[dev.simd] cmd/compile: handle rematerialized op for incompatible reg constraint

This CL fixes an issue raised by contributor dominikh@.

Change-Id: I941b330a6ba6f6c120c69951ddd24933f2f0b3ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/704056
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -2576,7 +2576,26 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 			e.s.f.Fatalf("can't find source for %s->%s: %s\n", e.p, e.b, v.LongString())
 		}
 		if dstReg {
-			x = v.copyInto(e.p)
+			// Handle incompatible registers.
+			// For #70451.
+			if e.s.regspec(v).outputs[0].regs&regMask(1<<register(loc.(*Register).num)) == 0 && c != nil {
+				_, srcReg := src.(*Register)
+				if !srcReg {
+					// We need a tmp register
+					x = v.copyInto(e.p)
+					r := e.findRegFor(x.Type)
+					e.erase(r)
+					// Rematerialize to a tmp register
+					e.set(r, vid, x, false, pos)
+					// Copy from tmp to the desired register
+					x = e.p.NewValue1(pos, OpCopy, x.Type, x)
+				} else {
+					// It exist in a valid register already, so just copy it to the desired register
+					x = e.p.NewValue1(pos, OpCopy, c.Type, c)
+				}
+			} else {
+				x = v.copyInto(e.p)
+			}
 		} else {
 			// Rematerialize into stack slot. Need a free
 			// register to accomplish this.

test/simd/bug2.go

@@ -0,0 +1,26 @@
+// compile
+
+//go:build amd64 && goexperiment.simd
+
+// 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.
+
+// Test case for rematerialization ignoring the register constraint
+// during regalloc's shuffle phase.
+
+package p
+
+import (
+	"simd"
+)
+
+func PackComplex(b bool) {
+	for {
+		if b {
+			var indices [4]uint32
+			simd.Uint32x4{}.ShiftAllRight(20).Store(&indices)
+			_ = indices[indices[0]]
+		}
+	}
+}