fix(asm): reject the operand-starved riscv64 spellings instead of panicking

Assisted-by: GLM 5.3

asm/assembler_fuzz_test.go

@@ -232,6 +232,11 @@ func FuzzAssembleRISCV64(f *testing.F) {
 	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tCSRRW $0x1000, X5, X6\n\tRET\n")
 	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tSLLI $64, X5, X6\n\tRET\n")
 	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tVLE8V (X10), V32\n\tRET\n")
+	// Operand-starved spellings that used to panic the layout and encode
+	// passes; each must come back as a diagnostic.
+	f.Add("TEXT ·f(SB), $0\n\tJALR\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tROR $3\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tVLE8V (X10)\n\tRET\n")
 	f.Add("#define A A\nA\n")
 
 	f.Fuzz(func(t *testing.T, src string) {

asm/riscv_assemble.go

@@ -570,6 +570,11 @@ func riscvRevShiftSize(mnem string, ops []*ast.Operand) int {
 	if mnem != "ROR" && mnem != "RORI" {
 		return 4 // SLLIW has no compressed form
 	}
+	if len(ops) < 2 {
+		// A malformed one-operand form: encoding rejects it with a
+		// diagnostic, and the layout pass only needs a word count.
+		return 4
+	}
 	imm := int(immFromOperand(ops[0]))
 	rs1 := regFromOperand(ops[1])
 	rd := rs1
@@ -4830,6 +4835,9 @@ func encodeRISCVVecLS(mnem string, ops []*ast.Operand) ([]byte, bool, error) {
 	if !ok {
 		return nil, true, fmt.Errorf("unsupported vector load/store %q", mnem)
 	}
+	if len(ops) < 2 {
+		return nil, true, fmt.Errorf("%s expects at least 2 operands, got %d", mnem, len(ops))
+	}
 	op := uint32(0x27)
 	if v.load {
 		op = 0x07

asm/riscv_frame.go

@@ -96,8 +96,10 @@ func riscvIsLeaf(t *ast.Text) bool {
 		case "JALR":
 			// JALR rd, offset(rs1) links when the destination register (the
 			// first operand) is X1; JALR rs1, rd links when the second
-			// register is X1; JALR offset(rs1) always links to X1.
-			if len(in.Operands) == 1 {
+			// register is X1; JALR offset(rs1) always links to X1.  A bare
+			// operand-less spelling is malformed and encoding rejects it;
+			// conservatively count it as a link so the frame stays honest.
+			if len(in.Operands) <= 1 {
 				return false
 			}
 			if isMemOperand(in.Operands[1]) {