Machine-Level Programming II: Control: loopsComp 21000: Introduction to Computer Organization & SystemsMarch 2014Systems book chapter 3* * Modified slides from the book “Computer Systems: a Programmer’s Perspective”, Randy Bryant & David O’Hallaron, 2011
Today • Complete addressing mode, address computation (leal) • Arithmetic operations • x86-64 • Control: Condition codes • Conditional branches and moves • Loops
“Do-While” Loop Example • Count number of 1’s in argument x (“popcount”) • Use conditional branch to either continue looping or to exit loop C Code Goto Version intpcount_do(unsigned x) { int result = 0; do { result += x & 0x1; x >>= 1; } while (x); return result; } intpcount_do(unsigned x) { int result = 0; loop: result += x & 0x1; x >>= 1; if (x) goto loop; return result; }
“Do-While” Loop Compilation Goto Version • Registers: %edxx %ecxresult intpcount_do(unsigned x){ int result = 0; loop: result += x & 0x1; x >>= 1; if (x) goto loop; return result; } movl $0, %ecx # result = 0 .L2: # loop: movl %edx, %eax andl $1, %eax # t = x & 1 addl %eax, %ecx # result += t shrl %edx # x >>= 1 jne .L2 # If !0, goto loop
General “Do-While” Translation Goto Version C Code • Body: • Test returns integer • = 0 interpreted as false • ≠ 0 interpreted as true loop: Body if (Test) gotoloop do Body while (Test); { Statement1; Statement2; … Statementn; }
“While” Loop Example C Code GotoVersion • Is this code equivalent to the do-while version? • Must jump out of loop if test fails intpcount_while(unsigned x) { int result = 0; while (x) { result += x & 0x1; x >>= 1; } return result; } intpcount_do(unsigned x){ int result = 0; if (!x) goto done; loop: result += x & 0x1; x >>= 1; if (x) goto loop; done: return result; }
“While” Loop Example GotoVersion Assembly intpcount_do(unsigned x){ int result = 0; if (!x) goto done; loop: result += x & 0x1; x >>= 1; if (x) goto loop; done: return result; } _pcount_do: 00001d30 pushl %ebp 00001d31 movl %esp, %ebp 00001d33 subl $16, %esp 00001d36 movl 8(%ebp), %eax 00001d39 movl %eax, -4(%ebp) 00001d3c movl $0, -16(%ebp) 00001d43 jmp 0x1d5b 00001d45 movl -4(%ebp), %eax 00001d48 andl $1, %eax 00001d4b movl -16(%ebp), %ecx 00001d4e addl %ecx, %eax 00001d50 movl %eax, -16(%ebp) 00001d53 movl -4(%ebp), %eax 00001d56 shrl %eax 00001d5ecmpl $0, %eax 00001d61 jne 0x1d45 00001d63 movl -16(%ebp), %eax 00001d72addl $16, %esp 00001d75 popl %ebp 00001d76 ret Note that some redundant instructions have been removed so the memory addresses don’t quite work.
General “While” Translation While version while (Test) Body Goto Version Do-While Version if (!Test) gotodone; loop: Body if (Test) gotoloop; done: if (!Test) gotodone; do Body while(Test); done:
Is this code equivalent to other versions? “For” Loop Example C Code #define WSIZE 8*sizeof(int) intpcount_for(unsigned x) { inti; int result = 0; for (i = 0; i < WSIZE; i++) { unsigned mask = 1 << i; result += (x & mask) != 0; } return result; }
“For” Loop Form Init i = 0 General Form Test for (Init; Test; Update ) Body i < WSIZE Update i++ for (i = 0; i < WSIZE; i++) { unsigned mask = 1 << i; result += (x & mask) != 0; } Body { unsigned mask = 1 << i; result += (x & mask) != 0; }
“For” Loop While Loop For Version for (Init; Test; Update) Body While Version Init; while (Test ) { Body Update; }
“For” Loop … Goto Init; if (!Test) gotodone; loop: Body Update if (Test) gotoloop; done: For Version for (Init; Test; Update) Body While Version Init; while (Test ) { Body Update; } Init; if (!Test) gotodone; do Body Update while(Test); done:
Initial test can be optimized away “For” Loop Conversion Example Goto Version C Code intpcount_for_gt(unsigned x) { inti; int result = 0; i = 0; if (!(i < WSIZE)) goto done; loop: { unsigned mask = 1 << i; result += (x & mask) != 0; } i++; if (i < WSIZE) goto loop; done: return result; } #define WSIZE 8*sizeof(int) intpcount_for(unsigned x) { inti; int result = 0; for (i = 0; i < WSIZE; i++) { unsigned mask = 1 << i; result += (x & mask) != 0; } return result; } Init !Test Body Update Test
Redundant instructions have been removed, so addresses are not correct. “For” Loop Conversion Example Assembly Version _pcount_for: 00001d10 pushl %ebp 00001d11 movl %esp, %ebp 00001d13 subl $24, %esp 00001d16 movl 8(%ebp), %eax 00001d19 movl %eax, -4(%ebp) 00001d1c movl $0, -20(%ebp) 00001d23 movl $0, -16(%ebp) 00001d2a jmp 0x1d5f 00001d2c movl -16(%ebp), %eax 00001d2f movl %eax, %ecx 00001d31 movl $1, %eax 00001d36 shll %cl, %eax 00001d38 movl %eax, -24(%ebp) 00001d3b movl -4(%ebp), %eax 00001d3e movl -24(%ebp), %edx 00001d41 andl %edx, %eax 00001d43cmpl $0, %eax 00001d46 setne %al 00001d49 andb $1, %al 00001d4b movzbl %al, %eax 00001d4e movl -20(%ebp), %edx 00001d51 addl %edx, %eax 00001d53 movl %eax, -20(%ebp) 00001d56 movl -16(%ebp), %eax 00001d59 addl $1, %eax 00001d62cmpl $31, %eax 00001d65 jbe 0x1d2c 00001d67 movl -20(%ebp), %eax 00001d76addl $24, %esp 00001d79 popl %ebp 00001d7a ret
Summary • Today • Complete addressing mode, address computation (leal) • Arithmetic operations • Control: Condition codes • Conditional branches & conditional moves • Loops • Next Time • Switch statements • Stack • Call / return • Procedure call discipline
