Arm64 multireg passing: keep in regs

[sandreenko]

For such code:

struct Struct
{
    int i1;
    int i2;
    int i3;
    int i4;
}

[MethodImpl(MethodImplOptions.NoInlining)]
static void Caller(Struct s)
{
    Callee(s);
}

we currently generate

IN0004: 000000      stp     fp, lr, [sp,#-32]!
IN0005: 000004      mov     fp, sp
N0006: 000008      str     x0, [fp,#16]
IN0007: 00000C      str     x1, [fp,#24]
IN0001: 000010      ldr     x0, [fp,#16]
IN0002: 000014      ldr     x1, [fp,#24]
IN0003: 000018      bl      Runtime_52320.C:Callee(Struct)
IN0008: 00001C      ldp     fp, lr, [sp],#32
IN0009: 000020      ret     lr

when, of course, we want something like:

IN0004: 000000      stp     fp, lr, [sp,#-32]!
IN0005: 000004      mov     fp, sp
IN0003: 000018      bl      Runtime_52320.C:Callee(Struct)
IN0008: 00001C      ldp     fp, lr, [sp],#32
IN0009: 000020      ret     lr

not a major thing but lets keep track of it and see if we can support it with the new morph logic.

category:cq
theme:register-allocator