[PATCH v3 2/2] riscv: Optimize bitops with Zbb extension
Charlie Jenkins
charlie at rivosinc.com
Thu Oct 26 23:24:39 PDT 2023
On Tue, Sep 26, 2023 at 05:46:55PM +0800, Xiao Wang 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 NO_ALTERNATIVE for this purpose.
>
> Signed-off-by: Xiao Wang <xiao.w.wang at intel.com>
> ---
> 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..c97e774cb647 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(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;
That's so clean!
> +
> +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;
> +
I generally like to move as much code out of asm as possible. You are
also able to remove one of the branches if you rearrange as follows:
if (!x)
return 0;
asm volatile (
".option push\n"
".option arch,+zbb\n"
CTZW "%0, %1\n"
".option pop\n"
: "=r" (r) : "r" (x) :);
return r + 1;
You could then also extract out the "if (!x)" at the beginning of the
legacy section at put it at the top of the function.
> +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;
> +
Same comment as above but with appropriate changes.
> +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
As a nit, it's nice when large #ifdef blocks have a comment like
/* !defined(CONFIG_RISCV_ISA_ZBB) || defined(NO_ALTERNATIVE) */
Overall great changes, just that small optimization to get rid of the
jump.
- Charlie
> +
> +#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 a1157c2a7170..d68cacd4e3af 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-$(CONFIG_RISCV) += -fpic -DNO_ALTERNATIVE
> cflags-$(CONFIG_LOONGARCH) += -fpie
>
> cflags-$(CONFIG_EFI_PARAMS_FROM_FDT) += -I$(srctree)/scripts/dtc/libfdt
> --
> 2.25.1
>
>
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