Support global variable initialization

Close #12

Change test bench

README.md

@@ -165,19 +165,18 @@ int fib(int n)      fib:                        Reserve stack frame for function
 
 1. Any non-zero value is NOT treated as logical truth by all ops.
    That is, the expression `0 == strcmp(ptr, "hello")` is not equivalent to `!strcmp(ptr, "hello")`.
-2. Global variable initialization is not supported. 
-   Therefore, you can not initialize a global such as `int i = [expr]`.
-3. Dereference is incomplete. Consider `int x = 5; int *ptr = &x;` and it is forbidden to use `*ptr`.
+2. Dereference is incomplete. Consider `int x = 5; int *ptr = &x;` and it is forbidden to use `*ptr`.
    However, it is valid to use `ptr[0]`, which behaves the same of `*ptr`.
-4. The support of varying number of function arguments is incomplete. No `<stdarg.h>` can be used.
+3. The support of varying number of function arguments is incomplete. No `<stdarg.h>` can be used.
    Alternatively, check the implementation `printf` in source `lib/c.c` for `var_arg`.
-5. The memory region allocated by `malloc` can not be released. In fact, there is no `free` function.
+4. The memory region allocated by `malloc` can not be released. In fact, there is no `free` function.
    Memory allocator should be introduced in embedded libc implementation.
-6. If you attempt to return values in `main` function, the value will not be reserved after the
+5. If you attempt to return values in `main` function, the value will not be reserved after the
    program exited. You have to modify the `return` statement to `exit` call. Check the test items in
    file `tests/driver.sh` for details.
-7. The C front-end is a bit dirty because there is no effective AST.
-8. No function pointer is supported.
+6. The C front-end is a bit dirty because there is no effective AST.
+7. No function pointer is supported.
+8. ELF lacks of .bss and .rodata section
 
 ## License
 

src/cfront.c

@@ -548,6 +548,7 @@ int read_numeric_constant(char buffer[])
 
 void read_inner_var_decl(var_t *vd)
 {
+    vd->init_val = 0;
     if (lex_accept(T_asterisk))
         vd->is_ptr = 1;
     else
@@ -1308,6 +1309,30 @@ int read_body_assignment(char *token, block_t *parent)
     return 0;
 }
 
+int read_global_assignment(char *token)
+{
+    var_t *var = find_global_var(token);
+    if (var) {
+        char buffer[10];
+        int isneg = 0;
+        /* global initialization must be constant */
+        /* TODO: support rvalue as expression e.g. int a = 3 + 2; */
+        if (lex_accept(T_minus))
+            isneg = 1;
+        if (lex_peek(T_numeric, buffer))
+            var->init_val = read_numeric_constant(buffer);
+        else
+            error("Invalid value after assignment");
+        if (isneg)
+            var->init_val = -1 * var->init_val;
+        lex_expect(T_numeric);
+        lex_expect(T_semicolon);
+
+        return 1;
+    }
+    return 0;
+}
+
 int break_exit_ir_index[MAX_NESTING];
 
 void read_code_block(func_t *func, block_t *parent);
@@ -1696,10 +1721,14 @@ void read_global_decl(block_t *block)
     /* is a variable */
     memcpy(&block->locals[block->next_local++], &_temp_var, sizeof(var_t));
 
-    if (lex_accept(T_assign))
-        /* global variable initialization is not supported at the moment. */
-        error("Global initialization not supported");
-    else if (lex_accept(T_comma))
+    if (lex_accept(T_assign)) {
+        if (_temp_var.is_ptr == 0 && _temp_var.array_size == 0) {
+            read_global_assignment(_temp_var.var_name);
+            return;
+        }
+        /* TODO: support global initialization for array and pointer */
+        error("Global initialization for array and pointer not supported");
+    } else if (lex_accept(T_comma))
         /* TODO: continuation */
         error("Global continuation not supported");
     else if (lex_accept(T_semicolon))

src/codegen.c

@@ -75,7 +75,12 @@ void size_funcs(int data_start)
         blk->locals[i].offset = elf_data_idx; /* set offset in data section */
         elf_add_symbol(blk->locals[i].var_name, strlen(blk->locals[i].var_name),
                        data_start + elf_data_idx);
-        elf_data_idx += size_var(&blk->locals[i]);
+        /* TODO: add .bss section */
+        if (strcmp(blk->locals[i].type_name, "int") == 0 &&
+            blk->locals[i].init_val != 0)
+            elf_write_data_int(blk->locals[i].init_val);
+        else
+            elf_data_idx += size_var(&blk->locals[i]);
     }
 }
 

src/defs.h

@@ -106,7 +106,8 @@ typedef struct {
     char var_name[MAX_VAR_LEN];
     int is_ptr;
     int array_size;
-    int offset; /* offset from stack or frame */
+    int offset;   /* offset from stack or frame */
+    int init_val; /* for global initialization */
 } var_t;
 
 /* function definition */

src/elf.c

@@ -19,6 +19,14 @@ void elf_write_data_str(char *vals, int len)
         elf_data[elf_data_idx++] = vals[i];
 }
 
+void elf_write_data_int(int val)
+{
+    elf_data[elf_data_idx++] = (val & 0x000000FF);
+    elf_data[elf_data_idx++] = (val & 0x0000FF00) >> 8;
+    elf_data[elf_data_idx++] = (val & 0x00FF0000) >> 16;
+    elf_data[elf_data_idx++] = (val & 0xFF000000) >> 24;
+}
+
 void elf_write_header_byte(int val)
 {
     elf_header[elf_header_idx++] = val;

tests/driver.sh

@@ -247,6 +247,16 @@ int main() {
 }
 EOF
 
+# global initialization
+try_ 41 << EOF
+int a = 44;
+int b = -3;
+int main()
+{
+    exit(a + b);
+}
+EOF
+
 # conditional operator
 # expr 10 "1 ? 10 : 5"
 # expr 25 "0 ? 10 : 25"