm beside its `--dev` one, asserting the breakpoint resolves and both Flan frames name `debug.flan`, and that
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the program still prints what it printed. It claims no `frame variable`, which is the gap.
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**Unwinding out of the C runtime works**, which is the case that matters for this project's error paths and
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which the brief did not ask for but should have. `bounds.flan` built `--x86 --debug`, breaking on the C symbol:
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```
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Breakpoint 1, flan_bounds_error (loc=0x40a880 "test/programs/bounds.flan:11:44", ...) at flan_rt.c:534
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#0 flan_bounds_error (...) at flan_rt.c:534
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#1 0x00000000004006be in flan.main () at bounds.flan:11
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#2 0x000000000040162e in main ()
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```
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**The compile unit's range is exact.** `DW_AT_low_pc` 0x400628 is `flan.tick`, the first function;
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`DW_AT_high_pc` 0x5cf puts the end at 0x400bf7, and `nm -S` says `main` — the last thing this object puts in
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`.text` — ends at 0x400bc1 + 0x36 = 0x400bf7. So `.Ldwtext` / `.Ldwtext_end` bracket the four functions this
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unit emitted and nothing else.
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`--x86 --dev --debug` together, which is a reachable combination and was never exercised by anything else,
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builds, runs and gives a line table `readelf` reads with no warnings.
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Cross-checked on a spread of the corpus — `debug`, `generics`, `vec`, `maps`, `strings`, `conditions`,
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`bounds-condition`, `algorithms`, `handles`, `registry` — all build with `--x86 --debug`, run, and produce
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`.debug_line` that `readelf --debug-dump=decodedline` reads with **zero warnings**. `generics` is the
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`bounds-condition`, `algorithms`, `handles`, `registry`, `destructure`, `edn`, twelve in all — every one
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builds with `--x86 --debug`, runs, and produces `.debug_line` that `readelf --debug-dump=decodedline` reads
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with **zero warnings**. `edn` and `generics` are the large ones, at 40 and 36 subprograms. `generics` is the
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multi-file case: its directory table has one entry and its file table two, `generics.flan` and `<prelude>`.
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`<prelude>` has no path on disk and a debugger simply finds no source for it, which is the truth.
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## Baseline, measured on this tree
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| | before | after |
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| | before (`957ba07`) | after |
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|---|---|---|
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| `spike/x86/survey.sh` | 99 MATCH / 0 DIFFER / 0 REFUSED / 0 NOX86, skips 28 + 6 + 2 | (see below) |
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| `spike/x86/cells.sh` | 4/4 ok | (see below) |
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| `dune test --root .` | exit 0 | (see below) |
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| `spike/x86/survey.sh` | 99 MATCH / 0 DIFFER / 0 REFUSED / 0 NOX86, skips 28 + 6 + 2 | **99 / 0 / 0 / 0**, skips **28 + 6 + 2** — identical |
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| `spike/x86/cells.sh` | 4/4 ok | **4/4 ok** |
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| `dune test --root .` | exit 0 | **exit 0**, run without a pipe; acceptance reports 232 checks, 0 failures |
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The survey is protected structurally as well as measured: every piece of this lane's work on the release path
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is behind `debug`, so `--x86` without `--debug` emits the assembly it emitted before — `fnctx.dw` is `None`,
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`dwline` does nothing, the `.Ldwtext` label is not written and neither are the `.cfi` directives.
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Run `dune test --root .` **without a pipe** — `HANDOFF-x86-redef.md` is emphatic and right: piping gives you
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`tail`'s exit status, and the `dev-robust` fixture puts `ld` and `clang` failure text in the output either way
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@ -187,14 +233,18 @@ while the run still exits 0.
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reachable: `DW_TAG_formal_parameter` and `DW_TAG_variable` for `snames`-named slots only, with
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`DW_AT_frame_base` as `DW_OP_call_frame_cfa` or `DW_OP_reg6`. That needs a type-DIE emitter, which is the
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part `emit.ml` spends most of its debug lines on.
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2. **`.cfi`, and it is now known to be cheap.** Nothing here emits it, so gdb unwinds by its prologue
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analyser — which works, because `push rbp; mov rbp,rsp; sub rsp,N` is the canonical pattern and this
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backend's header guarantees `rsp` is written exactly twice. The backtraces above are that analyser
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working. But the experiment at the top of this file says `.cfi` against `.byte` output is exact, and the
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frame model makes the content textbook: CFA is `rsp+16` after the `push`, `rbp`-based after the `mov`, and
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unchanged for the whole body. It would want `.cfi_startproc` / `.cfi_endproc` in `emit_fn` and three
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directives in the prologue. Worth doing; it is what makes a backtrace survive an unwind from inside the
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runtime's C.
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2. **`.cfi` is emitted, but only in a `--debug` build.** Five directives at three sites — `emit_fn`,
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`emit_main`, `emit_globals_init`, which share a prologue. `readelf --debug-dump=frames` gives the exact
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advances. `emit_main` gets no closing rule because it has no epilogue: it leaves through `flan_exit` and
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the `ud2` after that is unreachable, so the `rbp` rule holds to the last byte.
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gdb did not need it — its prologue analyser already unwound out of `flan_bounds_error`, because
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`push rbp; mov rbp,rsp; sub rsp,N` is the pattern it recognises — so this is a description stated rather
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than guessed, plus a correct unwind from the very first byte of a function, before the `push`.
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**The open decision is whether to make it unconditional.** The description is correct in every build and a
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release build is where a crash would most want it. What stopped it here is only that nothing measures the
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`.eh_frame` it would add, and another lane is measuring backend cost right now. One `if` in three places.
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3. **`X86.redefinition` emits no debug information.** It is passed no `dwarf`, so a redefinition module's
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bodies have no lines. Nothing reaches that yet — `flan dev` does not build `--x86` at all
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(`HANDOFF-x86-redef.md` §"What remains" item 3) — but when it does, a break loop stopping in a reloaded
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@ -203,9 +253,10 @@ while the run still exits 0.
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`high_pc` range, which is honest — a debugger finds no line for an address in them and says so — but a
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backtrace through the globals' initialiser names nothing. `emit_globals_init` does lower expressions that
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carry locations, so this is a small piece of work: give it a `dwsub` the way `emit_fn` has one.
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5. **Nothing tests this in `dune test`.** The corpus's source-level debugging case (`test/programs/debug.flan`
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and `debug-permuted.flan`) runs under lldb against the LLVM backend. An `--x86` arm of it would be the
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right regression test and does not exist; the verification above is a transcript in a handoff, which rots.
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5. **`debug-permuted.flan` has no `--x86` arm.** The new acceptance case covers `debug.flan` only. The
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permuted fixture exists to catch a member offset that does not follow the declaration, and that is a
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*types* claim — there are no type DIEs here to get wrong, so it would test nothing today. It becomes the
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right test the moment item 1 above is attempted, and it is already written.
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## Open questions for the author
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