fix(asm): reject the arm64 REGTMP spellings the toolchain refuses

Assisted-by: GLM 5.3

asm/arm64_encode_test.go

@@ -2267,6 +2267,52 @@ func TestArm64NoopVsNop(t *testing.T) {
 	}
 }
 
+// TestArm64RegtmpBoundaries pins the REGTMP acceptance the toolchain holds
+// (probed against `go tool asm`, Go 1.27, arm64): the plain imm12 and the
+// ±4095 offsets tolerate a REGTMP source or pair member, while every
+// lowering that itself writes REGTMP (the constant materialisations, the
+// pool path, the pair store expansions) refuses one, and the pool path
+// refuses a REGTMP base on both the loads and the stores.
+func TestArm64RegtmpBoundaries(t *testing.T) {
+	accept := []string{
+		"\tADD\t$5, R27, R3\n",              // imm12, no REGTMP sequence
+		"\tLDP\t700(R2), (R26, R27)\n",      // load add/sub path: unchecked pair
+		"\tMOVD\tR27, 4000(R2)\n",           // misaligned ±4095: unchecked data
+		"\tSTP\t(R26, R3), 700(R2)\n",       // store add/sub path, no pair member
+		"\tLDP\t0x1234567(R2), (R27, R3)\n", // pool load: base alone decides
+		"\tMOVD\t$0x1000000(R27), R1\n",     // lacon: case 34 reads the base
+	}
+	reject := []string{
+		"\tADD\t$0x1234567, R27, R3\n",      // materialisation, REGTMP source
+		"\tAND\t$0x22220000, R27, R4\n",     // logical materialisation, ditto
+		"\tSTP\t(R26, R27), 700(R2)\n",      // store add/sub path, pair member
+		"\tSTP\t(R3, R4), 0x1234567(R27)\n", // pool store, REGTMP base
+		"\tLDP\t0x1234567(R27), (R3, R4)\n", // pool load, REGTMP base
+		"\tSTP\t(R27, R3), 0x1234567(R2)\n", // pool store, REGTMP pair member
+		"\tMOVD\tR27, 0x1234567(R2)\n",      // pool store, REGTMP data
+		"\tMOVD\t0x1234567(R2), R27\n",      // pool load, REGTMP data
+	}
+	for _, src := range accept {
+		f, errs := parser.Parse("test_arm64.s", "#include \"textflag.h\"\n\nTEXT ·f(SB), NOSPLIT, $0-0\n"+src+"\tRET\n")
+		if len(errs) > 0 {
+			t.Errorf("expected acceptance for %q, got parse rejection", strings.TrimSpace(src))
+			continue
+		}
+		if _, err := AssembleFileARM64(f); err != nil {
+			t.Errorf("expected acceptance for %q, got %v", strings.TrimSpace(src), err)
+		}
+	}
+	for _, src := range reject {
+		f, errs := parser.Parse("test_arm64.s", "#include \"textflag.h\"\n\nTEXT ·f(SB), NOSPLIT, $0-0\n"+src+"\tRET\n")
+		if len(errs) > 0 {
+			continue // a parse rejection is a rejection
+		}
+		if _, err := AssembleFileARM64(f); err == nil {
+			t.Errorf("expected rejection for %q, got nil", strings.TrimSpace(src))
+		}
+	}
+}
+
 // TestArm64VLDSTPostIndexContract pins the structure-load post-index rules
 // against `go tool asm` (Go 1.27, arm64): the implicit by-size increment of
 // an unspelled or zero offset (both load and replicate forms, the words