Minor revision

- Add rule for single fixed-length vector or fixed-length vector array
  with size 1.
- Add rule for zero-length fixed-length arrays.
- Add explicitly rule for fixed-length vector struct as vector tuple type:
  pass by ref if no enough arg register.

riscv-cc.adoc

@@ -495,10 +495,21 @@ it will pass by reference and is replaced in the argument list with the address.
 Otherwise, it will use the rule defined in the hardware floating-point calling
 convention.
 
+A struct containing just one fixed-length vector or on fixed-length vector
+array of length one, it will flattening as a single fixed-length vector argument
+if the size of the vector is less than or equal to 8×ABI_VLEN bit.
+
+Struct with zero-length fixed-length arrays use the rule defined in the hardware
+floating-point calling convention, which means it won't consume vector argument
+register eitehr in C or {Cpp}.
+
 A struct containing just one fixed-length vector array is passed as though it
 were a vector tuple type if the size of the base element for the array is less
 or equal to 8×ABI_VLEN bit, and the size of the array is less than 8×ABI_VLEN
-bit. Otherwise, it will use the rule defined in the hardware floating-point
+bit.
+If there are not enough vector argument registers to pass the entire struct,
+it will pass by reference and is replaced in the argument list with the address.
+Otherwise, it will use the rule defined in the hardware floating-point
 calling convention.
 
 Unions with fixed-length vectors are always passed according to the integer