Howto reserve global register variable gcc?

[MM..1]

I try optimize to maximum MCU offload this operation:

I2S DMA stores sample LRLRLRLR... circular

i need reorder it to LLLLLLRRRRR in other memory place.

I mean DMA or other hw methods cant copy mem to mem with skip defined, mean every other.

Then i try write ISR and asm result is

08000c00 <SPI2_IRQHandler>:
  * @brief This function handles SPI2 global interrupt.
  */
void SPI2_IRQHandler(void)
{
  /* USER CODE BEGIN SPI2_IRQn 0 */
	*(int *)a = *(int *)b++;
 8000c00:	f859 3b01 	ldr.w	r3, [r9], #1
 8000c04:	f8c8 3000 	str.w	r3, [r8]
	*(int *)(a+256) = *(int *)b++;
 8000c08:	f859 3b01 	ldr.w	r3, [r9], #1
 8000c0c:	f8c8 3100 	str.w	r3, [r8, #256]	; 0x100
	a++;
 8000c10:	f108 0801 	add.w	r8, r8, #1
  /* USER CODE END SPI2_IRQn 0 */
//  HAL_I2S_IRQHandler(&hi2s2);
  /* USER CODE BEGIN SPI2_IRQn 1 */
 
  /* USER CODE END SPI2_IRQn 1 */
}
 8000c14:	4770      	bx	lr
 8000c16:	bf00      	nop

Registers r8 and r9 i declare in main.h as

register unsigned int a asm("r8");
register unsigned int b asm("r9");

Seems ISR is compiled good, but in all asm i found, that r8 and r9 is used on other places in many functions. Howto prevent this ???

[MM..1]

08000c00 <SPI2_IRQHandler>:
  * @brief This function handles SPI2 global interrupt.
  */
void SPI2_IRQHandler(void)
{
  /* USER CODE BEGIN SPI2_IRQn 0 */
	*(int *)a = *(int *)b++;
 8000c00:	4b08      	ldr	r3, [pc, #32]	; (8000c24 <SPI2_IRQHandler+0x24>)
 8000c02:	4a09      	ldr	r2, [pc, #36]	; (8000c28 <SPI2_IRQHandler+0x28>)
 8000c04:	6819      	ldr	r1, [r3, #0]
 8000c06:	1c48      	adds	r0, r1, #1
 8000c08:	6018      	str	r0, [r3, #0]
 8000c0a:	6810      	ldr	r0, [r2, #0]
 8000c0c:	6809      	ldr	r1, [r1, #0]
 8000c0e:	6001      	str	r1, [r0, #0]
	*(int *)(a++ +256) = *(int *)b++;
 8000c10:	6819      	ldr	r1, [r3, #0]
 8000c12:	1c48      	adds	r0, r1, #1
 8000c14:	6018      	str	r0, [r3, #0]
 8000c16:	6813      	ldr	r3, [r2, #0]
 8000c18:	1c58      	adds	r0, r3, #1
 8000c1a:	6010      	str	r0, [r2, #0]
 8000c1c:	680a      	ldr	r2, [r1, #0]
 8000c1e:	f8c3 2100 	str.w	r2, [r3, #256]	; 0x100
  /* USER CODE END SPI2_IRQn 0 */
//  HAL_I2S_IRQHandler(&hi2s2);
  /* USER CODE BEGIN SPI2_IRQn 1 */
 
  /* USER CODE END SPI2_IRQn 1 */
}
 8000c22:	4770      	bx	lr
 8000c24:	20000288 	.word	0x20000288
 8000c28:	20000284 	.word	0x20000284

Nobody helps ? This is for compare normal variable build. As you can see register code use 5 instruction and normal code 16 when variables is __IO.

Without __IO build add 2 next instruction complete 18.

[Tesla DeLorean]

You know some of us have other jobs, and can't be chasing bizarre corner cases people paint themselves into. You might need to dig into the specifics of GNU GCC/GAS as it relates to the features you're trying to use, and the system level expectations they are based on.

R9 is often used in relocatable or address-independent code, when the LOADER is responsible for setting R9 to the beginning of the DATA section for a specific instance of the code, think DLLs, think thread-safe. It is going to be less prevalent on ROM based code, and will be based on settings you've used related to the compiler/linker.

Why have these variables as IO/volatile ? If they only change in this interrupt, presumably that's not going to enter multiple times.

[MM..1]

Bizzarre maybe ... Speed difference x3 is realy bizzare. I read all i found about register variables, but STCubeIDE gcc result with code , that use register too on other places for example

0800026c <__udivmoddi4>:
 800026c:	e92d 47f0 	stmdb	sp!, {r4, r5, r6, r7, r8, r9, sl, lr}
 8000270:	9d08      	ldr	r5, [sp, #32]
 8000272:	4604      	mov	r4, r0
 ...

Next bizzare , that i dont understand for example register sl here is r10 in arm debuger.

Variable is changed on other place , because need reset , and check it on every step isnt effective.