Topic 6: Code Generation

Transcription

1 Compilers 2008 Topic 6: 6.4 Conditional Instructions Status Flags Conditionals A special register exists called the FLAGS register Consists of a sequence of bits, which are set (1) or unset (0). These flags are set as the result of various operations, e.g., the ZF (Zero Flag) is set when the last operation produced a zero value. The flags can be referenced in conditional jump operations, e.g., jz L100 jump to L100 if last op resulted in zero 2 1

2 Conditionals Integer Comparison Operation The CMP operation subtracts the second operand from the first Does not store the result of the subtraction anywhere. But affects the flags as if the subtraction had taken place, ZF is set if the subtraction produced zero, unset otherwise. SF (Sign Flag) is set if the subtraction was less than zero, 1 otherwise. Other operators exist for comparisons of floats, strings and booleans 3 If-then statement Conditionals How do we map an if-statement onto assembler code? Assume code like: if A == B then <stmt1> Firstly, we generate code for the comparison, e.g., CMP [A], [B] Then we generate code to jump over the code for <stmt1> if test fails JZ L1 jump if cmp result was zero Then we put the code for <stmt1>, e.g. ADD X, Y On the line following this, we put the label from above L1: 4 2

3 Conditionals The code together CMP [A], [B] JZ L1 ADD [X], [Y] L1: jump if ecx is zero 5 if statement: semantic actions <stmt> :- if <exp> then <instructions> : Generate code to jump if exp non-zero : Generate line with label <exp> jz end_then <instructions> end_then: 6 3

4 Conditionals If then-else statement How do we map an if-then-else statement onto assembler code? Assume code like: if A == B then <stmt1> else <stmt2> 7 if statement: semantic actions <stmt> :- if <exp> then <instruct1> else <instruct2> #A3 : Generate code to jump L1 if exp non-zero <exp> jz end_then <instruct1> end_then: jmp end_ifelse <instruct2> end_ifelse: 8 4

5 if statement: semantic actions <stmt> :- if <exp> then <instruct1> else <instruct2> #A3 : Generate code to jump if exp non-zero : At end of then code, generate jmp to end Also generate label for start_else <exp> jz start_else <instruct1> jmp end_ifelse start_else: <instruct2> end_ifelse: 9 if statement: semantic actions <stmt> :- if <exp> then <instruct1> else <instruct2> #A3 : Generate code to jump if exp non-zero : At end of then code, generate jmp to end Also generate label for start_else #A3: Generate label for end_ifelse <exp> jz start_else <instruct1> jmp end_ifelse start_else: <instruct2> #A3 end_ifelse: 10 5

6 if statement: sample code generated Code: CMP [A], [B] JZ L1 <stmt1 code> JMP L2 L1: <stmt2 code> L2: jump if cmp result was zero 11 Topic 6: 6.5 6

7 While statement While loops map onto assembler much as for an if-statement E.g., for while <exp> do <instructions> end while_start: <exp> jz while_end <instruccions> while_end: jmp while_start 13 While statement: semantic actions <loop> :- while <exp> do <instructions> end #A3 : Generate line with a unique label for loop start while_start: <exp> jz while_end <instruccions> while_end: jmp while_start 14 7

8 While statement: semantic actions <loop> :- while <exp> do <instructions> end #A3 : Generate line with a unique label for loop start : Generate line with jump to end if expr fails while_start: <exp> jz while_end <instruccions> while_end: jmp while_start 15 While statement: semantic actions <loop> :- while <exp> do <instructions> end #A3 : Generate line with a unique label for loop start : Generate line with jump to end if expr fails #A3: Generate jump back to start, and label for loop end while_start: <exp> jz while_end #A3 while_end: <instruccions> jmp while_start 16 8

9 While statement: A Real example topwhile: a label to mark the top of this WHILE loop mov eax, 3 planning to invoke function 3 read from a file mov ebx, [infileid] the file ID must be placed into register ebx mov ecx, mybyte the address of memory to receive file content must be placed into register ecx mov edx, 1 the number of bytes to read is placed in edx, int 80h invokes a kernel function according to the number in register eax cmp eax, 0 check whether a byte was read je dunwhile skip the body if no bytes were read xor byte [mybyte], b [] dereferences, thereby refers to the contents at mybytes mov eax, 4 planning to invoke function 4 write to a file mov ebx, [outfileid] the file ID must be placed into register ebx mov ecx, mybyte the address of memory to write from must be placed into register ecx mov edx, 1 the no. of bytes to write must be placed in edx int 80h invokes a kernel function according to no. in eax jmp topwhile go back to the top of the loop dunwhile: jump to here if no byte is read 17 Repeat statement: semantic actions <loop> :- repeat <instructions> until <exp> 18 9

10 Repeat statement: semantic actions <loop> :- repeat <instructions> until <exp> : Generate unique label for loop start repeat_start: 19 Repeat statement: semantic actions <loop> :- repeat <instructions> until <exp> : Generate unique label for loop start <instr> Code for <instr> generated by other productions <i> repeat_start: <instruccions> 20 10

11 Repeat statement: semantic actions <loop> :- repeat <instructions> until <exp> : Generate unique label for loop start <instr> Code for <instr> generated by other productions <exp>: Code for <exp> generated by other productions <i> <exp> repeat_start: <instruccions> <exp> 21 Repeat statement: semantic actions <loop> :- repeat <instructions> until <exp> : Generate unique label for loop start <instr> Code for <instr> generated by other productions <exp>: Code for <exp> generated by other productions : Generate jump to end if last result zero Generate unconditional jump back to beginning Generate label for loop end <i> repeat_start: <instruccions> <exp> <exp> jz repeat_end repeat_end: jmp repeat_start 22 11