Only pass the minimum number of syscall arguments

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

@@ -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)
 {
-  register long a0 asm("a0") = _a0;
-  register long a1 asm("a1") = _a1;
-  register long a2 asm("a2") = _a2;
-  register long a3 asm("a3") = _a3;
-  register long a4 asm("a4") = _a4;
-  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"
+  register long a0 asm("a0") = _a0;
-		: "+r"(a0) : "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5), "r"(syscall_id));
+  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;
+
+  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);

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);
 }

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);
 }

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);
 }

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);
 }

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;
 }

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;
 }

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);
 }

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);
 }

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);
 }

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;
 }

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);
 }

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);
 }

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);
 }

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;

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;
 }

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);
 }

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);
 }