Re: [PATCH] riscv: Fix build with CONFIG_CC_OPTIMIZE_FOR_SIZE=y

Jisheng Zhang
2022-10-06T06:58:21Z

On Thu, Sep 22, 2022 at 01:09:58AM -0500, Samuel Holland wrote:
> commit 8eb060e10185 ("arch/riscv: add Zihintpause support") broke
> building with CONFIG_CC_OPTIMIZE_FOR_SIZE enabled (gcc 11.1.0):
>
> CC arch/riscv/kernel/vdso/vgettimeofday.o
> In file included from <command-line>:
> ./arch/riscv/include/asm/jump_label.h: In function 'cpu_relax':
> ././include/linux/compiler_types.h:285:33: warning: 'asm' operand 0 probably does not match constraints
> 285 | #define asm_volatile_goto(x...) asm goto(x)
> | ^~~
> ./arch/riscv/include/asm/jump_label.h:41:9: note: in expansion of macro 'asm_volatile_goto'
> 41 | asm_volatile_goto(
> | ^~~~~~~~~~~~~~~~~
> ././include/linux/compiler_types.h:285:33: error: impossible constraint in 'asm'
> 285 | #define asm_volatile_goto(x...) asm goto(x)
> | ^~~
> ./arch/riscv/include/asm/jump_label.h:41:9: note: in expansion of macro 'asm_volatile_goto'
> 41 | asm_volatile_goto(
> | ^~~~~~~~~~~~~~~~~
> make[1]: *** [scripts/Makefile.build:249: arch/riscv/kernel/vdso/vgettimeofday.o] Error 1
> make: *** [arch/riscv/Makefile:128: vdso_prepare] Error 2
>
> Having a static branch in cpu_relax() is problematic because that
> function is widely inlined, including in some quite complex functions
> like in the VDSO. A quick measurement shows this static branch is
> responsible by itself for around 40% of the jump table.
>
> Drop the static branch, which ends up being the same number of
> instructions anyway. If Zihintpause is supported, we trade the nop from
> the static branch for a div. If Zihintpause is unsupported, we trade the
> jump from the static branch for (what gets interpreted as) a nop.

Hi Samuel,

I'm not sure whether it's correct to remove static branch usage from
cpu_relax, but your report inspired my patch of constifying arguments
of arch_static_branch() and arch_static_branch_jump() [1]. Could you
please also test it?

Thanks very much

[1]https://lore.kernel.org/linux-riscv/20221006064028.548-1-jszhang@kernel.org/T/#u

>
> Fixes: 8eb060e10185 ("arch/riscv: add Zihintpause support")
> Signed-off-by: Samuel Holland <samuel@sholland.org>
> ---
>
> arch/riscv/include/asm/hwcap.h | 3 ---
> arch/riscv/include/asm/vdso/processor.h | 25 ++++++++++---------------
> 2 files changed, 10 insertions(+), 18 deletions(-)
>
> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> index 6f59ec64175e..b21d46e68386 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -68,7 +68,6 @@ enum riscv_isa_ext_id {
> */
> enum riscv_isa_ext_key {
> RISCV_ISA_EXT_KEY_FPU, /* For 'F' and 'D' */
> - RISCV_ISA_EXT_KEY_ZIHINTPAUSE,
> RISCV_ISA_EXT_KEY_MAX,
> };
>
> @@ -88,8 +87,6 @@ static __always_inline int riscv_isa_ext2key(int num)
> return RISCV_ISA_EXT_KEY_FPU;
> case RISCV_ISA_EXT_d:
> return RISCV_ISA_EXT_KEY_FPU;
> - case RISCV_ISA_EXT_ZIHINTPAUSE:
> - return RISCV_ISA_EXT_KEY_ZIHINTPAUSE;
> default:
> return -EINVAL;
> }
> diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
> index 1e4f8b4aef79..789bdb8211a2 100644
> --- a/arch/riscv/include/asm/vdso/processor.h
> +++ b/arch/riscv/include/asm/vdso/processor.h
> @@ -4,30 +4,25 @@
>
> #ifndef __ASSEMBLY__
>
> -#include <linux/jump_label.h>
> #include <asm/barrier.h>
> -#include <asm/hwcap.h>
>
> static inline void cpu_relax(void)
> {
> - if (!static_branch_likely(&riscv_isa_ext_keys[RISCV_ISA_EXT_KEY_ZIHINTPAUSE])) {
> #ifdef __riscv_muldiv
> - int dummy;
> - /* In lieu of a halt instruction, induce a long-latency stall. */
> - __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> + int dummy;
> + /* In lieu of a halt instruction, induce a long-latency stall. */
> + __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> #endif
> - } else {
> - /*
> - * Reduce instruction retirement.
> - * This assumes the PC changes.
> - */
> + /*
> + * Reduce instruction retirement.
> + * This assumes the PC changes.
> + */
> #ifdef __riscv_zihintpause
> - __asm__ __volatile__ ("pause");
> + __asm__ __volatile__ ("pause");
> #else
> - /* Encoding of the pause instruction */
> - __asm__ __volatile__ (".4byte 0x100000F");
> + /* Encoding of the pause instruction */
> + __asm__ __volatile__ (".4byte 0x100000F");
> #endif
> - }
> barrier();
> }
>
> --
> 2.35.1