[PATCH] include: riscv_asm: Optimize csr_xyz() macros to reduce stack usage

[Anup Patel]

On Tue, Dec 16, 2025 at 10:57 AM Bo Gan <ganboing at gmail.com> wrote:
>
> When using debug builds, aka., DEBUG=1, csr_write_num() function can
> trigger stack overflow. This is caused by the large amount of macro
> expansion of csr_write(...), which, under debug builds, will generate
> massive amount of stack variables (tested with GCC 13.2.0). The issue
> is masked previously as we didn't have too many csr_write()'s before
> commit 55296fd27c0c, but now, it does overflow the default 4KB stack.
>
> The csr_read(relaxed) macros already use the "register" modifier to
> optimize stack usage (perhaps unknowingly?), so this patch just
> follows suit.
>
> Fixes: 55296fd27c0c ("lib: Allow custom CSRs in csr_read_num() and csr_write_num()")
> Signed-off-by: Bo Gan <ganboing at gmail.com>

LGTM.

Reviewed-by: Anup Patel <anup at brainfault.org>

Applied this patch to the riscv/opensbi repo.

Thanks,
Anup

> ---
>  include/sbi/riscv_asm.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/sbi/riscv_asm.h b/include/sbi/riscv_asm.h
> index ef48dc89..0cf3fc37 100644
> --- a/include/sbi/riscv_asm.h
> +++ b/include/sbi/riscv_asm.h
> @@ -83,7 +83,7 @@
>
>  #define csr_swap(csr, val)                                              \
>         ({                                                              \
> -               unsigned long __v = (unsigned long)(val);               \
> +               register unsigned long __v = (unsigned long)(val);      \
>                 __asm__ __volatile__("csrrw %0, " __ASM_STR(csr) ", %1" \
>                                      : "=r"(__v)                        \
>                                      : "rK"(__v)                        \
> @@ -111,7 +111,7 @@
>
>  #define csr_write(csr, val)                                        \
>         ({                                                         \
> -               unsigned long __v = (unsigned long)(val);          \
> +               register unsigned long __v = (unsigned long)(val); \
>                 __asm__ __volatile__("csrw " __ASM_STR(csr) ", %0" \
>                                      :                             \
>                                      : "rK"(__v)                   \
> @@ -120,7 +120,7 @@
>
>  #define csr_read_set(csr, val)                                          \
>         ({                                                              \
> -               unsigned long __v = (unsigned long)(val);               \
> +               register unsigned long __v = (unsigned long)(val);      \
>                 __asm__ __volatile__("csrrs %0, " __ASM_STR(csr) ", %1" \
>                                      : "=r"(__v)                        \
>                                      : "rK"(__v)                        \
> @@ -130,7 +130,7 @@
>
>  #define csr_set(csr, val)                                          \
>         ({                                                         \
> -               unsigned long __v = (unsigned long)(val);          \
> +               register unsigned long __v = (unsigned long)(val); \
>                 __asm__ __volatile__("csrs " __ASM_STR(csr) ", %0" \
>                                      :                             \
>                                      : "rK"(__v)                   \
> @@ -139,7 +139,7 @@
>
>  #define csr_read_clear(csr, val)                                        \
>         ({                                                              \
> -               unsigned long __v = (unsigned long)(val);               \
> +               register unsigned long __v = (unsigned long)(val);      \
>                 __asm__ __volatile__("csrrc %0, " __ASM_STR(csr) ", %1" \
>                                      : "=r"(__v)                        \
>                                      : "rK"(__v)                        \
> @@ -149,7 +149,7 @@
>
>  #define csr_clear(csr, val)                                        \
>         ({                                                         \
> -               unsigned long __v = (unsigned long)(val);          \
> +               register unsigned long __v = (unsigned long)(val); \
>                 __asm__ __volatile__("csrc " __ASM_STR(csr) ", %0" \
>                                      :                             \
>                                      : "rK"(__v)                   \
> --
> 2.34.1
>
>
> --
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