C: float (single) vararg prints 0.000000 on i386/arm32/riscv32 — default argument promotion missing

• Type: bug (backend/call lowering — 32-bit targets). Track A.
• Found: 2026-07-08 by the new C cross-conformance matrix (feature-c-cross-target-feature-coverage): 00174 + 00175 fail identically on i386, arm32, riscv32; aarch64 and x86-64 pass.

Repro

float a = 12.34 + 56.78;
printf("%f\n", a);            /* 32-bit: 0.000000; expected 69.120003 */
printf("%f\n", 12.34 + 56.78); /* fine everywhere (double expr) */

Also any float PARAM forwarded to printf (00175's floatfunc(float a) prints float: 0.000000 for all inputs on the three targets).

Symptom scope

• Double literals/exprs to varargs: correct on all targets.
• float variable or parameter to varargs: 0.000000 on i386/arm32/riscv32. C99 6.5.2.2p6 default argument promotion (float → double for ...) is not applied (or applied with a wrong 4-byte slot) on the 32-bit call paths.
• x86-64 + aarch64 OK, so the shared C frontend promotes somewhere the 64-bit paths honor; the 32-bit variadic marshalling passes the raw single (or a half-filled pair) and __pxx_va_arg reads garbage/zero.

Where to look

Per-target variadic call marshalling (the v178 arc: project_cross_variadic_arm32_riscv32_v178) — check where a tySingle argument to a variadic C call gets widened; likely needs an explicit single→double convert before slotting on the 32-bit targets (2 words), mirroring whatever the x86-64 path does via xmm.

Gate

00174 + 00175 pass under tools/run_c_conformance.sh --target {i386,arm32,riscv32}; drop their lines from test/c-conformance/pxx.skip.{i386,arm32,riscv32}; existing variadic guards stay green; self-host byte-identical.

Log

• 2026-07-08 — resolved, commit 648dfd5b.