The psABI is documented here:
https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md

Structs of containing one int and one fp value are passed in int+fp registers, as described here:
"""
A struct containing one floating-point real and one integer, in either order, is passed in a floating-point register and an integer register, with the integer zero- or sign-extended as though it were a scalar, provided the floating-point real is no more than FLEN bits wide and the integer is no more than XLEN bits wide, and at least one floating-point argument register and at least one integer argument register is available. Otherwise, it is passed according to the integer calling convention.
"""

Scalar sign/zero-extension is specified as "When passed in registers, scalars narrower than XLEN bits are widened according to the sign of their type up to 32 bits, then sign-extended to XLEN bits."

As for return, the document states "Values are returned in the same manner as a first named argument of the same type would be passed."

Assume a struct consisting of a uint8 and a float:
struct s_u8_f { uint8_t i; float f; };

By the above rules if passed as the first argument to a function, we would expect to see i in a0 (zero-extended), and f in fa0. As specified above, we expect the same if this struct is returned by value.

GCC does not sign or zero-extend as it should as demonstrated in the program below.

$ cat foo.c 
#include <stdint.h>

struct s_u8_f { uint8_t i; float f; };
struct s_i8_f { int8_t i;  float f; };

struct s_u8_f should_zext_arg(int8_t i, float f) {
  struct s_u8_f s;
  s.i = i;
  s.f = f;
  return s;
}

struct s_i8_f should_sext_arg(uint8_t i, float f) {
  struct s_i8_f s;
  s.i = i;
  s.f = f;
  return s;
}

$ ./riscv32-unknown-elf-gcc -march=rv32imf -mabi=ilp32f foo.c -S -o - -O1
	.file	"foo.c"
	.option nopic
	.text
	.align	2
	.globl	should_zext_arg
	.type	should_zext_arg, @function
should_zext_arg:
	addi	sp,sp,-32
	addi	sp,sp,32
	jr	ra
	.size	should_zext_arg, .-should_zext_arg
	.align	2
	.globl	should_sext_arg
	.type	should_sext_arg, @function
should_sext_arg:
	addi	sp,sp,-32
	addi	sp,sp,32
	jr	ra
	.size	should_sext_arg, .-should_sext_arg
	.ident	"GCC: (GNU) 9.0.0 20180703 (experimental)"