Re: [PATCH] RISC-V: setjmp: reduce code size for register load/store with Zilsd

Hi puranikvinit:

Thanks for your patch! it's generally LGTM, but I am concerned about
the alignment issue, zilsd spec only guarantees that 16-byte alignment
won't cause a misaligned trap, so it may be guarded with
__riscv_misaligned_fast?

[1] https://github.com/riscv/riscv-zilsd/blob/main/zilsd.adoc#loadstore-pair-instructions-zilsd

On Wed, May 28, 2025 at 1:45 PM puranikvinit <[email protected]> wrote:
>
> This patch optimizes the RISC-V setjmp implementation in
> newlib/libc/machine/riscv/setjmp.S for 32-bit targets. It reduces code
> size by using doubleword store/load instructions (sd/ld) when the Zilsd
> or Zclsd extensions are available for saving and
> restoring s0-s11 registers, while preserving the original
> single-word instructions (REG_S/REG_L) for compatibility with other
> configurations.
>
> Signed-off-by: puranikvinit <[email protected]>
> Reviewed-by: Christian Herber <[email protected]>
>
> RISC-V: setjmp: remove redundant checks for Zclsd
> ---
>  newlib/libc/machine/riscv/setjmp.S | 74 ++++++++++++++++++++----------
>  1 file changed, 49 insertions(+), 25 deletions(-)
>
> diff --git a/newlib/libc/machine/riscv/setjmp.S b/newlib/libc/machine/riscv/setjmp.S
> index 2d4ab6cfc..d5b132466 100644
> --- a/newlib/libc/machine/riscv/setjmp.S
> +++ b/newlib/libc/machine/riscv/setjmp.S
> @@ -16,21 +16,33 @@
>    .type   setjmp, @function
>  setjmp:
>         REG_S ra,  0*SZREG(a0)
> -       REG_S s0,  1*SZREG(a0)
> -       REG_S s1,  2*SZREG(a0)
> +  #if __riscv_xlen == 32 && (__riscv_zilsd)
> +         sd    s0,  1*SZREG(a0)
> +  #else
> +         REG_S s0,  1*SZREG(a0)
> +         REG_S s1,  2*SZREG(a0)
> +  #endif
>
>  #ifndef __riscv_abi_rve
> -       REG_S s2,  3*SZREG(a0)
> -       REG_S s3,  4*SZREG(a0)
> -       REG_S s4,  5*SZREG(a0)
> -       REG_S s5,  6*SZREG(a0)
> -       REG_S s6,  7*SZREG(a0)
> -       REG_S s7,  8*SZREG(a0)
> -       REG_S s8,  9*SZREG(a0)
> -       REG_S s9, 10*SZREG(a0)
> -       REG_S s10,11*SZREG(a0)
> -       REG_S s11,12*SZREG(a0)
> -       REG_S sp, 13*SZREG(a0)
> +  #if __riscv_xlen == 32 && (__riscv_zilsd)
> +         sd    s2,  3*SZREG(a0)
> +         sd    s4,  5*SZREG(a0)
> +         sd    s6,  7*SZREG(a0)
> +         sd    s8,  9*SZREG(a0)
> +         sd    s10,11*SZREG(a0)
> +  #else
> +         REG_S s2,  3*SZREG(a0)
> +         REG_S s3,  4*SZREG(a0)
> +         REG_S s4,  5*SZREG(a0)
> +         REG_S s5,  6*SZREG(a0)
> +         REG_S s6,  7*SZREG(a0)
> +         REG_S s7,  8*SZREG(a0)
> +         REG_S s8,  9*SZREG(a0)
> +         REG_S s9, 10*SZREG(a0)
> +         REG_S s10,11*SZREG(a0)
> +         REG_S s11,12*SZREG(a0)
> +  #endif
> +  REG_S sp, 13*SZREG(a0)
>  #else
>         REG_S sp, 3*SZREG(a0)
>  #endif
> @@ -59,19 +71,31 @@ setjmp:
>    .type   longjmp, @function
>  longjmp:
>         REG_L ra,  0*SZREG(a0)
> -       REG_L s0,  1*SZREG(a0)
> -       REG_L s1,  2*SZREG(a0)
> +  #if __riscv_xlen == 32 && (__riscv_zilsd)
> +    ld s0, 1*SZREG(a0)
> +  #else
> +    REG_L s0,  1*SZREG(a0)
> +    REG_L s1,  2*SZREG(a0)
> +  #endif
>  #ifndef __riscv_abi_rve
> -       REG_L s2,  3*SZREG(a0)
> -       REG_L s3,  4*SZREG(a0)
> -       REG_L s4,  5*SZREG(a0)
> -       REG_L s5,  6*SZREG(a0)
> -       REG_L s6,  7*SZREG(a0)
> -       REG_L s7,  8*SZREG(a0)
> -       REG_L s8,  9*SZREG(a0)
> -       REG_L s9, 10*SZREG(a0)
> -       REG_L s10,11*SZREG(a0)
> -       REG_L s11,12*SZREG(a0)
> +  #if __riscv_xlen == 32 && (__riscv_zilsd)
> +         ld    s2,  3*SZREG(a0)
> +         ld    s4,  5*SZREG(a0)
> +         ld    s6,  7*SZREG(a0)
> +         ld    s8,  9*SZREG(a0)
> +         ld    s10,11*SZREG(a0)
> +  #else
> +         REG_L s2,  3*SZREG(a0)
> +         REG_L s3,  4*SZREG(a0)
> +         REG_L s4,  5*SZREG(a0)
> +         REG_L s5,  6*SZREG(a0)
> +         REG_L s6,  7*SZREG(a0)
> +         REG_L s7,  8*SZREG(a0)
> +         REG_L s8,  9*SZREG(a0)
> +         REG_L s9, 10*SZREG(a0)
> +         REG_L s10,11*SZREG(a0)
> +         REG_L s11,12*SZREG(a0)
> +  #endif
>         REG_L sp, 13*SZREG(a0)
>  #else
>         REG_L sp, 3*SZREG(a0)
> --
> 2.49.0
>