Re: [PATCH 1/1] Only pass the minimum number of syscall arguments

Hi Georg:

LGTM, great job, it reduce the code size for libgloss.

Tested rv32gc/rv64gc on qemu.

Thanks :)

On Tue, Feb 11, 2020 at 4:20 AM Georg Sauthoff <[email protected]> wrote:
>
> Previously, __internal_syscall() compiled into asm-code that unconditionally
> sets the syscall argument registers a0 to a5.
>
> For example, the instruction sequence for a exit syscall looked like
> this:
>
>     li    a0, 1   # in ther caller of exit()
>     # ...         # in newlib:
>     li    a1, 0   # unused arguments
>     li    a2, 0
>     li    a3, 0
>     li    a4, 0
>     li    a5, 0
>     li    a7, 93  # exit syscall number
>
> (i.e. the binary contains then 5 superfluous instructions for this
> one argument syscall)
>
> This commit changes the RISC-V syscall code such that only the required
> syscall argument registers are set.
>
> GCC detects that argc is known at compile time and thus evaluates all the
> if-statements where argc is used at compile time (tested with -O2 and -Os).
> ---
>  libgloss/riscv/internal_syscall.h | 41 ++++++++++++++++++++++---------
>  libgloss/riscv/sys_access.c       |  2 +-
>  libgloss/riscv/sys_close.c        |  2 +-
>  libgloss/riscv/sys_exit.c         |  2 +-
>  libgloss/riscv/sys_faccessat.c    |  2 +-
>  libgloss/riscv/sys_fstat.c        |  2 +-
>  libgloss/riscv/sys_fstatat.c      |  2 +-
>  libgloss/riscv/sys_gettimeofday.c |  2 +-
>  libgloss/riscv/sys_link.c         |  2 +-
>  libgloss/riscv/sys_lseek.c        |  2 +-
>  libgloss/riscv/sys_lstat.c        |  2 +-
>  libgloss/riscv/sys_open.c         |  2 +-
>  libgloss/riscv/sys_openat.c       |  2 +-
>  libgloss/riscv/sys_read.c         |  2 +-
>  libgloss/riscv/sys_sbrk.c         |  4 +--
>  libgloss/riscv/sys_stat.c         |  2 +-
>  libgloss/riscv/sys_unlink.c       |  2 +-
>  libgloss/riscv/sys_write.c        |  2 +-
>  18 files changed, 48 insertions(+), 29 deletions(-)
>
> diff --git a/libgloss/riscv/internal_syscall.h b/libgloss/riscv/internal_syscall.h
> index e5d5594..1f1f42f 100644
> --- a/libgloss/riscv/internal_syscall.h
> +++ b/libgloss/riscv/internal_syscall.h
> @@ -22,31 +22,50 @@ __syscall_error(long a0)
>  }
>
>  static inline long
> -__internal_syscall(long n, long _a0, long _a1, long _a2, long _a3, long _a4, long _a5)
> +__internal_syscall(long n, int argc, long _a0, long _a1, long _a2, long _a3, long _a4, long _a5)
>  {
> +#ifdef __riscv_32e
> +  register long syscall_id asm("t0") = n;
> +#else
> +  register long syscall_id asm("a7") = n;
> +#endif
> +
>    register long a0 asm("a0") = _a0;
> +  if (argc < 2) {
> +      asm volatile ("ecall" : "+r"(a0) : "r"(syscall_id));
> +      return a0;
> +  }
>    register long a1 asm("a1") = _a1;
> +  if (argc == 2) {
> +      asm volatile ("ecall" : "+r"(a0) : "r"(a1), "r"(syscall_id));
> +      return a0;
> +  }
>    register long a2 asm("a2") = _a2;
> +  if (argc == 3) {
> +      asm volatile ("ecall" : "+r"(a0) : "r"(a1), "r"(a2), "r"(syscall_id));
> +      return a0;
> +  }
>    register long a3 asm("a3") = _a3;
> +  if (argc == 4) {
> +      asm volatile ("ecall" : "+r"(a0) : "r"(a1), "r"(a2), "r"(a3), "r"(syscall_id));
> +      return a0;
> +  }
>    register long a4 asm("a4") = _a4;
> +  if (argc == 5) {
> +      asm volatile ("ecall" : "+r"(a0) : "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(syscall_id));
> +      return a0;
> +  }
>    register long a5 asm("a5") = _a5;
>
> -#ifdef __riscv_32e
> -  register long syscall_id asm("t0") = n;
> -#else
> -  register long syscall_id asm("a7") = n;
> -#endif
> -
> -  asm volatile ("scall"
> -               : "+r"(a0) : "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5), "r"(syscall_id));
> +  asm volatile ("ecall" : "+r"(a0) : "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5), "r"(syscall_id));
>
>    return a0;
>  }
>
>  static inline long
> -syscall_errno(long n, long _a0, long _a1, long _a2, long _a3, long _a4, long _a5)
> +syscall_errno(long n, int argc, long _a0, long _a1, long _a2, long _a3, long _a4, long _a5)
>  {
> -  long a0 = __internal_syscall (n, _a0, _a1, _a2, _a3, _a4, _a5);
> +  long a0 = __internal_syscall (n, argc, _a0, _a1, _a2, _a3, _a4, _a5);
>
>    if (a0 < 0)
>      return __syscall_error (a0);
> diff --git a/libgloss/riscv/sys_access.c b/libgloss/riscv/sys_access.c
> index ef446d2..45bedb3 100644
> --- a/libgloss/riscv/sys_access.c
> +++ b/libgloss/riscv/sys_access.c
> @@ -5,5 +5,5 @@
>  int
>  _access(const char *file, int mode)
>  {
> -  return syscall_errno (SYS_access, file, mode, 0, 0, 0, 0);
> +  return syscall_errno (SYS_access, 2, file, mode, 0, 0, 0, 0);
>  }
> diff --git a/libgloss/riscv/sys_close.c b/libgloss/riscv/sys_close.c
> index 80b10c6..c1691d8 100644
> --- a/libgloss/riscv/sys_close.c
> +++ b/libgloss/riscv/sys_close.c
> @@ -5,5 +5,5 @@
>  int
>  _close(int file)
>  {
> -  return syscall_errno (SYS_close, file, 0, 0, 0, 0, 0);
> +  return syscall_errno (SYS_close, 1, file, 0, 0, 0, 0, 0);
>  }
> diff --git a/libgloss/riscv/sys_exit.c b/libgloss/riscv/sys_exit.c
> index 03e1c34..995dd65 100644
> --- a/libgloss/riscv/sys_exit.c
> +++ b/libgloss/riscv/sys_exit.c
> @@ -5,6 +5,6 @@
>  void
>  _exit(int exit_status)
>  {
> -  syscall_errno (SYS_exit, exit_status, 0, 0, 0, 0, 0);
> +  syscall_errno (SYS_exit, 1, exit_status, 0, 0, 0, 0, 0);
>    while (1);
>  }
> diff --git a/libgloss/riscv/sys_faccessat.c b/libgloss/riscv/sys_faccessat.c
> index e966a4a..6418ef4 100644
> --- a/libgloss/riscv/sys_faccessat.c
> +++ b/libgloss/riscv/sys_faccessat.c
> @@ -4,5 +4,5 @@
>  /* Permissions of a file (by name) in a given directory.  */
>  int _faccessat(int dirfd, const char *file, int mode, int flags)
>  {
> -  return syscall_errno (SYS_faccessat, dirfd, file, mode, flags, 0, 0);
> +  return syscall_errno (SYS_faccessat, 4, dirfd, file, mode, flags, 0, 0);
>  }
> diff --git a/libgloss/riscv/sys_fstat.c b/libgloss/riscv/sys_fstat.c
> index 13a0bca..d97ba44 100644
> --- a/libgloss/riscv/sys_fstat.c
> +++ b/libgloss/riscv/sys_fstat.c
> @@ -9,7 +9,7 @@ int
>  _fstat(int file, struct stat *st)
>  {
>    struct kernel_stat kst;
> -  int rv = syscall_errno (SYS_fstat, file, &kst, 0, 0, 0, 0);
> +  int rv = syscall_errno (SYS_fstat, 2, file, &kst, 0, 0, 0, 0);
>    _conv_stat (st, &kst);
>    return rv;
>  }
> diff --git a/libgloss/riscv/sys_fstatat.c b/libgloss/riscv/sys_fstatat.c
> index 0e4ea42..bf03355 100644
> --- a/libgloss/riscv/sys_fstatat.c
> +++ b/libgloss/riscv/sys_fstatat.c
> @@ -8,7 +8,7 @@ int
>  _fstatat(int dirfd, const char *file, struct stat *st, int flags)
>  {
>    struct kernel_stat kst;
> -  int rv = syscall_errno (SYS_fstatat, dirfd, file, &kst, flags, 0, 0);
> +  int rv = syscall_errno (SYS_fstatat, 4, dirfd, file, &kst, flags, 0, 0);
>    _conv_stat (st, &kst);
>    return rv;
>  }
> diff --git a/libgloss/riscv/sys_gettimeofday.c b/libgloss/riscv/sys_gettimeofday.c
> index 457dcbc..daa14e4 100644
> --- a/libgloss/riscv/sys_gettimeofday.c
> +++ b/libgloss/riscv/sys_gettimeofday.c
> @@ -6,5 +6,5 @@
>  int
>  _gettimeofday(struct timeval *tp, void *tzp)
>  {
> -  return syscall_errno (SYS_gettimeofday, tp, 0, 0, 0, 0, 0);
> +  return syscall_errno (SYS_gettimeofday, 1, tp, 0, 0, 0, 0, 0);
>  }
> diff --git a/libgloss/riscv/sys_link.c b/libgloss/riscv/sys_link.c
> index eaeb22b..83cd1b2 100644
> --- a/libgloss/riscv/sys_link.c
> +++ b/libgloss/riscv/sys_link.c
> @@ -4,5 +4,5 @@
>  /* Establish a new name for an existing file.  */
>  int _link(const char *old_name, const char *new_name)
>  {
> -  return syscall_errno (SYS_link, old_name, new_name, 0, 0, 0, 0);
> +  return syscall_errno (SYS_link, 2, old_name, new_name, 0, 0, 0, 0);
>  }
> diff --git a/libgloss/riscv/sys_lseek.c b/libgloss/riscv/sys_lseek.c
> index 7486a3a..8eb8b81 100644
> --- a/libgloss/riscv/sys_lseek.c
> +++ b/libgloss/riscv/sys_lseek.c
> @@ -6,5 +6,5 @@
>  off_t
>  _lseek(int file, off_t ptr, int dir)
>  {
> -  return syscall_errno (SYS_lseek, file, ptr, dir, 0, 0, 0);
> +  return syscall_errno (SYS_lseek, 3, file, ptr, dir, 0, 0, 0);
>  }
> diff --git a/libgloss/riscv/sys_lstat.c b/libgloss/riscv/sys_lstat.c
> index 2eeabcc..dd5dc52 100644
> --- a/libgloss/riscv/sys_lstat.c
> +++ b/libgloss/riscv/sys_lstat.c
> @@ -7,7 +7,7 @@
>  int _lstat(const char *file, struct stat *st)
>  {
>    struct kernel_stat kst;
> -  int rv = syscall_errno (SYS_lstat, file, &kst, 0, 0, 0, 0);
> +  int rv = syscall_errno (SYS_lstat, 2, file, &kst, 0, 0, 0, 0);
>    _conv_stat (st, &kst);
>    return rv;
>  }
> diff --git a/libgloss/riscv/sys_open.c b/libgloss/riscv/sys_open.c
> index 4fd5d67..eb1a99b 100644
> --- a/libgloss/riscv/sys_open.c
> +++ b/libgloss/riscv/sys_open.c
> @@ -5,5 +5,5 @@
>  int
>  _open(const char *name, int flags, int mode)
>  {
> -  return syscall_errno (SYS_open, name, flags, mode, 0, 0, 0);
> +  return syscall_errno (SYS_open, 3, name, flags, mode, 0, 0, 0);
>  }
> diff --git a/libgloss/riscv/sys_openat.c b/libgloss/riscv/sys_openat.c
> index cf429b7..652ab2e 100644
> --- a/libgloss/riscv/sys_openat.c
> +++ b/libgloss/riscv/sys_openat.c
> @@ -4,5 +4,5 @@
>  /* Open file relative to given directory.  */
>  int _openat(int dirfd, const char *name, int flags, int mode)
>  {
> -  return syscall_errno (SYS_openat, dirfd, name, flags, mode, 0, 0);
> +  return syscall_errno (SYS_openat, 4, dirfd, name, flags, mode, 0, 0);
>  }
> diff --git a/libgloss/riscv/sys_read.c b/libgloss/riscv/sys_read.c
> index 7367e26..dd3bc33 100644
> --- a/libgloss/riscv/sys_read.c
> +++ b/libgloss/riscv/sys_read.c
> @@ -5,5 +5,5 @@
>  /* Read from a file.  */
>  ssize_t _read(int file, void *ptr, size_t len)
>  {
> -  return syscall_errno (SYS_read, file, ptr, len, 0, 0, 0);
> +  return syscall_errno (SYS_read, 3, file, ptr, len, 0, 0, 0);
>  }
> diff --git a/libgloss/riscv/sys_sbrk.c b/libgloss/riscv/sys_sbrk.c
> index f91c2c5..086509e 100644
> --- a/libgloss/riscv/sys_sbrk.c
> +++ b/libgloss/riscv/sys_sbrk.c
> @@ -41,13 +41,13 @@ _sbrk(ptrdiff_t incr)
>
>    if (heap_end == 0)
>      {
> -      long brk = __internal_syscall (SYS_brk, 0, 0, 0, 0, 0, 0);
> +      long brk = __internal_syscall (SYS_brk, 1, 0, 0, 0, 0, 0, 0);
>        if (brk == -1)
>          return (void *)__syscall_error (-ENOMEM);
>        heap_end = brk;
>      }
>
> -  if (__internal_syscall (SYS_brk, heap_end + incr, 0, 0, 0, 0, 0) != heap_end + incr)
> +  if (__internal_syscall (SYS_brk, 1, heap_end + incr, 0, 0, 0, 0, 0) != heap_end + incr)
>      return (void *)__syscall_error (-ENOMEM);
>
>    heap_end += incr;
> diff --git a/libgloss/riscv/sys_stat.c b/libgloss/riscv/sys_stat.c
> index a193b10..1e03700 100644
> --- a/libgloss/riscv/sys_stat.c
> +++ b/libgloss/riscv/sys_stat.c
> @@ -8,7 +8,7 @@ int
>  _stat(const char *file, struct stat *st)
>  {
>    struct kernel_stat kst;
> -  int rv = syscall_errno (SYS_stat, file, &kst, 0, 0, 0, 0);
> +  int rv = syscall_errno (SYS_stat, 2, file, &kst, 0, 0, 0, 0);
>    _conv_stat (st, &kst);
>    return rv;
>  }
> diff --git a/libgloss/riscv/sys_unlink.c b/libgloss/riscv/sys_unlink.c
> index b55fe1e..1cf6bbe 100644
> --- a/libgloss/riscv/sys_unlink.c
> +++ b/libgloss/riscv/sys_unlink.c
> @@ -5,5 +5,5 @@
>  int
>  _unlink(const char *name)
>  {
> -  return syscall_errno (SYS_unlink, name, 0, 0, 0, 0, 0);
> +  return syscall_errno (SYS_unlink, 1, name, 0, 0, 0, 0, 0);
>  }
> diff --git a/libgloss/riscv/sys_write.c b/libgloss/riscv/sys_write.c
> index b972734..ce2edd3 100644
> --- a/libgloss/riscv/sys_write.c
> +++ b/libgloss/riscv/sys_write.c
> @@ -6,5 +6,5 @@
>  ssize_t
>  _write(int file, const void *ptr, size_t len)
>  {
> -  return syscall_errno (SYS_write, file, ptr, len, 0, 0, 0);
> +  return syscall_errno (SYS_write, 3, file, ptr, len, 0, 0, 0);
>  }
> --
> 2.24.1
>