[dev.simd] cmd/compile: make condtion of CanSSA smarter for SIMD fields

This CL tires to improve a situation pointed out by
https://github.com/golang/go/issues/73787#issuecomment-3305494947.

Change-Id: Ic23c80fe71344fc25383ab238ad6631e0f0cd22e
Reviewed-on: https://go-review.googlesource.com/c/go/+/705416
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/ssa/value.go

@@ -9,6 +9,7 @@ import (
 	"cmd/compile/internal/types"
 	"cmd/internal/src"
 	"fmt"
+	"internal/buildcfg"
 	"math"
 	"sort"
 	"strings"
@@ -615,11 +616,14 @@ func CanSSA(t *types.Type) bool {
 	if t.IsSIMD() {
 		return true
 	}
-	if t.Size() > int64(4*types.PtrSize) {
+	sizeLimit := int64(MaxStruct * types.PtrSize)
+	if t.Size() > sizeLimit {
 		// 4*Widthptr is an arbitrary constant. We want it
 		// to be at least 3*Widthptr so slices can be registerized.
 		// Too big and we'll introduce too much register pressure.
-		return false
+		if !buildcfg.Experiment.SIMD {
+			return false
+		}
 	}
 	switch t.Kind() {
 	case types.TARRAY:
@@ -639,7 +643,17 @@ func CanSSA(t *types.Type) bool {
 				return false
 			}
 		}
-		return true
+		// Special check for SIMD. If the composite type
+		// contains SIMD vectors we can return true
+		// if it pass the checks below.
+		if !buildcfg.Experiment.SIMD {
+			return true
+		}
+		if t.Size() <= sizeLimit {
+			return true
+		}
+		i, f := t.Registers()
+		return i+f <= MaxStruct
 	default:
 		return true
 	}

src/cmd/compile/internal/test/value_test.go

@@ -0,0 +1,41 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"cmd/compile/internal/ssa"
+	"cmd/compile/internal/types"
+	"internal/buildcfg"
+	"testing"
+)
+
+// This file contains tests for ssa values, types and their utility functions.
+
+func TestCanSSA(t *testing.T) {
+	i64 := types.Types[types.TINT64]
+	v128 := types.TypeVec128
+	s1 := mkstruct(i64, mkstruct(i64, i64, i64, i64))
+	if ssa.CanSSA(s1) {
+		// Test size check for struct.
+		t.Errorf("CanSSA(%v) returned true, expected false", s1)
+	}
+	a1 := types.NewArray(s1, 1)
+	if ssa.CanSSA(a1) {
+		// Test size check for array.
+		t.Errorf("CanSSA(%v) returned true, expected false", a1)
+	}
+	if buildcfg.Experiment.SIMD {
+		s2 := mkstruct(v128, v128, v128, v128)
+		if !ssa.CanSSA(s2) {
+			// Test size check for SIMD struct special case.
+			t.Errorf("CanSSA(%v) returned false, expected true", s2)
+		}
+		a2 := types.NewArray(s2, 1)
+		if !ssa.CanSSA(a2) {
+			// Test size check for SIMD array special case.
+			t.Errorf("CanSSA(%v) returned false, expected true", a2)
+		}
+	}
+}

test/codegen/simd.go

@@ -27,3 +27,33 @@ func vptest2() bool {
 	// amd64:`SETEQ\s(.*)$`
 	return v1.And(v2).IsZero()
 }
+
+type Args2 struct {
+	V0 simd.Uint8x32
+	V1 simd.Uint8x32
+	x  string
+}
+
+//go:noinline
+func simdStructNoSpill(a Args2) simd.Uint8x32 {
+	// amd64:-`VMOVDQU\s.*$`
+	return a.V0.Xor(a.V1)
+}
+
+func simdStructWrapperNoSpill(a Args2) simd.Uint8x32 {
+	// amd64:-`VMOVDQU\s.*$`
+	a.x = "test"
+	return simdStructNoSpill(a)
+}
+
+//go:noinline
+func simdArrayNoSpill(a [1]Args2) simd.Uint8x32 {
+	// amd64:-`VMOVDQU\s.*$`
+	return a[0].V0.Xor(a[0].V1)
+}
+
+func simdArrayWrapperNoSpill(a [1]Args2) simd.Uint8x32 {
+	// amd64:-`VMOVDQU\s.*$`
+	a[0].x = "test"
+	return simdArrayNoSpill(a)
+}