Fix simd12_t size

[EgorBo]

sizeof(simd12_t) used to return 16 - that could produce some garbage values in upper 4 bytes, e.g.:

public Vector3 Test() => Vector3.Reflect(
    new Vector3(3.0f, 1.0f, 1.0f), 
    new Vector3(3.0f, 1.0f, 1.0f));

; Method Program:Test():System.Numerics.Vector3:this
G_M16498_IG01:
       vzeroupper 
						;; size=3 bbWeight=1 PerfScore 1.00

G_M16498_IG02:
       vmovups  xmm0, xmmword ptr [reloc @RWD00]
       vmovups  xmm1, xmmword ptr [reloc @RWD00]
       vdpps    xmm0, xmm0, xmm1, 127
       vmulps   xmm0, xmm0, xmm1
       vmovups  xmm1, xmmword ptr [reloc @RWD16]
       vmulps   xmm0, xmm0, xmm1
       vmovups  xmm1, xmmword ptr [reloc @RWD00]
       vsubps   xmm0, xmm1, xmm0
       vmovsd   qword ptr [rdx], xmm0
       vextractps dword ptr [rdx+08H], xmm0, 2
       mov      rax, rdx
						;; size=64 bbWeight=1 PerfScore 39.25

G_M16498_IG03:
       ret      
						;; size=1 bbWeight=1 PerfScore 1.00
-RWD00  	dq	3F80000040400000h, DDDDDDDD3F800000h
+RWD00  	dq	3F80000040400000h, 000000003F800000h
-RWD16  	dq	4000000040000000h, DDDDDDDD40000000h
+RWD16  	dq	4000000040000000h, 0000000040000000h
; Total bytes of code: 68

so I applied the #include "pshpack4.h" fix @tannergooding suggested

[ghost]

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Issue Details

---

sizeof(simd12_t) used to return 16 - that could produce some garbage values in upper 4 bytes, e.g.:

public Vector3 Test() => Vector3.Reflect(
    new Vector3(3.0f, 1.0f, 1.0f), 
    new Vector3(3.0f, 1.0f, 1.0f));

; Method Program:Test():System.Numerics.Vector3:this
G_M16498_IG01:
       vzeroupper 
						;; size=3 bbWeight=1 PerfScore 1.00

G_M16498_IG02:
       vmovups  xmm0, xmmword ptr [reloc @RWD00]
       vmovups  xmm1, xmmword ptr [reloc @RWD00]
       vdpps    xmm0, xmm0, xmm1, 127
       vmulps   xmm0, xmm0, xmm1
       vmovups  xmm1, xmmword ptr [reloc @RWD16]
       vmulps   xmm0, xmm0, xmm1
       vmovups  xmm1, xmmword ptr [reloc @RWD00]
       vsubps   xmm0, xmm1, xmm0
       vmovsd   qword ptr [rdx], xmm0
       vextractps dword ptr [rdx+08H], xmm0, 2
       mov      rax, rdx
						;; size=64 bbWeight=1 PerfScore 39.25

G_M16498_IG03:
       ret      
						;; size=1 bbWeight=1 PerfScore 1.00
-RWD00  	dq	3F80000040400000h, DDDDDDDD3F800000h
+RWD00  	dq	3F80000040400000h, 000000003F800000h
-RWD16  	dq	4000000040000000h, DDDDDDDD40000000h
+RWD16  	dq	4000000040000000h, 0000000040000000h
; Total bytes of code: 68

Author:	EgorBo
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

Failure is #76454