Variadic C on windowed xtensa

Found 2026-09-24 (frankS) bringing up feature-c-esp-conformance-coverage.

Measured

compiler/pascal26 --target=xtensa --xtensa-abi=windowed --platform=esp -Ilib/crtl/include -Ilib/crtl/src hello.c hello.o for a printf hello:

error: variadic C functions on xtensa: only the call0 ABI is implemented.
  The windowed frame pointer, argument window and incoming-stack bias all
  differ ...   in: lib/crtl/src/fcntl.c

The first wall is crtl's own fcntl/open (int open(const char *, int, ...)), reached before the program's printf. So the unit of work is va_start/va_arg over the windowed incoming area, not anything in the user's program.

What it unblocks

The S3 leg of tools/run_c_conformance_esp.sh, which is written for esp32c3 only and says so; adding it is a chip switch plus the hello-s3 project once this lands. For scale, the C3 at the same tree: 218 pass, 1 fail, 1 skip of 220.

2026-09-24 (frankS): DONE

• cparser.inc variadic prologue: windowed arm addresses the save area off a15 explicitly (EmitFrameAddrXtensa answers a7, which is still arg word 5 at that point), overflow at a15+32. Refused: variadic + aggregate result.
• cparser.inc: windowed ESP-object entry stub (entry a1,64; argc=0/argv; init; main; finalizers; retw) at .text+0, where the writer exports app_main.
• Proven by booting, not by building: tools/run_c_conformance_esp.sh --chip esp32s3 (new; Makefile test-c-conformance-esp32s3). First full run at compiler 17be23f99ad8: 209/10/1. Every red but 00053 was a real bug, fixed in the same commit and re-run at f8f9d3f827d3: xtensa signed-only integer compare and quos/rems for unsigned; the windowed fenv stub lacking entry (%f truncated); 64-bit branch truthiness testing the low word only; and a harness artefact (renamed main loses C99's fall-off-returns-0). Left red: 00053 (p50 struct-tag bug), 00175 (C implicit double->int, next), 00207 (VLA: alloca unsupported on xtensa).
• Guards: test-emit-obj rows (variadic builds, entry bytes 36 81 00, the aggregate refusal); pinned v416 refuses the variadic row.