1. Code Generation Compiler Baojian Hua bjhua@ustc.edu.cn

2. Front End source code abstract syntax tree lexical analyzer parser tokens IR semantic analyzer

3. Back End IR TempMap instruction selector register allocator Assem instruction scheduler

4. Code Generation Generating code for some ISA this course uses x86 Many components instruction selection, register allocation, scheduling, … Many different strategies for this time, we concentrate on a simple one: stack machine and later in this course, we ’ d turn to more advanced (and sophisticated) ones

5. What ’ s a stack machine? A stack machine has only an operand stack and no (or few) registers all computation performed on the operand stack architecture very simple and uniform Long history: Date back at least to 70 ’ s last century Renew industry ’ s interest in the recent decade Sun ’ s JVM and Microsoft ’ s CLR, etc.

6. Stack Machine ISA: s86 prog -> instr prog -> instr -> push v -> pop id -> add -> sub -> times -> divide v -> num -> id // Sample Program push 8 push 2 push x times sub

7. The simple expression lang ’ // recall our simple // expression language exp -> num -> id -> exp + exp -> exp – exp -> exp * exp -> exp / exp -> (exp) // or in ML datatype exp = Int of int | Id of string | Add of exp * exp | Sub of exp * exp | Times of exp * exp | Divide of exp * exp // Sample Program 8-2*x

8. Code gen ’ from exp to s86 C (num) = push num C (id) = push id C (e1 + e2) = C (e1); C (e2); add C (e1 – e2) = C (e1); C (e2); sub C (e1 * e2) = C (e1); C (e2); times C (e1 / e2) = C (e1); C (e2); divide

9. Code gen ’ from exp to s86 // or in ML fun C (e) = case e of Num i => push i | Id s => push s | Add (e1, e2) => C (e1); C (e2); add | … => (* similar *)

10. Example C (8-2*x) = C(8); C(2*x); sub = push 8; C(2*x); sub = push 8; C(2); C(x); times; sub = …

11. Moral Code generation for stack machine is dirty simple recursive equation from point view of math recursive function from point view of CS think before hack! But we ’ d have more to say about: variable storage more language features statement, declarations, functions, etc..

12. Address space Address space is the way how programs use memory highly architecture and OS dependent right is the typical layout of 32-bit x86/Linux OS heap data text BIOS, VGA 0x00100000 stack 0xc00000000 0x08048000 0x00000000 0xffffffff

13. Static Storage Static storage is an area of space in data section a typical use is to hold C/C++ file scope variables (static) and extern variable (global) Exp lang ’ has only static variables, all can be stored to static section so require a pass to collect all variables

14. Declarations // scale exp a bit prog -> decs exp decs -> int id; decs -> exp -> … // or in ML datatype decs = T of {var: string, ty: tipe} list // Sample Program int x; 8-2*x;

15. Code gen ’ rules D (int id; decs) = id:.int 0 D (decs) D ( ) =

16. Statement // scale the exp a by adding the following: s -> id = e; -> if (e) s else s // compile: CS (id = e;) = C (e); pop id

17. Statement, cont ’ // s86 should also be modified! // compile: CS (if e s1 s2) = C(e); jz.Lfalse.Ltrue: CS(s1) jmp.Lend.Lfalse: CS(s2).Lend e s1s2 …

18. Moral It ’ s also straightforward to translate other control structure in this style while, for, switch, etc.. This kind of code generation is called recursive decedent may be done at parsing time adopted in many compilers read the offered article on Borland Turbo Pascal 3.0 you may safely ignore the Pascal-specific features

19. From s86 to x86 Run the generated s86 code? design a virtual machine as we did in lab #1 this is also the way of JVM or CLR translate to native code and then exec ’ it so-called just-in-time (JIT) the dominant OO method today … Next, we discuss the 2nd method by mapping s86 to x86

20. Operand Stack // x86 does not have a dedicated operand stack? // Solution 1: use the control stack: ebp, esp // leave to you. // Solution 2: make a fake operand stack, as in:.set PAGE, 4096.data opStack:.space PAGE, 0xcc top:.int opStack+PAGE // “top” points to stack top, and stack grows // down to lower address

21. Instructions // map fake s86 instructions to x86’s:.macro s86push x sub dword ptr [top], 4 mov ebx, [top] mov eax, \x mov [ebx], eax.endm // others are similar. // Care must be taken to take account of the // machine constraints. For instance, mem-mem // move is illegal on x86.