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The xtensa windowed ABI is compiled twice and executed never

The gap, counted
executed rows in test-xtensa 107, all Call0
--xtensa-abi=windowed anywhere in the Makefile 2
windowed programs ever RUN and compared 0

Both windowed rows assert that compilation succeeds:

./$(COMPILER) --target=xtensa --xtensa-abi=windowed test/test_emit_obj.pas ...o
readelf -h ...o | grep -q 'REL (Relocatable file)'



# the windowed ABI picks a7 as the frame pointer, not a15 — it must at least
# lower (no runner: windowed images link through xtensa-esp-elf-gcc)
@./$(COMPILER) ... >/dev/null && echo "xtensa windowed inline asm lowers ok"

A compile-only row cannot fail for any reason a running program can fail for, so the entire windowed backend has been gated on "the compiler did not crash".

The justification in that comment is no longer true

no runner: windowed images link through xtensa-esp-elf-gcc

That was the reason there is no execution row, and it has been overtaken. Hosted windowed programs run today under plain tools/run_target.sh xtensa — that is how the 129-source differential measured windowed at 53 and then 94 of 129 matching the x86-64 oracle. No gcc, no ESP toolchain, the same runner every Call0 row already uses. Whatever was true when that comment was written, the runner exists now and nothing was updated to notice.

What the gap cost, concretely

bug-a-a-perf-commit-silently-fixed-41-xtensa-windowed-divergences-and-nobody-knows-why: 41 windowed programs were SIGBUSing on an unaligned data section — not producing subtly wrong values, dying on signal 7 — and were fixed as an accidental side effect of a qemu performance commit. Nobody noticed the breakage and nobody noticed the repair. Both are invisible to every gated suite in this repo, because the only instrument that has ever executed a windowed program is a scratch harness in one agent's /tmp.

That harness dies with the session. This ticket exists so the finding does not.

Do NOT scope this as "port the 129 rows"

One executed windowed row in a gated suite is worth more than a plan for all of them, and it is the row that would have caught this. The cheapest useful version is a single program that reads a multi-word data descriptor — the shape that faults on a misaligned data section:

./$(COMPILER) --target=xtensa --platform=posix --xtensa-soft-mulhigh \
    --xtensa-abi=windowed test/test_cross_record.pas $(TESTTMP)/xt_win_record
./$(COMPILER) test/test_cross_record.pas $(TESTTMP)/xt_win_record_x64
tools/run_target.sh xtensa $(TESTTMP)/xt_win_record > $(TESTTMP)/xt_win_record.out 2>&1; \
  tools/expect_same.sh xtensa-windowed/test_cross_record-rc "$$?" "0"
tools/expect_same.sh xtensa-windowed/test_cross_record \
  "$$(cat $(TESTTMP)/xt_win_record.out)" "$$($(TESTTMP)/xt_win_record_x64)"

Two rows, not one, and the first draft of this ticket got it wrong. It wrapped the runner in $$(...), which keeps stdout and discards the status — and a SIGBUS is a status, not a string. That row would have compared truncated output and reported a value mismatch for a signal death: the right verdict for the wrong reason, and the wrong diagnosis handed to whoever read it. Corrected by frank-optimize-b4, which had fixed the same defect in 20 cross-target rows the same night.

Worth stating plainly because of where it landed: the row written to catch a crash was itself written with one outcome slot for a subject that has two. The existing test_xtensa_sigdfl row two screens up already uses the correct form (> /dev/null 2>&1; expect_same "$$?" "143") — the pattern was in this same target and was not copied.

The sharpest instance of the family, also b4's, is worth carrying here for the next person writing a row: the two test_asmcore_* rows ran binary | tail -1 | grep -q, and a pipe eats the status while the success line is printed before the crash — so they were green through an entire window in which the binary was segfaulting. Both slots wrong, in the direction that reads as healthy.

Measured: that exact row FAILS at 75d2ba662^ (SIGBUS) and PASSES at 75d2ba662. It is a real canary, not a hopeful one.

Named here so it can be folded into the alignment fix in one pass rather than sequenced behind it — frank-optimize-b4 is in elfwriter.inc now, and this row is the only pre-merge check that exists for the property its change moves.

After the one row

Sensible growth, in order, each worth doing only once the one above is green:

  1. Three or four more from the aggregate/RTTI family (test_cross_dynarray, test_interfaces, test_cross_sets) — the shapes that actually faulted.
  2. A windowed section in test-xtensa rather than rows scattered among Call0 ones, so the count is visible and a future reader can see it is 4 and not 107.
  3. Only then consider whether the differential belongs in the repo as a tool.

Why prio 60

Same number as the bug it protects, and for the reason the two are one problem from opposite ends: a correctness property propped up by a perf constant, and a correctness property with no gate, are the same exposure. Fixing the alignment without adding the row leaves the next person exactly where this one started.

Note on the track letter

Filed A+S rather than T. The gap is in test-xtensa, a Makefile target owned by the lane that owns the backend, not in Track T's testmgr/twatch tooling. If it later grows into a differential runner it becomes T's; one Makefile row is not that.

RESOLVED 2026-08-30 (frankS) — five executed windowed rows in test-xtensa

test-xtensa now ends with a windowed section: 107 Call0 rows + 5 windowed, the first executed windowed programs in the repo's history.

The five, one per aggregate/RTTI family, each picked because it was measured red rather than because it looked relevant — all five are in the set of 41 that flipped between 75d2ba662^ and 75d2ba662:

source family
test_cross_record records — multi-word descriptors
test_cross_dynarray dynamic arrays — length/refcount header
test_interfaces interfaces — vtable/RTTI through two indirections
test_cross_sets sets — multi-word bitmaps
test_cross_variant variants — payload word after a byte tag

Two outcome slots each, per the correction b4 made to this ticket's own draft.

The rows are proven able to go red — and the failed attempt is the useful half

Against the pre-repair compiler (41e452a55913 = 75d2ba662^) all five die on signal 7 and both slots fire: rc reports exit 135, value reports differing output. Note which slot carries the diagnosis. Without the rc slot the failure reads as a value mismatch and the SIGBUS is invisible — which is precisely the defect b4 caught in the draft.

What did NOT reproduce it: setting ELF_DATA_ALIGN = 1 at HEAD leaves all five GREEN. I expected a red and got a green, and the green is the finding. readelf -lW on a windowed binary shows the data section in its own PT_LOAD at offset 0x30000 — b4's split (3b8d1039e) page-aligns it independently, so AlignCodeForData is currently redundant on the executable path and CheckDataBaseAligned cannot fire there.

That is belt-and-braces on a property that cost 41 programs, not a defect. But it means these rows are not measuring the explicit invariant today, and anyone who later removes the PT_LOAD split must not read their green as evidence the alignment call still holds the property up. Recorded in the Makefile beside the rows, where that reader will be standing.

Had I not run the negative experiment I would have written "proven red" on the strength of the old-compiler run alone, which is true and would have implied something false.

A second finding, filed separately

bug-t-the-esp-bare-suite-is-in-no-tier-so-nothing-ever-runs-it [T+S p45].

b4 had already added the executed windowed canary — into test-esp-bare, which appears in zero testmgr tiers and in no script. So the row existed, was correct, used the right two slots, and still could not fail anything. The same defect one level up: this ticket's complaint was a suite whose pass and whose skip print the same thing; that is a row whose pass and whose non-execution print the same thing. Adding a correct row to an unenrolled target moves the invisibility instead of removing it, and a Makefile target looks gated when you are reading the Makefile. That is why the five went into test-xtensa, which is enrolled in full.

Scope held

Only the test-xtensa target was edited. test-esp-bare was left alone — b4's row there is a harmless duplicate of one of these five and is b4's to keep or drop. Nothing in compiler/** was committed; the ELF_DATA_ALIGN change was a local experiment, reverted, and the tree rebuilt to a clean fixedpoint.

Items 1-3 of "After the one row"

Item 1 (three or four more from the aggregate/RTTI family) is done — that is what these five are. Item 2 (a windowed section rather than scattered rows, so the count is visible) is done: the section header states the count and why it is 5 and not 107. Item 3 (whether the differential belongs in the repo as a tool) is deliberately not done and should stay a separate decision.

Log

APPENDIX — the harness's provenance, banked because the harness does not survive

Track S stands down with this ticket, and the 129-source differential lives in a session scratch directory that dies with it. The five rows above are the part that survives as code; this is the part that would otherwise have to be re-derived from scratch. Everything here is a measurement, not a recollection.

The binaries the baselines were taken at

tag sha256 (12) what it is
cc-preelfpage 41e452a55913 75d2ba662^ — before the accidental repair
cc-elfpage a3f0f9e3325f 75d2ba662 — the perf commit that masked it
cc-align 62cfb924053f df98fea47 — b4's explicit alignment

Sweeps ran per-ABI with flags --target=xtensa --platform=posix --xtensa-soft-mulhigh [--xtensa-abi=windowed], each program's stdout compared against the same source compiled for x86-64 by the same binary (self-oracle, so a compiler-wide regression cannot hide by moving both sides).

The partition that matters: 53 -> 94 windowed
windowed MATCH at 75d2ba662^ 53
windowed MATCH at 75d2ba662 and at df98fea47 94
flipped red -> green 41
lost in either direction 0

The 41 are not "unlucky" programs, and this is the sentence to keep: the misalignment was universal and always was — the other 53 were never safe, only untested. A predicted code mod 4 split between the two groups was run and refuted (both groups are congruent to 3 mod 4), which is what replaced "41 unlucky programs" with the universal reading and changed what the repair had to be.

The 41, by name, so the canary set can be extended without re-deriving it

Still not matching under windowed at df98fea47 (35 of 129)

Call0 and riscv32, for completeness

Call0 104/129 and riscv32 111/129 at 62cfb924053f, both unchanged across the alignment work with lost=0 gained=0 by set difference. Those two targets tolerate unaligned word loads, so they could never have gained from the fix — they were measuring its cost, and it is zero.

How to rebuild the harness if it is ever wanted

It is roughly 60 lines of Python. Take the cross-target Pascal sources listed above as the corpus; compile each one twice with the same compiler binary (once with the xtensa flags named above, once bare); run the xtensa image through tools/run_target.sh xtensa; compare stdout; bucket into MATCH / DIFF / CFAIL. The two traps that cost real time, both worth stealing:

  1. Stamp every result file with target, ABI, and the compiler's sha256. A tag collision silently overwrote an xtensa result with a riscv32 one and produced 13 regressions that did not exist.
  2. Compare row SETS, never counts, and assert matches_before - lost + gained == matches_after. An equal count hides an equal swap, and the windowed row is exactly where that would have mattered.

Whether a differential like this belongs in the repo as a real tool is item 3 of the growth list above and is deliberately still open.