c(x86-64 SysV): a variadic function with a named long double parameter is refused (definition and calls)

[davidgmbb]

Observed

long double g(int n, long double first, ...) { return first; }
long double call(void) { return g(1, 2.0L, 3); }

$ build/Release/ide cc -c -target x86_64-linux v.c
cc: error: v.c:1:13: C IR lowering does not yet support the parameter or return value types of function 'g'

Clang 18 (--target=x86_64-linux-gnu) accepts it. The refusal is deliberate. c_ir_signature_body_supported in src/buster/lib/compiler/frontend/c/c_gen.c rejects any wide-float fixed parameter of a variadic signature ("The canonical SysV va_start path cannot account for a fixed x87 parameter yet"), and c_parse_validate_signature in c_parse.c mirrors it. Passing long double variadically already works on SysV. Only a named one before ... is refused.

On System V a named x87 long double occupies a 16-byte slot in the overflow (stack) area, so va_start must start its overflow_arg_area after it. gp_offset/fp_offset are unaffected.

#1720 lifted the same restriction for AAPCS64 binary128, where a named binary128 uses a V register like a 16-byte vector. This issue is the x87 half.

Expected

The definition and the call compile and match Clang objects in both call directions. That includes a named long double that spills to the stack after other stack arguments.

Details

• Revision c3f93e8 (branch of aarch64: lower binary128 long double through the compiler runtime #1720, unchanged for x86-64); Linux x86-64.

Done when

The mirror and lowering gates admit the x87 shape, the canonical and MIR va_start account for the overflow slot, and a mixed-compiler fixture (Buster caller ↔ Clang callee and back) reads variadic arguments correctly after one or two named long doubles.

[davidgmbb]

Closing as completed after current-main verification. The x87 half was implemented in PR #1800, integrated into #1720 and then merged to main at 0e7760893c1b8e59097293c2897343c4c99aa6db.

At current main c105c8895894b68fecd3ddb74ad4b285f3c4b2d1, both c_parse_validate_signature and c_ir_signature_body_supported admit non-atomic SysV x87 named parameters. The canonical placement aligns the overflow stack cursor and consumes two eightbyte slots. The registered compiler_driver_test_sysv_named_f80_varargs remains present in src/buster/tests/compiler/driver/driver_test.c (blob 56569630d7c663430122a034f342fa5053f8a410): one/two named long doubles, seven GPR arguments and stack spill, precision-sensitive values, host↔Buster calls, four allocators, both frontend forms and PIC/non-PIC. This directly covers the issue's completion criteria.

The integrated #1720 source head ebfa159ce3124a5cd4cd714364c17130bf2c70eb has 40 successful checks and two expected optional skips, including successful CI complete. Source identities were rechecked now; no new local compiler run or new execution is claimed. The current stale comment about an unsupported fixed x87 shape describes the old state, not the current predicate.

License: Buster's first-party license is unselected, verified from LICENSES/README.md at the current inspected main and #621. No external implementation is imported.