Re: [PATCH] RISC-V: Optimize bitops with Zbb extension

[Ard Biesheuvel]

On Sun, 6 Aug 2023 at 04:39, Xiao Wang <xiao.w.wang@xxxxxxxxx> wrote:
>
> This patch leverages the alternative mechanism to dynamically optimize
> bitops (including __ffs, __fls, ffs, fls) with Zbb instructions. When
> Zbb ext is not supported by the runtime CPU, legacy implementation is
> used. If Zbb is supported, then the optimized variants will be selected
> via alternative patching.
>
> The legacy bitops support is taken from the generic C implementation as
> fallback.
>
> If the parameter is a build-time constant, we leverage compiler builtin to
> calculate the result directly, this approach is inspired by x86 bitops
> implementation.
>
> EFI stub runs before the kernel, so alternative mechanism should not be
> used there, this patch introduces a macro EFI_NO_ALTERNATIVE for this
> purpose.
>

Why? The unpatched sequences work fine, no?


> Signed-off-by: Xiao Wang <xiao.w.wang@xxxxxxxxx>
> ---
>  arch/riscv/include/asm/bitops.h       | 266 +++++++++++++++++++++++++-
>  drivers/firmware/efi/libstub/Makefile |   2 +-
>  2 files changed, 264 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/include/asm/bitops.h b/arch/riscv/include/asm/bitops.h
> index 3540b690944b..f727f6489cd5 100644
> --- a/arch/riscv/include/asm/bitops.h
> +++ b/arch/riscv/include/asm/bitops.h
> @@ -15,13 +15,273 @@
>  #include <asm/barrier.h>
>  #include <asm/bitsperlong.h>
>
> +#if !defined(CONFIG_RISCV_ISA_ZBB) || defined(EFI_NO_ALTERNATIVE)
>  #include <asm-generic/bitops/__ffs.h>
> -#include <asm-generic/bitops/ffz.h>
> -#include <asm-generic/bitops/fls.h>
>  #include <asm-generic/bitops/__fls.h>
> +#include <asm-generic/bitops/ffs.h>
> +#include <asm-generic/bitops/fls.h>
> +
> +#else
> +#include <asm/alternative-macros.h>
> +#include <asm/hwcap.h>
> +
> +#if (BITS_PER_LONG == 64)
> +#define CTZW   "ctzw "
> +#define CLZW   "clzw "
> +#elif (BITS_PER_LONG == 32)
> +#define CTZW   "ctz "
> +#define CLZW   "clz "
> +#else
> +#error "Unexpected BITS_PER_LONG"
> +#endif
> +
> +static __always_inline unsigned long variable__ffs(unsigned long word)
> +{
> +       int num;
> +
> +       asm_volatile_goto(
> +               ALTERNATIVE("j %l[legacy]", "nop", 0, RISCV_ISA_EXT_ZBB, 1)
> +               : : : : legacy);
> +
> +       asm volatile (
> +               ".option push\n"
> +               ".option arch,+zbb\n"
> +               "ctz %0, %1\n"
> +               ".option pop\n"
> +               : "=r" (word) : "r" (word) :);
> +
> +       return word;
> +
> +legacy:
> +       num = 0;
> +#if BITS_PER_LONG == 64
> +       if ((word & 0xffffffff) == 0) {
> +               num += 32;
> +               word >>= 32;
> +       }
> +#endif
> +       if ((word & 0xffff) == 0) {
> +               num += 16;
> +               word >>= 16;
> +       }
> +       if ((word & 0xff) == 0) {
> +               num += 8;
> +               word >>= 8;
> +       }
> +       if ((word & 0xf) == 0) {
> +               num += 4;
> +               word >>= 4;
> +       }
> +       if ((word & 0x3) == 0) {
> +               num += 2;
> +               word >>= 2;
> +       }
> +       if ((word & 0x1) == 0)
> +               num += 1;
> +       return num;
> +}
> +
> +/**
> + * __ffs - find first set bit in a long word
> + * @word: The word to search
> + *
> + * Undefined if no set bit exists, so code should check against 0 first.
> + */
> +#define __ffs(word)                            \
> +       (__builtin_constant_p(word) ?           \
> +        (unsigned long)__builtin_ctzl(word) :  \
> +        variable__ffs(word))
> +
> +static __always_inline unsigned long variable__fls(unsigned long word)
> +{
> +       int num;
> +
> +       asm_volatile_goto(
> +               ALTERNATIVE("j %l[legacy]", "nop", 0, RISCV_ISA_EXT_ZBB, 1)
> +               : : : : legacy);
> +
> +       asm volatile (
> +               ".option push\n"
> +               ".option arch,+zbb\n"
> +               "clz %0, %1\n"
> +               ".option pop\n"
> +               : "=r" (word) : "r" (word) :);
> +
> +       return BITS_PER_LONG - 1 - word;
> +
> +legacy:
> +       num = BITS_PER_LONG - 1;
> +#if BITS_PER_LONG == 64
> +       if (!(word & (~0ul << 32))) {
> +               num -= 32;
> +               word <<= 32;
> +       }
> +#endif
> +       if (!(word & (~0ul << (BITS_PER_LONG-16)))) {
> +               num -= 16;
> +               word <<= 16;
> +       }
> +       if (!(word & (~0ul << (BITS_PER_LONG-8)))) {
> +               num -= 8;
> +               word <<= 8;
> +       }
> +       if (!(word & (~0ul << (BITS_PER_LONG-4)))) {
> +               num -= 4;
> +               word <<= 4;
> +       }
> +       if (!(word & (~0ul << (BITS_PER_LONG-2)))) {
> +               num -= 2;
> +               word <<= 2;
> +       }
> +       if (!(word & (~0ul << (BITS_PER_LONG-1))))
> +               num -= 1;
> +       return num;
> +}
> +
> +/**
> + * __fls - find last set bit in a long word
> + * @word: the word to search
> + *
> + * Undefined if no set bit exists, so code should check against 0 first.
> + */
> +#define __fls(word)                                                    \
> +       (__builtin_constant_p(word) ?                                   \
> +        (unsigned long)(BITS_PER_LONG - 1 - __builtin_clzl(word)) :    \
> +        variable__fls(word))
> +
> +static __always_inline int variable_ffs(int x)
> +{
> +       int r;
> +
> +       asm_volatile_goto(
> +               ALTERNATIVE("j %l[legacy]", "nop", 0, RISCV_ISA_EXT_ZBB, 1)
> +               : : : : legacy);
> +
> +       asm volatile (
> +               ".option push\n"
> +               ".option arch,+zbb\n"
> +               "bnez %1, 1f\n"
> +               "li %0, 0\n"
> +               "j 2f\n"
> +               "1:\n"
> +               CTZW "%0, %1\n"
> +               "addi %0, %0, 1\n"
> +               "2:\n"
> +               ".option pop\n"
> +               : "=r" (r) : "r" (x) :);
> +
> +       return r;
> +
> +legacy:
> +       r = 1;
> +       if (!x)
> +               return 0;
> +       if (!(x & 0xffff)) {
> +               x >>= 16;
> +               r += 16;
> +       }
> +       if (!(x & 0xff)) {
> +               x >>= 8;
> +               r += 8;
> +       }
> +       if (!(x & 0xf)) {
> +               x >>= 4;
> +               r += 4;
> +       }
> +       if (!(x & 3)) {
> +               x >>= 2;
> +               r += 2;
> +       }
> +       if (!(x & 1)) {
> +               x >>= 1;
> +               r += 1;
> +       }
> +       return r;
> +}
> +
> +/**
> + * ffs - find first set bit in a word
> + * @x: the word to search
> + *
> + * This is defined the same way as the libc and compiler builtin ffs routines.
> + *
> + * ffs(value) returns 0 if value is 0 or the position of the first set bit if
> + * value is nonzero. The first (least significant) bit is at position 1.
> + */
> +#define ffs(x) (__builtin_constant_p(x) ? __builtin_ffs(x) : variable_ffs(x))
> +
> +static __always_inline int variable_fls(unsigned int x)
> +{
> +       int r;
> +
> +       asm_volatile_goto(
> +               ALTERNATIVE("j %l[legacy]", "nop", 0, RISCV_ISA_EXT_ZBB, 1)
> +               : : : : legacy);
> +
> +       asm volatile (
> +               ".option push\n"
> +               ".option arch,+zbb\n"
> +               "bnez %1, 1f\n"
> +               "li %0, 0\n"
> +               "j 2f\n"
> +               "1:\n"
> +               CLZW "%0, %1\n"
> +               "neg %0, %0\n"
> +               "addi %0, %0, 32\n"
> +               "2:\n"
> +               ".option pop\n"
> +               : "=r" (r) : "r" (x) :);
> +
> +       return r;
> +
> +legacy:
> +       r = 32;
> +       if (!x)
> +               return 0;
> +       if (!(x & 0xffff0000u)) {
> +               x <<= 16;
> +               r -= 16;
> +       }
> +       if (!(x & 0xff000000u)) {
> +               x <<= 8;
> +               r -= 8;
> +       }
> +       if (!(x & 0xf0000000u)) {
> +               x <<= 4;
> +               r -= 4;
> +       }
> +       if (!(x & 0xc0000000u)) {
> +               x <<= 2;
> +               r -= 2;
> +       }
> +       if (!(x & 0x80000000u)) {
> +               x <<= 1;
> +               r -= 1;
> +       }
> +       return r;
> +}
> +
> +/**
> + * fls - find last set bit in a word
> + * @x: the word to search
> + *
> + * This is defined in a similar way as ffs, but returns the position of the most
> + * significant set bit.
> + *
> + * fls(value) returns 0 if value is 0 or the position of the last set bit if
> + * value is nonzero. The last (most significant) bit is at position 32.
> + */
> +#define fls(x)                                                         \
> +       (__builtin_constant_p(x) ?                                      \
> +        (int)(((x) != 0) ?                                             \
> +         (sizeof(unsigned int) * 8 - __builtin_clz(x)) : 0) :          \
> +        variable_fls(x))
> +
> +#endif
> +
> +#include <asm-generic/bitops/ffz.h>
>  #include <asm-generic/bitops/fls64.h>
>  #include <asm-generic/bitops/sched.h>
> -#include <asm-generic/bitops/ffs.h>
>
>  #include <asm-generic/bitops/hweight.h>
>
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index 16d64a34d1e1..b0f8c495c10f 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -28,7 +28,7 @@ cflags-$(CONFIG_ARM)          += -DEFI_HAVE_STRLEN -DEFI_HAVE_STRNLEN \
>                                    -DEFI_HAVE_MEMCHR -DEFI_HAVE_STRRCHR \
>                                    -DEFI_HAVE_STRCMP -fno-builtin -fpic \
>                                    $(call cc-option,-mno-single-pic-base)
> -cflags-$(CONFIG_RISCV)         += -fpic
> +cflags-$(CONFIG_RISCV)         += -fpic -DEFI_NO_ALTERNATIVE
>  cflags-$(CONFIG_LOONGARCH)     += -fpie
>
>  cflags-$(CONFIG_EFI_PARAMS_FROM_FDT)   += -I$(srctree)/scripts/dtc/libfdt
> --
> 2.25.1
>