JIT: Introduce LclVarDsc::lvIsMultiRegDest

[jakobbotsch]

With recent work to expand returned promoted locals into FIELD_LIST the only "whole references" of promoted locals we should see is when stored from a multi-reg node. This is the only knowledge the backend should need for correctness purposes, so introduce a bit to track this property, and switch the backend to check this instead.

The existing lvIsMultiRegRet is essentially this + whether the local is returned. We should be able to remove this, but it is currently used for some outdated correctness checks in old promotion, so keep it around for now.

This makes the new bitwise combination of returns kick in much more often for multireg returns. For example on win-arm64:

static Memory<int> ToMemory(int[] arr)
{
    return arr.AsMemory();
}

 G_M45070_IG01:  ;; offset=0x0000
-            stp     fp, lr, [sp, #-0x20]!
+            stp     fp, lr, [sp, #-0x10]!
             mov     fp, sp
-            str     xzr, [fp, #0x10]    // [V03 tmp2]
-                                                ;; size=12 bbWeight=1 PerfScore 2.50
-G_M45070_IG02:  ;; offset=0x000C
-            cbz     x0, G_M45070_IG04
-                                                ;; size=4 bbWeight=1 PerfScore 1.00
-G_M45070_IG03:  ;; offset=0x0010
-            str     x0, [fp, #0x10]     // [V07 tmp6]
-            str     wzr, [fp, #0x18]    // [V08 tmp7]
-            ldr     x0, [fp, #0x10]     // [V07 tmp6]
-            ldr     w0, [x0, #0x08]
-            str     w0, [fp, #0x1C]     // [V09 tmp8]
-            b       G_M45070_IG05
-                                                ;; size=24 bbWeight=0.50 PerfScore 4.50
-G_M45070_IG04:  ;; offset=0x0028
-            str     xzr, [fp, #0x10]    // [V07 tmp6]
-            str     xzr, [fp, #0x18]
-                                                ;; size=8 bbWeight=0.50 PerfScore 1.00
-G_M45070_IG05:  ;; offset=0x0030
-            ldp     x0, x1, [fp, #0x10] // [V03 tmp2], [V03 tmp2+0x08]
-                                                ;; size=4 bbWeight=1 PerfScore 3.00
-G_M45070_IG06:  ;; offset=0x0034
-            ldp     fp, lr, [sp], #0x20
+						;; size=8 bbWeight=1 PerfScore 1.50
+G_M45070_IG02:  ;; offset=0x0008
+            cbz     x0, G_M45070_IG06
+						;; size=4 bbWeight=1 PerfScore 1.00
+G_M45070_IG03:  ;; offset=0x000C
+            ldr     w1, [x0, #0x08]
+						;; size=4 bbWeight=0.80 PerfScore 2.40
+G_M45070_IG04:  ;; offset=0x0010
+            mov     w1, w1
+            mov     x2, xzr
+            orr     x1, x2, x1,  LSL #32
+						;; size=12 bbWeight=1 PerfScore 2.00
+G_M45070_IG05:  ;; offset=0x001C
+            ldp     fp, lr, [sp], #0x10
             ret     lr
-                                                ;; size=8 bbWeight=1 PerfScore 2.00
+						;; size=8 bbWeight=1 PerfScore 2.00
+G_M45070_IG06:  ;; offset=0x0024
+            mov     x0, xzr
+            mov     w1, wzr
+            b       G_M45070_IG04
+						;; size=12 bbWeight=0.20 PerfScore 0.40
 
-; Total bytes of code 60, prolog size 12, PerfScore 14.00, instruction count 15, allocated bytes for code 60 (MethodHash=b2994ff1) for method Program:ToMemory(int[]):System.Memory`1[int] (FullOpts)
+; Total bytes of code 48, prolog size 8, PerfScore 9.30, instruction count 12, allocated bytes for code 48 (MethodHash=b2994ff1) for method Program:ToMemory(int[]):System.Memory`1[int] (FullOpts)

(which still can be improved)

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[Copilot]

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @EgorBo

Diffs