Not enough register for reading expression
eecheng87 opened this issue · 2 comments
There's a test case:
a = 1 + (2 + (3 + (4 + (5 + (6 + (7 + (8 + (9 + (10 + (11 + (12 + (13 + (14 + (15 + (16 + (17 + (18 + (19 + 20))))))))))))))))))
In shecc, it will store all constants in register. Obviously, there's not enough register.
In modern compiler, I found they just load an immediate. e.g.
li a4, 210
Even I mix identifier into expression, modern compiler still load an immediate value. Do we need to rewrite read expression? Maybe we need to evaluate expression before run time. Also, I think we need symbol table to store these identifier.
An effective evaluator is required for parsed expression. Which parts of C front-end would be modified to fit?
Maybe I need to modify read_expr which also contains read_operand (most complicate part).
It seems modern compiler will also evaluate result of function in expression during compile time, should we follow this? Following is example:
int f(int b){
int c = 10;
return b * c + 1;
}
void _start(){
int a;
int c = 10;
a = 1 + ( f(1) + (1 + (1 + (1 + (1 + (1 + (1 + (1 + (1 + (1 + (20 + (3 + (1 + (1 + (6 + (1 + (1 + (1 + 20))))))))))))))))));
print(a);corresponding asm
00010068 <_start>:
10068: 400027b7 lui a5,0x40002
1006c: 04b00713 li a4,75
10070: 00e78023 sb a4,0(a5) # 40002000 <__global_pointer$+0x3fff0788>