[PATCH 2/4] RISC-V: reduce mcount save space on RV32

[Andrew Jones]

On Thu, Oct 27, 2022 at 06:24:33PM +0100, Jamie Iles wrote:
> For RV32 we can reduce the size of the ABI save+restore state by using
> SZREG so that register stores are packed rather than on an 8 byte
> boundary.
> 
> Signed-off-by: Jamie Iles <jamie at jamieiles.com>
> ---
>  arch/riscv/kernel/mcount.S | 32 ++++++++++++++++----------------
>  1 file changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/riscv/kernel/mcount.S b/arch/riscv/kernel/mcount.S
> index 9cf0904afd6d..4166909ce7b3 100644
> --- a/arch/riscv/kernel/mcount.S
> +++ b/arch/riscv/kernel/mcount.S
> @@ -15,8 +15,8 @@
>  
>  	.macro SAVE_ABI_STATE
>  	addi	sp, sp, -16
> -	REG_S	s0, 0(sp)
> -	REG_S	ra, 8(sp)
> +	REG_S	s0, 0*SZREG(sp)
> +	REG_S	ra, 1*SZREG(sp)
>  	addi	s0, sp, 16
>  	.endm

Shouldn't we only be adjusting sp by 2*SZREG?. Indeed RESTORE_ABI_STATE
below is getting modified that way.

>  
> @@ -25,24 +25,24 @@
>  	 * register if a0 was not saved.
>  	 */
>  	.macro SAVE_RET_ABI_STATE
> -	addi	sp, sp, -32
> -	REG_S	s0, 16(sp)
> -	REG_S	ra, 24(sp)
> -	REG_S	a0, 8(sp)
> +	addi	sp, sp, -4*SZREG
> +	REG_S	s0, 2*SZREG(sp)
> +	REG_S	ra, 3*SZREG(sp)
> +	REG_S	a0, 1*SZREG(sp)
>  	addi	s0, sp, 32

This should be 'addi s0, sp, -4*SZREG'

>  	.endm
>  
>  	.macro RESTORE_ABI_STATE
> -	REG_L	ra, 8(sp)
> -	REG_L	s0, 0(sp)
> -	addi	sp, sp, 16
> +	REG_L	ra, 1*SZREG(sp)
> +	REG_L	s0, 0*SZREG(sp)
> +	addi	sp, sp, 2*SZREG
>  	.endm
>  
>  	.macro RESTORE_RET_ABI_STATE
> -	REG_L	ra, 24(sp)
> -	REG_L	s0, 16(sp)
> -	REG_L	a0, 8(sp)
> -	addi	sp, sp, 32
> +	REG_L	ra, 3*SZREG(sp)
> +	REG_L	s0, 2*SZREG(sp)
> +	REG_L	a0, 1*SZREG(sp)
> +	addi	sp, sp, 4*SZREG
>  	.endm
>  
>  ENTRY(ftrace_stub)
> @@ -101,10 +101,10 @@ ENTRY(MCOUNT_NAME)
>   * prepare_to_return(&ra_to_caller_of_caller, ra_to_caller)
>   */
>  do_ftrace_graph_caller:
> -	addi	a0, s0, -8
> +	addi	a0, s0, -SZREG
>  	mv	a1, ra
>  #ifdef HAVE_FUNCTION_GRAPH_FP_TEST
> -	REG_L	a2, -16(s0)
> +	REG_L	a2, -2*SZREG(s0)
>  #endif
>  	SAVE_ABI_STATE
>  	call	prepare_ftrace_return
> @@ -117,7 +117,7 @@ do_ftrace_graph_caller:
>   * (*ftrace_trace_function)(ra_to_caller, ra_to_caller_of_caller)
>   */
>  do_trace:
> -	REG_L	a1, -8(s0)
> +	REG_L	a1, -SZREG(s0)
>  	mv	a0, ra
>  
>  	SAVE_ABI_STATE
> -- 
> 2.34.1
>

Thanks,
drew