C99 6.5.2.2p6 requires float->double promotion for floats in the
variadic region of a call; w6c emitted MOVSS (4B), so a C variadic's
va_arg(double) read 8B of garbage. Promote at push (CVTSS2SD) so the
8B slot holds a real double, covering the XMM-reg and >8-float spill
cases uniformly; fi/AL XMM-count is unchanged.
cstage only; the wwstage twin rides C2 (#10), which adds wwstage's
C-variadic-call codegen.
SysV §3.5.7 requires a variadic call to set AL = number of vector (XMM) regs used for the variable float args; the C callee gates its xmm-save-area stores on `test %al,%al`, so the old hardcoded XORQ AX,AX (AL=0) made va_arg(double) read garbage for any float-bearing C variadic call. Emit MOVQ $fi,AX (fi = the in-scope XMM cursor, ≤8); w6a has no MOVL-immediate encoding so MOVQ is the assemblable form and sets AL=fi identically. fi==0 keeps XORQ → byte-identical to pre-fix for no-float variadic calls. Runtime test 989_ffivariadic links a cc-compiled va_arg(double) fixture (zero relocs/undefined, w6l-linkable) and sweeps N=3/5/8 floats (N=2 is vacuous via stale-stack aliasing). C1 of the C-FFI-variadic align-up (USER ruling); C2 wwstage + C3 bodiless gate follow. ref/qbe/amd64/sysv.c:384. 454 green.
