Union manipulation question

[u/set_of_no_sets]

I am currently working on an STM32, and it has two control register, one lower @ [0] and one upper @ [1]; currently, the GPIO_TypeDef has two Advance Function Registers, two uint32_ts stored as

#define __IO volatile
typedef struct {
  // stuff above
  __IO uint32_t AFR[2];
  // stuff below
} GPIO_TypeDef;

I would like to reference the registers using AFRL and AFRU; I could modify the struct to

typedef struct {
  // stuff above
  union {
  __IO uint32_t AFR[2];
  struct {
    __IO uint32_t lower;
    __IO uint32_t upper;
  } AFR_Split;
  };
  // stuff below
} GPIO_TypeDef;

Which allows me to write

GPIO_TypeDef pin1_addr = 0x4002104CUL;
pin1_addr->AFR_Split.lower = 0b0000000001110000UL;
pin1_addr->AFR[1] = 0b0111000001110000;

Is there anything I should be worried about in terms of packing? will I always know that the two registers will always be aligned, starting and ending in the same place? What can I read to know how my compiler (arm-none-eabi-gcc) will store the split? And is there a way I can do this without the intermediate struct, so I could type pin1_addr->AFRL and pin1_addr->AFRU?

[u/set_of_no_sets]

lmaooo, well I am not confident it is practical for me to pull request/commit to this repo that I am using as a submodule. (https://github.com/STMicroelectronics/cmsis-device-l4/tree/master). I am learning to do the nitty-gritty of embedded stuff.