fix(asm): read the riscv64 raw-data immediates at full width

WORD and BYTE read their immediate through the truncating helper, so the
int32 wrap turned WORD $0xffffffff into -1 and rejected it, while WORD
$0x100000000 and BYTE $0x100000001 arrived pre-truncated and slipped
past the range check as small values.  Both statements now read the
immediate the source wrote and bound it at the toolchain's own limits:
[0, 0xffffffff] for WORD, [0, 0xff] for BYTE, with the bounds pinned in
a test.

Assisted-by: GLM 5.3 Flash

asm/riscv_assemble.go

@@ -1042,11 +1042,15 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 		}
 		return nil, nil
 	case "WORD":
-		// WORD $w lays down a raw 32-bit little-endian word.
+		// WORD $w lays down a raw 32-bit little-endian word, in the range
+		// [0, 0xffffffff] exactly as the toolchain's validation bounds it.
 		if len(ops) != 1 {
 			return nil, fmt.Errorf("WORD expects 1 operand, got %d", len(ops))
 		}
-		w := int64(immFromOperand(ops[0]))
+		w, ok := riscvRawImm(ops[0])
+		if !ok {
+			return nil, fmt.Errorf("WORD expects an immediate")
+		}
 		if w < 0 || w > 0xFFFFFFFF {
 			return nil, fmt.Errorf("WORD: immediate %d does not fit a 32-bit word", w)
 		}
@@ -1055,7 +1059,10 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 		// BYTE $b lays down one raw byte per operand.
 		var out []byte
 		for _, op := range ops {
-			b := int64(immFromOperand(op))
+			b, ok := riscvRawImm(op)
+			if !ok {
+				return nil, fmt.Errorf("BYTE expects immediates")
+			}
 			if b < 0 || b > 0xFF {
 				return nil, fmt.Errorf("BYTE: immediate %d does not fit a byte", b)
 			}
@@ -3249,6 +3256,22 @@ func immFromOperand(op *ast.Operand) int32 {
 	return 0
 }
 
+// riscvRawImm reads an immediate at its full written width: the raw-data
+// statements (WORD, BYTE) validate against their own ranges, so a value the
+// source spelled wider than int32 must reach the check whole, never truncated
+// through an int32 read (WORD $0xffffffff is in range, WORD $0x100000000 is
+// not, and neither may arrive disguised as the other).
+func riscvRawImm(op *ast.Operand) (int64, bool) {
+	if !op.Imm.HasVal {
+		return 0, false
+	}
+	v := op.Imm.Val
+	if op.Imm.Neg {
+		v = -v
+	}
+	return v, true
+}
+
 // riscvImm32FromOperand reads an immediate for the MOV/I-type paths as a
 // signed 32-bit value.  The toolchain materialises wider constants through
 // its SLLI expansion, which this assembler does not implement, so values

asm/riscv_encode_test.go

@@ -1340,3 +1340,52 @@ TEXT ·v(SB), NOSPLIT, $0
 		0xEA056207, // vlsseg8e32.v v4, (x10), x0
 	)
 }
+
+// TestRISCV_rawDataRange pins the WORD and BYTE immediate ranges at the
+// toolchain's own boundaries: WORD takes [0, 0xffffffff] and BYTE [0, 0xff],
+// and a value the source spells wider must reach the check whole.  The read
+// once truncated through int32, which rejected WORD $0xffffffff as -1 while
+// accepting WORD $0x100000000 as 0.
+func TestRISCV_rawDataRange(t *testing.T) {
+	asmOne := func(t *testing.T, stmt string) ([]byte, error) {
+		t.Helper()
+		fn := firstTextRISCV(t, "#include \"textflag.h\"\nTEXT ·w(SB), NOSPLIT, $0\n\t"+stmt+"\n\tRET\n")
+		code, _, _, _, _, _, err := assembleRISCV(fn)
+		return code, err
+	}
+	t.Run("word bounds", func(t *testing.T) {
+		code, err := asmOne(t, "WORD $0")
+		if err != nil {
+			t.Fatalf("WORD $0: %v", err)
+		}
+		if string(code[:4]) != "\x00\x00\x00\x00" {
+			t.Errorf("WORD $0 = % x", code[:4])
+		}
+		code, err = asmOne(t, "WORD $0xffffffff")
+		if err != nil {
+			t.Fatalf("WORD $0xffffffff must assemble, as go tool asm accepts it: %v", err)
+		}
+		if string(code[:4]) != "\xff\xff\xff\xff" {
+			t.Errorf("WORD $0xffffffff = % x", code[:4])
+		}
+		for _, w := range []string{"$-1", "$0x100000000", "$-4294967296"} {
+			if _, err := asmOne(t, "WORD "+w); err == nil {
+				t.Errorf("WORD %s must be rejected, as go tool asm rejects it", w)
+			}
+		}
+	})
+	t.Run("byte bounds", func(t *testing.T) {
+		code, err := asmOne(t, "BYTE $255")
+		if err != nil {
+			t.Fatalf("BYTE $255: %v", err)
+		}
+		if code[0] != 0xff {
+			t.Errorf("BYTE $255 = % x", code[:1])
+		}
+		for _, b := range []string{"$256", "$0x100000001", "$-1"} {
+			if _, err := asmOne(t, "BYTE "+b); err == nil {
+				t.Errorf("BYTE %s must be rejected: out of the byte range", b)
+			}
+		}
+	})
+}