fix(bloat): make ESP32-S3 flash totals independent of linker-marker attribution

[zackees]

Context

FastLED's ESP32-S3 Blink bloat gate reads fbuild symbols report.json.total_flash. That field is a sum of attributed symbol sizes, not the firmware image size. On the same FastLED commit (01caa0a89f, FastLED/FastLED#4468), CI reported 383,120 B and a local NixOS build reported 387,217 B, while fbuild's image-flash values differed by only 64 B (473,588 vs 473,524 B). This makes the local gate fail on source that CI passes.

The symbol diff in FastLED/FastLED#4468 identified the main mechanism: local nm contributes a sized _stext row (~10.9 KB) at the start of .flash.text; CI has no such row. _stext is a linker-script section-start marker, not a function or object. The local report also synthesized only 276 map-derived rows versus 1,026 in CI, so the marker's span suppresses real .literal.*/.text.* ownership and the net reported difference is ~4 KB, not 10.9 KB. Some framework archive totals differ too; the current evidence does not establish that filtering _stext alone closes the entire gap.

Reproduced with fbuild 2.5.26 on current FastLED source: a clean base worktree (only an unrelated Python import change) reports total_flash=387,289 B, image flash 473,588 B, _stext=10,912 B, 276 map-derived rows. A response-LUT feature worktree reports 387,363 B, image flash 473,680 B, _stext=10,916 B, and no response-LUT/pipeline symbols in Blink. The local feature delta is 74 B; the ~4 KB baseline failure is already present on the base worktree.

Source inspection points at fbuild-core/src/symbol_analysis/mod.rs: parse_nm_output accepts any nonzero sized symbol; build_fine_grained_map_with_synth inserts all nm ranges into nm_covered before map synthesis; retain_loaded_symbols filters by PT_LOAD containment, which cannot reject an in-range linker marker such as _stext. The total_flash rollup then sums these attribution rows.

Proposal

Treat linker-script boundary labels as non-owning markers before both symbol rollup and nm_covered construction. Keep per-symbol attribution useful, but expose or use a stable, explicitly named image/allocated-section flash metric for cross-machine regression gates; diagnostic attributed-symbol sums should not masquerade as physical flash usage. Preserve the per-input-section map synthesis when a marker overlaps real sections. Coordinate the FastLED gate's metric change under FastLED/FastLED#4468.

Acceptance criteria

• Add a focused RED fixture with _stext at the start of a loaded .flash.text segment and live .literal.* sections under its inferred nm size; show that the current analyzer counts _stext and suppresses those map-derived owners. Make it GREEN with the fix.
• Add a paired-fixture test for the same loadable bytes with/without a sized linker marker; the gate-facing flash metric agrees, and attribution does not double count or lose the literal owners.
• Document the semantics of image flash versus attributed-symbol flash in report.json and CLI output; do not silently change an existing field without a migration path for consumers.
• Re-run FastLED's ESP32-S3 Blink gate on one unchanged commit in CI and locally (or equivalent distinct toolchain environments) and show that it no longer produces a false ~4 KB regression. If framework objects actually differ, report that separately from attribution drift.

Decisions

• P2 tooling correctness: this blocks reliable local preflight and can invite unjustified baseline bumps, but does not alter shipped firmware.
• File upstream in fbuild because the marker filtering and report contract live in the symbol analyzer; keep bloat gate: local ESP32-S3 Blink total_flash reads 4 KB higher than CI for the same commit FastLED#4468 as the downstream gate tracker.
• Do not raise FastLED's pinned baseline to the local number: the clean base already fails, and the firmware image sizes nearly agree.

Open questions

• Which exact CI/local toolchain or linker-map difference makes _stext appear as a sized nm row only locally? The differing reports establish the failure mode; an archived CI firmware.map and nm output for the same commit would isolate the input difference.

Related issues

• bloat gate: local ESP32-S3 Blink total_flash reads 4 KB higher than CI for the same commit FastLED#4468 tracks the same false local-versus-CI gate failure and contains the original same-commit report diff.
• Symbol/size analysis: detect or prevent map-file 0x00000000 false positives (~95 KB phantom-bloat reported on FastLED#2473) #417 fixed a different map-file false-positive class (address-zero ranges); this one involves a loaded linker boundary marker.

[zackees]

Fix landed as c95d0d2. Process note: the commit reached main without a PR, pushed by accident. Another agent switched the shared local checkout to main in the middle of the task, so the push went to main with admin bypass. It was not force-reverted. CI on that commit: https://github.com/FastLED/fbuild/actions/runs/36228030912 (other gates on that SHA already green). The release goes through PR #1472.

Result on the same FastLED ESP32-S3 Blink ELF and map, comparing host nm 2.46 with xtensa nm 2.43.1:

• before the fix, total_flash was 394,421 vs 390,344
• after the fix, total_flash is 390,344 vs 390,344 and the new image_flash is 448,835 vs 448,835 (firmware.bin is 448,944 B)

_stext and the 68 other zero-sized asm labels no longer get rows, and the map-derived owners are restored (1,040 rows).

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