Re: [RFC PATCH 06/19] riscv: Adding ISA string parser for environment variables

[David Abdurachmanov <david.abdurachmanov@gmail.com>]

On Tue, Feb 7, 2023 at 1:20 AM Christoph Muellner
<christoph.muellner@vrull.eu> wrote:
>
> From: Christoph Müllner <christoph.muellner@vrull.eu>
>
> RISC-V does not have a reliable mechanism to detect hart features
> like supported ISA extensions or cache block sizes at run-time
> as of now.
>
> Not knowing the hart features limits optimization strategies of glibc
> (e.g. ifunc support requires run-time hard feature knowledge).
>
> To circumvent this limitation this patch introduces a mechanism to
> get the hart features via environment variables:
> * RISCV_RT_MARCH represents a lower-case ISA string (-march string)
>   E.g. RISCV_RT_MARCH=rv64gc_zicboz
> * RISCV_RT_CBOM_BLOCKSIZE represents the cbom instruction block size
>   E.g. RISCV_RT_CBOZ_BLOCKSIZE=64
> * RISCV_RT_CBOZ_BLOCKSIZE represents the cboz instruction block size

Hi,

Non-expert here, but have you considered defining RISCV glibc
tunables? It's designed for this purpose.

See: https://www.gnu.org/software/libc/manual/html_node/Tunables.html
Especially hardware capability tunables:
https://www.gnu.org/software/libc/manual/html_node/Hardware-Capability-Tunables.html

It would be nice to have tunables defined for RISC-V that could stay
with us for a long time.

Cheers,
david

> These environment variables are parsed during startup and the found
> ISA extensions are stored a struct (hart_features) for evaluation
> by dynamic dispatching code.
>
> As the parser code is executed very early, we cannot call functions
> that have direct or indirect (via getenv()) dependencies to strlen()
> and strncmp(), as these functions cannot be called before the ifunc
> support is initialized. Therefore, this patch contains its own helper
> functions for strlen(), strncmp(), and getenv().
>
> Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
> ---
>  sysdeps/unix/sysv/linux/riscv/hart-features.c | 294 ++++++++++++++++++
>  .../unix/sysv/linux/riscv/macro-for-each.h    |  24 ++
>  2 files changed, 318 insertions(+)
>  create mode 100644 sysdeps/unix/sysv/linux/riscv/macro-for-each.h
>
> diff --git a/sysdeps/unix/sysv/linux/riscv/hart-features.c b/sysdeps/unix/sysv/linux/riscv/hart-features.c
> index 41111eff57..6de41a26cc 100644
> --- a/sysdeps/unix/sysv/linux/riscv/hart-features.c
> +++ b/sysdeps/unix/sysv/linux/riscv/hart-features.c
> @@ -17,12 +17,17 @@
>     <https://www.gnu.org/licenses/>.  */
>
>  #include <hart-features.h>
> +#include <macro-for-each.h>
> +#include <string_private.h>
>
>  /* The code in this file is executed very early, so we cannot call
>     indirect functions because ifunc support is not initialized.
>     Therefore this file adds a few simple helper functions to avoid
>     dependencies to functions outside of this file.  */
>
> +#define xstr(s) str(s)
> +#define str(s) #s
> +
>  static inline void
>  inhibit_loop_to_libcall
>  simple_memset (void *s, int c, size_t n)
> @@ -35,9 +40,298 @@ simple_memset (void *s, int c, size_t n)
>      }
>  }
>
> +static inline size_t
> +inhibit_loop_to_libcall
> +simple_strlen (const char *s)
> +{
> +  size_t n = 0;
> +  char c = *s;
> +  while (c != 0)
> +    {
> +      s++;
> +      n++;
> +      c = *s;
> +    }
> +  return n;
> +}
> +
> +static inline int
> +inhibit_loop_to_libcall
> +simple_strncmp (const char *s1, const char *s2, size_t n)
> +{
> +  while (n != 0)
> +    {
> +      if (*s1 == 0 || *s1 != *s2)
> +       return *((const unsigned char *)s1) - *((const unsigned char *)s2);
> +      n--;
> +      s1++;
> +      s2++;
> +    }
> +  return 0;
> +}
> +
> +extern char **__environ;
> +static inline char*
> +simple_getenv (const char *name)
> +{
> +  char **ep;
> +  uint16_t name_start;
> +
> +  if (__environ == NULL || name[0] == 0 || name[1] == 0)
> +    return NULL;
> +
> +  size_t len = simple_strlen (name);
> +#if _STRING_ARCH_unaligned
> +  name_start = *(const uint16_t *) name;
> +#else
> +  name_start = (((const unsigned char *) name)[0]
> +               | (((const unsigned char *) name)[1] << 8));
> +#endif
> +  len -= 2;
> +  name += 2;
> +
> +  for (ep = __environ; *ep != NULL; ++ep)
> +    {
> +#if _STRING_ARCH_unaligned
> +      uint16_t ep_start = *(uint16_t *) *ep;
> +#else
> +      uint16_t ep_start = (((unsigned char *) *ep)[0]
> +                          | (((unsigned char *) *ep)[1] << 8));
> +#endif
> +      if (name_start == ep_start && !simple_strncmp (*ep + 2, name, len)
> +         && (*ep)[len + 2] == '=')
> +       return &(*ep)[len + 3];
> +    }
> +  return NULL;
> +}
> +
> +/* Check if the given number is a power of 2.
> +   Return true if so, or false otherwise.  */
> +static inline int
> +is_power_of_two (unsigned long v)
> +{
> +  return (v & (v - 1)) == 0;
> +}
> +
> +/* Check if the given string str starts with
> +   the prefix pre.  Return true if so, or false
> +   otherwise.  */
> +static inline int
> +starts_with (const char *str, const char *pre)
> +{
> +  return simple_strncmp (pre, str, simple_strlen (pre)) == 0;
> +}
> +
> +/* Lower all characters of a string up to the
> +   first NUL-character in the string.  */
> +static inline void
> +strtolower (char *s)
> +{
> +  char c = *s;
> +  while (c != '\0')
> +    {
> +      if (c >= 'A' && c <= 'Z')
> +       *s = c + 'a' - 'A';
> +      s++;
> +      c = *s;
> +    }
> +}
> +
> +/* Count the number of detected extensions.  */
> +static inline unsigned long
> +count_extensions (struct hart_features *hart_features)
> +{
> +  unsigned long n = 0;
> +#define ISA_EXT(e)                                                     \
> +  if (hart_features->have_##e == 1)                                    \
> +    n++;
> +#define ISA_EXT_GROUP(g, ...)                                          \
> +  if (hart_features->have_##g == 1)                                    \
> +    n++;
> +#include "isa-extensions.def"
> +  return n;
> +}
> +
> +/* Check if the given charater is not '0'-'9'.  */
> +static inline int
> +notanumber (const char c)
> +{
> +  return (c < '0' || c > '9');
> +}
> +
> +/* Parse RISCV_RT_MARCH and store found extensions.  */
> +static inline void
> +parse_rt_march (struct hart_features *hart_features)
> +{
> +  const char* s = simple_getenv ("RISCV_RT_MARCH");
> +  if (s == NULL)
> +    goto end;
> +
> +  hart_features->rt_march = s;
> +
> +  /* "RISC-V ISA strings begin with either RV32I, RV32E, RV64I, or RV128I
> +      indicating the supported address space size in bits for the base
> +      integer ISA."  */
> +  if (starts_with (s, "rv32") && notanumber (*(s+4)))
> +    {
> +      hart_features->xlen = 32;
> +      s += 4;
> +    }
> +  else if (starts_with (s, "rv64") && notanumber (*(s+4)))
> +    {
> +      hart_features->xlen = 64;
> +      s += 4;
> +    }
> +  else if (starts_with (s, "rv128") && notanumber (*(s+5)))
> +    {
> +      hart_features->xlen = 128;
> +      s += 5;
> +    }
> +  else
> +    {
> +      goto fail;
> +    }
> +
> +  /* Parse the extensions.  */
> +  const char *s_old = s;
> +  while (*s != '\0')
> +    {
> +#define ISA_EXT(e)                                                     \
> +      else if (starts_with (s, xstr (e)))                              \
> +       {                                                               \
> +         hart_features->have_##e = 1;                                  \
> +         s += simple_strlen (xstr (e));                                \
> +       }
> +#define ISA_EXT_GROUP(g, ...)                                          \
> +      ISA_EXT (g)
> +      if (0);
> +#include "isa-extensions.def"
> +
> +      /* Consume optional version information.  */
> +      while (*s >= '0' && *s <= '9')
> +       s++;
> +      while (*s == 'p')
> +       s++;
> +      while (*s >= '0' && *s <= '9')
> +       s++;
> +
> +      /* Consume optional '_'.  */
> +      if (*s == '_')
> +       s++;
> +
> +      /* If we got stuck, bail out.  */
> +      if (s == s_old)
> +       goto fail;
> +    }
> +
> +  /* Propagate subsets (until we reach a fixpoint).  */
> +  unsigned long n = count_extensions (hart_features);
> +  while (1)
> +    {
> +      /* Forward-propagation.  E.g.:
> +      if (hart_features->have_g == 1)
> +       {
> +         hart_features->have_i = 1;
> +         ...
> +         hart_features->have_zifencei = 1;
> +       }  */
> +#define ISA_EXT_GROUP_HEAD(y)                                          \
> +      if (hart_features->have_##y)                                     \
> +       {
> +#define ISA_EXT_GROUP_SUBSET(s)                                                \
> +         hart_features->have_##s = 1;
> +#define ISA_EXT_GROUP_TAIL(z)                                          \
> +       }
> +#define ISA_EXT_GROUP(x, ...)                                          \
> +       ISA_EXT_GROUP_HEAD (x)                                          \
> +       FOR_EACH (ISA_EXT_GROUP_SUBSET, __VA_ARGS__)                    \
> +       ISA_EXT_GROUP_TAIL (x)
> +#include "isa-extensions.def"
> +#undef ISA_EXT_GROUP_HEAD
> +#undef ISA_EXT_GROUP_SUBSET
> +#undef ISA_EXT_GROUP_TAIL
> +
> +      /* Backward-propagation.  E.g.:
> +      if (1
> +         && hart_features->have_i == 1
> +         ...
> +         && hart_features->have_zifencei == 1
> +         )
> +       hart_features->have_g = 1;  */
> +#define ISA_EXT_GROUP_HEAD(y)                                          \
> +      if (1
> +#define ISA_EXT_GROUP_SUBSET(s)                                                \
> +         && hart_features->have_##s == 1
> +#define ISA_EXT_GROUP_TAIL(z)                                          \
> +         )                                                             \
> +       hart_features->have_##z = 1;
> +#define ISA_EXT_GROUP(x, ...)                                          \
> +       ISA_EXT_GROUP_HEAD (x)                                          \
> +       FOR_EACH (ISA_EXT_GROUP_SUBSET, __VA_ARGS__)                    \
> +       ISA_EXT_GROUP_TAIL (x)
> +#include "isa-extensions.def"
> +#undef ISA_EXT_GROUP_HEAD
> +#undef ISA_EXT_GROUP_SUBSET
> +#undef ISA_EXT_GROUP_TAIL
> +
> +      unsigned long n2 = count_extensions (hart_features);
> +      /* Stop if fix-point reached.  */
> +      if (n == n2)
> +       break;
> +      n = n2;
> +    }
> +
> +end:
> +  return;
> +
> +fail:
> +  hart_features->rt_march = NULL;
> +}
> +
> +/* Parse RISCV_RT_CBOM_BLOCKSIZE and store value.  */
> +static inline void
> +parse_rt_cbom_blocksize (struct hart_features *hart_features)
> +{
> +  hart_features->rt_cbom_blocksize = NULL;
> +  hart_features->cbom_blocksize = 0;
> +
> +  const char *s = simple_getenv ("RISCV_RT_CBOM_BLOCKSIZE");
> +  if (s == NULL)
> +    return;
> +
> +  uint64_t v = _dl_strtoul (s, NULL);
> +  if (!is_power_of_two (v))
> +    return;
> +
> +  hart_features->rt_cbom_blocksize = s;
> +  hart_features->cbom_blocksize = v;
> +}
> +
> +/* Parse RISCV_RT_CBOZ_BLOCKSIZE and store value.  */
> +static inline void
> +parse_rt_cboz_blocksize (struct hart_features *hart_features)
> +{
> +  hart_features->rt_cboz_blocksize = NULL;
> +  hart_features->cboz_blocksize = 0;
> +
> +  const char *s = simple_getenv ("RISCV_RT_CBOZ_BLOCKSIZE");
> +  if (s == NULL)
> +    return;
> +
> +  uint64_t v = _dl_strtoul (s, NULL);
> +  if (!is_power_of_two (v))
> +    return;
> +
> +  hart_features->rt_cboz_blocksize = s;
> +  hart_features->cboz_blocksize = v;
> +}
> +
>  /* Discover hart features and store them.  */
>  static inline void
>  init_hart_features (struct hart_features *hart_features)
>  {
>    simple_memset (hart_features, 0, sizeof (*hart_features));
> +  parse_rt_march (hart_features);
> +  parse_rt_cbom_blocksize (hart_features);
> +  parse_rt_cboz_blocksize (hart_features);
>  }
> diff --git a/sysdeps/unix/sysv/linux/riscv/macro-for-each.h b/sysdeps/unix/sysv/linux/riscv/macro-for-each.h
> new file mode 100644
> index 0000000000..524bef3c0a
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/riscv/macro-for-each.h
> @@ -0,0 +1,24 @@
> +/* Recursive macros implementation by David Mazières
> +   https://www.scs.stanford.edu/~dm/blog/va-opt.html  */
> +
> +#ifndef _MACRO_FOR_EACH_H
> +#define _MACRO_FOR_EACH_H
> +
> +#define EXPAND1(...) __VA_ARGS__
> +#define EXPAND2(...) EXPAND1 (EXPAND1 (EXPAND1 (EXPAND1 (__VA_ARGS__))))
> +#define EXPAND3(...) EXPAND2 (EXPAND2 (EXPAND2 (EXPAND2 (__VA_ARGS__))))
> +#define EXPAND4(...) EXPAND3 (EXPAND3 (EXPAND3 (EXPAND3 (__VA_ARGS__))))
> +#define EXPAND(...)  EXPAND4 (EXPAND4 (EXPAND4 (EXPAND4 (__VA_ARGS__))))
> +
> +#define FOR_EACH(macro, ...)                                           \
> +  __VA_OPT__ (EXPAND (FOR_EACH_HELPER (macro, __VA_ARGS__)))
> +
> +#define PARENS ()
> +
> +#define FOR_EACH_HELPER(macro, a1, ...)                                        \
> +  macro (a1)                                                           \
> +  __VA_OPT__ (FOR_EACH_AGAIN PARENS (macro, __VA_ARGS__))
> +
> +#define FOR_EACH_AGAIN() FOR_EACH_HELPER
> +
> +#endif /* _MACRO_FOR_EACH_H  */
> --
> 2.39.1
>