[pistoleer]

Swift solves this by using witness tables. Witness table is a skeleton of the desired record shape. It records offsets of desired fields as found in the actual supplied record. Each function call the actual record is not passed, but its corresponding witness table instead.

For instance, if the "prototype" of the argument is {int foo, float bar}, and I supply {int foo, int baz, float bar}, the table will be {foo: base+0 bytes, bar: base+8 bytes}.

[whatshisface]

Why doesn't it limit what it pushes on the stack to (foo,bar) instead of pushing (foo,baz,bar) along with an offset table? If the offset table is possible to construct at the callsite, compiling it into the call is also possible.

[Joker_vD]

Because the polymorphic function called may also need to pass the whole record it was given to some other polymorphic function a la "void storeInGlobalStorage(key: string, value: T)".