DRAFT riscv64: simplify assembly constructs not supported by tcc.

At the moment tcc does not support assembler instructions
with C expression operands. As a workaround read values
directly from stack.

* lib/linux/riscv64-mes-tcc/_exit.c
* lib/linux/riscv64-mes-tcc/_write.c
* lib/linux/riscv64-mes-tcc/syscall.c

lib/linux/riscv64-mes-tcc/_exit.c

@@ -28,12 +28,10 @@
 void
 _exit (int code)
 {
-  register long __a7 asm ("a7") = SYS_exit;
-  register long __a0 asm ("a0") = code;
   asm volatile (
+       "addi a7, zero, 93\n\t"
+       "ld a0, s0, -24\n\t"
        "ecall\n\t"
-       : // no outputs
-       : "r" (__a0), "r" (__a7)
        );
   // not reached
   _exit (0);

lib/linux/riscv64-mes-tcc/_write.c

@@ -28,14 +28,11 @@
 ssize_t
 _write (int filedes, void const *buffer, size_t size)
 {
-  register long __a7 asm ("a7") = (long) SYS_write;
-  register long __a0 asm ("a0") = (long) filedes;
-  register long __a1 asm ("a1") = (long) buffer;
-  register long __a2 asm ("a2") = (long) size;
   asm volatile (
+       "addi a7, zero, 64\n\t"
+       "ld a0, s0, -24\n\t"
+       "ld a1, s0, -32\n\t"
+       "ld a2, s0, -40\n\t"
        "ecall\n\t"
-       : "+r" (__a0)
-       : "r" (__a7), "r" (__a1), "r" (__a2)
        );
-  return (ssize_t)__a0;
 }

lib/linux/riscv64-mes-tcc/syscall.c

@@ -27,89 +27,70 @@
 long
 __sys_call (long sys_call)
 {
-  register long __a7 asm ("a7") = sys_call;
+    asm volatile (
-  register long __a0 asm ("a0");
+       "ld a7, s0, -24\n\t"
-  asm volatile (
        "ecall\n\t"
-       : "=r" (__a0)
-       : "r" (__a7)
        );
-  return __a0;
 }
 
 long
 __sys_call1 (long sys_call, long one)
 {
-  register long __a7 asm ("a7") = sys_call;
-  register long __a0 asm ("a0") = one;
   asm volatile (
+       "ld a7, s0, -24\n\t"
+       "ld a0, s0, -32\n\t"
        "ecall\n\t"
-       : "+r" (__a0)
-       : "r" (__a7)
        );
-  return __a0;
 }
 
 long
 __sys_call2 (long sys_call, long one, long two)
 {
-  register long __a7 asm ("a7") = sys_call;
-  register long __a0 asm ("a0") = one;
-  register long __a1 asm ("a1") = two;
   asm volatile (
+       "ld a7, s0, -24\n\t"
+       "ld a0, s0, -32\n\t"
+       "ld a1, s0, -40\n\t"
        "ecall\n\t"
-       : "+r" (__a0)
-       : "r" (__a7), "r" (__a1)
        );
-  return __a0;
 }
 
 long
 __sys_call3 (long sys_call, long one, long two, long three)
 {
-  register long __a7 asm ("a7") = sys_call;
-  register long __a0 asm ("a0") = one;
-  register long __a1 asm ("a1") = two;
-  register long __a2 asm ("a2") = three;
   asm volatile (
+       "ld a7, s0, -24\n\t"
+       "ld a0, s0, -32\n\t"
+       "ld a1, s0, -40\n\t"
+       "ld a2, s0, -48\n\t"
        "ecall\n\t"
-       : "+r" (__a0)
-       : "r" (__a7), "r" (__a1), "r" (__a2)
        );
-  return __a0;
 }
 
 long
 __sys_call4 (long sys_call, long one, long two, long three, long four)
 {
-  register long __a7 asm ("a7") = sys_call;
-  register long __a0 asm ("a0") = one;
-  register long __a1 asm ("a1") = two;
-  register long __a2 asm ("a2") = three;
-  register long __a3 asm ("a3") = four;
   asm volatile (
+       "ld a7, s0, -24\n\t"
+       "ld a0, s0, -32\n\t"
+       "ld a1, s0, -40\n\t"
+       "ld a2, s0, -48\n\t"
+       "ld a3, s0, -56\n\t"
        "ecall\n\t"
-       : "+r" (__a0)
-       : "r" (__a7), "r" (__a1), "r" (__a2), "r" (__a3)
        );
-  return __a0;
 }
 
 long
 __sys_call5 (long sys_call, long one, long two, long three, long four, long five)
 {
-  register long __a7 asm ("a7") = sys_call;
-  register long __a0 asm ("a0") = one;
-  register long __a1 asm ("a1") = two;
-  register long __a2 asm ("a2") = three;
-  register long __a3 asm ("a3") = four;
-  register long __a4 asm ("a4") = five;
   asm volatile (
+       "ld a7, s0, -24\n\t"
+       "ld a0, s0, -32\n\t"
+       "ld a1, s0, -40\n\t"
+       "ld a2, s0, -48\n\t"
+       "ld a3, s0, -56\n\t"
+       "ld a4, s0, -64\n\t"
        "ecall\n\t"
-       : "+r" (__a0)
-       : "r" (__a7), "r" (__a1), "r" (__a2), "r" (__a3), "r" (__a4)
        );
-  return __a0;
 }
 // *INDENT-ON*