[Arm64] HFA register arguments pushed to stack

[BruceForstall]

It appears that HFA register arguments are always pushed to the stack.

Example:

using System;

namespace hfaargs
{
    struct hfa1
    {
        public float f1, f2;
    }

    class Program
    {
        static float f(hfa1 a)
        {
            return a.f1 + a.f2;
        }

        static void Main(string[] args)
        {
            hfa1 h = new hfa1();
            h.f1 = 1.0F;
            h.f2 = 2.0F;
            float ret = f(h);

            Console.WriteLine("{0}", ret);
        }
    }
}

generates:

;  V00 arg0         [V00    ] (  4,  4   )  struct ( 8) [fp+0x18]   HFA(float)  do-not-enreg[XSFA] multireg-arg addr-exposed
;# V01 OutArgs      [V01    ] (  1,  1   )  lclBlk ( 0) [sp+0x00]   "OutgoingArgSpace"
;
; Lcl frame size = 16

G_M4231_IG01:
        A9BE7BFD          stp     fp, lr, [sp,#-32]!
        910003FD          mov     fp, sp
        BD001BA0          str     s0, [fp,#24]
        BD001FA1          str     s1, [fp,#28]
G_M4231_IG02:
        BD401BA0          ldr     s0, [fp,#24]
G_M4231_IG03:
        BD401FB0          ldr     s16, [fp,#28]
G_M4231_IG04:
        1E302800          fadd    s0, s0, s16
G_M4231_IG05:
        A8C27BFD          ldp     fp, lr, [sp],#32
        D65F03C0          ret     lr

This particular case could just be:

        stp     fp, lr, [sp,#-16]!
        mov     fp, sp
        fadd    s0, s0, s1          ; only one non-prolog/epilog instruction; no stack space needed for the args
        ldp     fp, lr, [sp],#16
        ret     lr

Related: #35631

category:cq
theme:register-allocator
skill-level:expert
cost:medium

[BruceForstall]

cc @kunalspathak @CarolEidt

[BruceForstall]

This appears to be fixed. The HFA is marked as struct promoted. (Repro with PMI and altjit):

; Assembly listing for method hfaargs.Program:f(hfaargs.hfa1):float
; Emitting BLENDED_CODE for generic ARM64 CPU - Windows
; optimized code
; fp based frame
; partially interruptible
; invoked as altjit
; Final local variable assignments
;
;* V00 arg0         [V00    ] (  0,  0   )  struct ( 8) zero-ref    HFA(float)  multireg-arg
;# V01 OutArgs      [V01    ] (  1,  1   )  lclBlk ( 0) [sp+0x00]   "OutgoingArgSpace"
;  V02 tmp1         [V02,T00] (  2,  2   )   float  ->   d0         V00.f1(offs=0x00) P-INDEP "field V00.f1 (fldOffset=0x0)"
;  V03 tmp2         [V03,T01] (  2,  2   )   float  ->   d1         V00.f2(offs=0x04) P-INDEP "field V00.f2 (fldOffset=0x4)"
;
; Lcl frame size = 0

G_M4231_IG01:              ;; offset=0000H
        A9BF7BFD          stp     fp, lr, [sp,#-16]!
        910003FD          mov     fp, sp
                        ;; bbWeight=1    PerfScore 1.50
G_M4231_IG02:              ;; offset=0008H
        1E212800          fadd    s0, s0, s1
                        ;; bbWeight=1    PerfScore 3.00
G_M4231_IG03:              ;; offset=000CH
        A8C17BFD          ldp     fp, lr, [sp],#16
        D65F03C0          ret     lr