Lecture 11 Code Generation Joey Paquet 2000 2014

Lecture 11 Code Generation Joey Paquet, 2000 -2014 1

Code Generation Examples • Variable declaration: x y dw 0 where x and y are the addresses of x and y, which are labels generated during the parse and stored in the symbol table • To load or change the content of a variable: lw r 1, x(r 0) sw x(r 0), r 1 where x is the label of variable x, r 1 is the register containing the value of variable x and r 0 is assumed to contain 0 (offset) Joey Paquet, 2000 -2014 2

Arithmetic Operations a+b t 1 a*b t 3 lw r 1, a(r 0) lw r 2, b(r 0) add r 3, r 1, r 2 dw 0 sw t 1(r 0), r 3 lw r 1, a(r 0) lw r 2, b(r 0) mul r 3, r 1, r 2 dw 0 sw t 3(r 0), r 3 a+8 t 2 a*8 t 4 Joey Paquet, 2000 -2014 lw r 1, a(r 0) addi r 2, r 1, 8 dw 0 sw t 2(r 0), r 2 lw r 1, a(r 0) muli r 2, r 1, 8 dw 0 sw t 4(r 0), r 2 3

Relational Operations a==b lw r 1, a(r 0) lw r 2, b(r 0) ceq r 3, r 1, r 2 t 5 dw 0 sw t 5(r 0), r 3 a==8 lw r 1, a(r 0) ceqi r 2, r 1, 8 t 6 dw 0 sw t 6(r 0), r 2 Joey Paquet, 2000 -2014 4

Logical Operations a and b t 7 zero 1 endand 1 not a lw r 1, a(r 0) lw r 2, b(r 0) and r 3, r 1, r 2 dw 0 bz r 3, zero 1 addi r 1, r 0, 1 sw t 7(r 0), r 1 j endand 1 sw t 7(r 0), r 0 t 8 zero 2 endnot 1 Joey Paquet, 2000 -2014 lw r 1, a(r 0) not r 2, r 1 dw 0 bz r 2, zero 2 addi r 1, r 0, 1 sw t 8(r 0), r 1 j endnot 1 sw t 8(r 0), r 0 5

Assignment Operation a : = b lw r 1, b(r 0) sw a(r 0), r 1 a : = 8 sub r 1, r 1 addi r 1, 8 sw a(r 0), r 1 a : = b+c {code for b+c. yields tn as a result} lw r 1, tn(r 0) sw a(r 0), r 1 Joey Paquet, 2000 -2014 6

Conditional Statements if a>b then a: =b else a: =0; [1] [2] [3] [4] [5] [6] else 1 [4] endif 1[6] {code for "a>b". yeilds tn as a result} lw r 1, tn(r 0) bz r 1, else 1 {code for "a: =b”} j endif 1 {code for "a: =0”} {code continuation} Joey Paquet, 2000 -2014 [1] [2] [3] [4] [5] 7

Loop Statements while a<b do a: =a+1; [1] [2] [3] [4] [5] gowhile 1 [1] {code for "a<b". yeilds tn as a result} lw r 1, tn(r 0) bz r 1, endwhile 1 {code for statblock (a: =a+1)} j gowhile 1 endwhile 1[5] {code continuation} Joey Paquet, 2000 -2014 [2] [3] [4] [5] 8

Translating Functions • There are two essential parts in translating functions: – translating function declarations – translating function calls • First, the compiler encounters a function header. It can either be a function prototype or the header of a full function declaration • In both cases, a record can be created in the global symbol table, and a local symbol table can be created for new functions • In the case of a full definition, the code is generated for the variable declarations and statements inside the body of the function Joey Paquet, 2000 -2014 9

Translating Functions • The address field in the symbol table entry of the function contains the address (or an ASM label) of the first memory cell assigned to the function. • This address will be used to jump to the function when a function call is encountered and translated • Function calls raise the need for semantic checking. The number and type of actual parameters must match with the information stored in the symbol table for that function • Once the semantic check is successful, semantic translation can occur for the function call Joey Paquet, 2000 -2014 10

Function Declarations int fn ( int a, int b ){ statlist }; [1] [2] [3] [4] [5] fn fnp 1 fnp 2 dw 0 [1] dw 0 [2] sw fnp 1(r 0), r 2 [2] dw 0 [3] sw fnp 2(r 0), r 3 [3] {code for local var. decl. & statement list} [4] lw r 2, fnp 1(r 0) [5] lw r 3, fnp 2(r 0) [5] jr r 15 [5] Joey Paquet, 2000 -2014 11

Function Declarations • fn corresponds both to the first instruction in the function and the address of the return value of fn • Parameters are copied to registers at function call and copied in the local variables when the function execution begins • Before resolution, the parameters are copied to registers and copied back to the actual parameters in the calling function upon returning from the function • This limits the possible number of parameters to the number of registers available • r 15 is reserved for link Joey Paquet, 2000 -2014 12

Function Calls • For languages not allowing recursive function calls, only one occurrence of any function can be running at any given time • In this case, all variables local to the function are statically allocated at compile time. The only things there are to manage are: – the jump to the function code – the passing of parameters upon calling and return value upon completion – the jump back to the instruction following the function call Joey Paquet, 2000 -2014 13

Function Call (no parameter) fn() fn . . . {code for calling function} jl R 15, fn {code continuation in the calling function}. . . {code for called function}. . . lw r 1, fn(r 0) jr R 15. . . Joey Paquet, 2000 -2014 14

Passing Parameters • Parameters are sometimes passed using registers. • In this case, the number of parameters passed cannot exceed the total number of registers • The return value can also be passed using a register, typically r 1 Joey Paquet, 2000 -2014 15

Passing Parameters (Registers) x = fn(p 1, p 2); . . . {code for the calling function} lw r 2, p 1(r 0) lw r 3, p 2(r 0) jl r 15, fn sw x(r 0), r 1 {code continuation in the calling function}. . . fn {refer to param[i] as ri+1 in fn code}. . . lw r 1, fn(r 0) jr r 15 Joey Paquet, 2000 -2014 16

Passing Parameters • To avoid the limitation of the allowed number of parameters to the number of registers, parameters can be stored following the function call jump • These methods are only usable for languages where recursive function calls are not allowed • With recursive function calls, the problem is that several occurences of the same function can be running at the same time • In this case, all the variables and parameters of a running function are stored in an activation record dynamically allocated on the process stack • This involves the elaboration of a run-time system as part of the compiled code Joey Paquet, 2000 -2014 17

Passing Parameters (by value) x = fn(a, b) {code before} lw r 1, a(r 0) sw fnp 1(r 0), r 1 lw r 1, b(r 0) sw fnp 2(r 0), r 1 jl r 15, fn sw x(r 0), r 1 {code after} fn fnp 1 fnp 2 dw 0 {code for called function} {refer to param[i] as fnpi} {put return value in r 1} jr r 15 Joey Paquet, 2000 -2014 18

Calling the Operating System • Some function calls interact with the operating system, e. g. when a program does input/output • In these cases, there are several possibilities depending on the resources offered by the operating system, e. g. : – treatment via special predefined ASM operations – access to the OS via calls or traps • In the MOON processor, we have two special operators: PUTC and GETC • They respectively output some data to the screen and input data from the keyboard • They are used to directly translate read() and write() function calls (see the MOON manual) Joey Paquet, 2000 -2014 19

Part II Hints for Assignment IV Joey Paquet, 2000 -2014 20

Hints for Assignment IV • Keep in mind that constructing the symbol tables is part of semantic checking. It should not be taken as a separate topic. • Your semantic actions should include symbol table construction function calls. • For assignment IV, the most important thing is to proceed in stages. Do not try to resolve all semantic checking and translation in your first try. Joey Paquet, 2000 -2014 21

Hints for Assignment IV • First resolve all semantic checking, but again by proceeding in stages. • Implement semantic checking for a small part of the grammar, use a complete test case to make sure this part works properly, and then resolve another part of semantic checking. • After all semantic checking is done, you can begin code generation, which should be relatively easy if semantic checking is done properly (semantic checking is a condition to semantic translation). • Again, proceed in stages for semantic translation. Joey Paquet, 2000 -2014 22

Hints for Assignment IV • Suggested sequence: – – – variable declarations expressions (one operator at a time) assignment statement conditional statement loop statement • Tricky parts: – – – expressions involving arrays (offset calculation) function calls floating point numbers recursive function calls user-defined data types (offset calculations) Joey Paquet, 2000 -2014 23

Hints for Assignment IV • You will not fail the project if you did not implement code generation for all aspects of the language. • But, you might fail if your compiler is not working. • This is why you should proceed in stages and make sure each stage is correct before going further. • Be careful to not break what was previously working. • Make sure you have a compiler that works properly for a subset of the problem. • For the part that you did not implement, think of a solution. You may get some marks if you are able to clearly explain how to do what is missing during your project demonstration. Joey Paquet, 2000 -2014 24