D268 - Floating-point arguments cross the trampoline in xmm registers

Status: decided Date: 2026-08-25

The trampoline spills all eight floating-point argument registers on every call and loads xmm0 from a slot the handler writes. A function answering in xmm0 is a separate GuestFloatFn type that returns raw bits.

Why: a System V double travels only in xmm0-xmm7, so an integer-only boundary makes every floating-point function unreachable and returns the guest's own argument. Eight unconditional stores are cheap beside the pushes and call already made. A separate type keeps the integer path, which carries the busiest imports, untouched, and raw bits let single- and double-precision functions state their true return type.

Rejected:

• A second trampoline chosen per import: kept in reserve if the stores ever measure badly.
• Widening GuestFn to carry floats: every integer call pays for it.