Analogue Electronics examInstructions:Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 1 Carnegie Mellon Assembly & C: Condi;onal Opera;ons Carnegie Mellon Instructors: Malcolm Heywood csci2122 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 2 Carnegie Mellon Today ! Control: Condi2on codes ! Condi2onal branches ! Loops Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 3 Carnegie Mellon Processor State (x86-64, Par2al) ! Informa2on about currently execu2ng program ! Temporary data ( %rax, … ) ! Loca;on of run;me stack ( %rsp ) ! Loca;on of current code control point ( %rip, … ) ! Status of recent tests ( CF, ZF, SF, OF ) %rip Registers Current stack top Instruc;on pointer CF ZF SF OF Condi;on codes %rsp %r8 %r9 %r10 %r11 %r12 %r13 %r14 %r15 %rax %rbx %rcx %rdx %rsi %rdi %rbp Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 4 Condi2on Code Registers Flag Condi2on Code Carry (CF) Current opera;on generated a carry from MSB. Detects overflow for unsigned operand. Zero (ZF) Current opera;on generated a zero. Sign (SF) Current opera;on generated a nega;ve value. Overflow (OF) Current opera;on generated a two’s complement overflow. Could be posi;ve or nega;ve. Can be set implicitly or explicitly. Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 5 Carnegie Mellon Condi2on Codes (Implicit SeGng) ! Implicitly set (think of it as side effect) by arithme2c opera2ons Example: addq Src,Dest t = a+b CF set if carry out from most significant bit (unsigned overflow) ZF set if t == 0 SF set if t < 0 (as signed) OF set if two’s-complement (signed) overflow (a>0 && b>0 && t<0) || (a<0 && b<0 && t>=0) ! Not set by leaq instruc2on Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 6 Carnegie Mellon Condi2on Codes (Explicit SeGng: Compare) ! Explicit SeGng by Compare Instruc2on !cmpq Src2, Src1 !cmpq b,a like compu;ng a-b without seYng des;na;on !CF set if carry out from most significant bit (used for unsigned comparisons) !ZF set if a == b !SF set if (a-b) < 0 (as signed) !OF set if two’s-complement (signed) overflow (a>0 && b<0 && (a-b)<0) || (a<0 && b>0 && (a-b)>0) Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 7 Carnegie Mellon Condi2on Codes (Explicit SeGng: Test) ! Explicit SeGng by Test instruc2on !testq Src2, Src1 !testq b,a like compu;ng a&b without seYng des;na;on !Sets condi;on codes based on value of Src1 & Src2 !Useful to have one of the operands be a mask !ZF set when a&b == 0 !SF set when a&b < 0 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 8 Carnegie Mellon Reading Condi2on Codes ! SetX Instruc2ons ! Query the outcome from: t = a –t w b ! i.e. two’s complement calcula;on with ‘w’ precision ! Set low-order byte of des;na;on to 0 or 1 based on combina;ons of condi;on codes SetX Condi;on Descrip;on sete ZF Equal / Zero setne ~ZF Not Equal / Not Zero sets SF Nega;ve setns ~SF Nonnega;ve setg ~(SF^OF)&~ZF Greater (Signed) setge ~(SF^OF) Greater or Equal (Signed) setl (SF^OF) Less (Signed) setle (SF^OF)|ZF Less or Equal (Signed) seta ~CF&~ZF Above (unsigned) setb CF Below (unsigned) Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 9 %rsp x86-64 Integer Registers ! Can reference low-order byte %al %bl %cl %dl %sil %dil %spl %bpl %r8b %r9b %r10b %r11b %r12b %r13b %r14b %r15b %r8 %r9 %r10 %r11 %r12 %r13 %r14 %r15 %rax %rbx %rcx %rdx %rsi %rdi %rbp Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 10 Example 1: SET instruc2on int gt (long x, long y) { return x > y; } Comp: 1. cmpq %rsi, %rdi 2. setl %al 3. movzbl %al, %eax 4. ret Register Use(s) %rdi Argument x %rsi Argument y %rax Return value Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 11 Example 2: type associa2on int comp(data_t a, data_t b) { return a COMP b; } ! Let data_t be defined by ‘type_def’ and COMP defined by ‘#define’ ! Let a and b appear in (some part) of %rdx and %rsi respec;vely ! What type does ‘type_def’ assume for the following: 1. Cmpl %esi, %edi Setl %al 2. Cmpb %sil, %dil Setbe %al 3. Cmpb %rsi, %rdi Set ne %a Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 12 Carnegie Mellon Today ! Control: Condi2on codes ! Condi2onal branches ! Loops Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 13 Carnegie Mellon Jumping ! jX Instruc2ons ! Jump to different part of code depending on condi;on codes jX Condi;on Descrip;on jmp 1 Uncondi;onal je ZF Equal / Zero jne ~ZF Not Equal / Not Zero js SF Nega;ve jns ~SF Nonnega;ve jg ~(SF^OF)&~ZF Greater (Signed) jge ~(SF^OF) Greater or Equal (Signed) jl (SF^OF) Less (Signed) jle (SF^OF)|ZF Less or Equal (Signed) ja ~CF&~ZF Above (unsigned) jb CF Below (unsigned) Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 14 Carnegie Mellon Example: Condi2onal Branch long absdiff(long x, long y) { long result; if (x > y) result = x-y; else result = y-x; return result; } absdiff: cmpq %rsi, %rdi jle .L2 movq %rdi, %rax subq %rsi, %rax ret .L2: # x <= y movq %rsi, %rax subq %rdi, %rax ret ! Genera;on gcc –c absdiff.c ! Disassembly objdump –d absdiff.o Register Use(s) %rdi Argument x %rsi Argument y %rax Return value Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 15 Carnegie Mellon Expressing with Goto Code long absdiff(long x, long y) { long result; if (x > y) result = x-y; else result = y-x; return result; } ! C allows goto statement ! Jump to posi;on designated by label long absdiff_j(long x, long y) { long result; if (x <= y) goto Else; result = x-y; return result; Else: result = y-x; return result; } Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 16 Condi2onal Branches with Condi2onal Control General form of C ‘if-else’ if (test-expression) then-statement(s) else else-statement(s) Corresponding Assembly (C syntax for ctrl [and alternate]) t = test-expression; if (!t) [(t)] goto false; [true] then-statement(s) goto done; false: [true:] else-statement(s) Done: return-statement Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 17 Condi2onal Branches with Condi2onal Move General form of C condi2onal expression v = test-expr ? then-state : else-state; Assembly – Case 1 if (! test-expr) goto false; v = then-state; goto done; False: else-state; Done: return-state; Assembly – Case 2 v = then-state; ve = else-state; t = test-expr; if (!t) v = ve; Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 18 Carnegie Mellon Condi2onal Move Example absdiff: movq %rdi, %rax # x subq %rsi, %rax # result = x-y movq %rsi, %rdx subq %rdi, %rdx # eval = y-x cmpq %rsi, %rdi # compare x:y cmovle %rdx, %rax # if <=, result = eval ret long absdiff (long x, long y) { long result; if (x > y) result = x-y; else result = y-x; return result; } Register Use(s) %rdi Argument x %rsi Argument y %rax Return value Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 19 Carnegie Mellon Expensive Computa;ons Bad Cases for Condi2onal Move ! Both values get computed ! Only makes sense when computa2ons are very simple val = Test(x) ? Hard1(x) : Hard2(x); Risky Computa;ons ! Both values get computed ! May have undesirable effects val = p ? *p : 0; Computa;ons with side effects ! Both values get computed ! Must be side-effect free val = x > 0 ? x*=7 : x+=3; NB: the ‘condi;onal move’ structure with memory access poten;ally results in vulnerabili;es. See for example, ‘Spectre’ and ‘Meltdown’ Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 20 Advantages of the ‘condi2onal move’ ! In a nutshell, ‘speed’ ! Register value is either wrinen over or not, that is the only difference between taken or not taken ! No change in program flow ! Penalty for branch miss predic;on is low ! Overhead of execu;ng ‘both paths’ of condi;on low ! Just a slot in instruc;on pipeline Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 21 Carnegie Mellon Today ! Control: Condi2on codes ! Condi2onal branches ! Loops Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 22 Carnegie Mellon C Code do Body while (Test-expr); Goto Version loop: Body t = Test-expr; if (t) goto loop General “Do-While” Transla2on ! Body: { Statement1; Statement2; … Statementn; } Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 23 Do-while example C code long dw_loop(long x) { long y = x*x; long *p = &x; long n = 2*x; do { x += y; (*p)++; n–; } while(n > 0); return x; } Assembly code 1. dw_loop: 2. movq %rdi, %rax 3. movq %rdi, %rcx 4. imulq %rdi, %rcx 5. leaq (%rdi,%rdi), %rdx 6. .L2: 7. leaq 1(%rcx,%rax), %rax 8. subq $1, %rdx 9. testq %rdx, %rdx 10.jg .L2 11.rep; ret Here’s ‘x’, but then what happens? Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 24 Carnegie Mellon C Code while (Test-expr) Body Goto Version goto test; loop: Body test: t = Test-expr; if (t) goto loop; General “While” Transla2on (1 of 2) – jump to middle version ! Body: { Statement1; Statement2; … Statementn; } Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 25 While ‘jump to middle’ example C code Long wloop(long a, long b) { long result = ………; while (…………) { result = …………; a = …………; } return result; } Assembly code 1. wloop: 2. movl $1, %eax 3. Jmp .L2 4. .L3: 5. Leaq (%rdi,%rsi), %rdx 6. imulq %rdx, %rax 7. Addq $1, %rdi 8. .L2 9. Cmpq %rsi, %rdi 10.jl .L3 11.rep; ret Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 26 Carnegie Mellon While version while (Test) Body Do-While Version if (!Test) goto done; do Body while(Test); done: General “While” Transla2on (2 of 2) – guarded do ! “Do-while” conversion ! Used with –O1 Goto Version if (!Test) goto done; loop: Body if (Test) goto loop; done: Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 27 While ‘guarded do’ example C code Long w2loop(long a, long b) { long result = ………; while (…………) { result = …………; a = …………; } return result; } Assembly code 1. w2loop: 2. testq %rsi, %rsi 3. jle .L8 4. movq %rsi, %rax 5. .L7: 6. imulq %rdi, %rax 7. subq %rdi, %rsi 8. testq %rsi, %rsi 9. jg .L7 10.rep; ret 11. .L8: 12.movq %rsi, %rax 13.ret Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 28 For loop for (init-expr; test-expr; update-expr) body-statement Jump-to-middle form init-expr; goto test; loop: body-statement; update-expr; test: t = test-expr; if (t) goto loop; Guarded-do form init-expr; t = test-expr; if (!t) goto done; loop: body-statement; update-expr; t = test-expr; if (t) goto loop; done: Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 29 While ‘guarded do’ example C code long something(long x) { long val = 0; long i; for (…………) { ………… } return val; } Assembly code 1. something: 2. movl $64, %edx 3. movl $0, %eax 4. .Loop: 5. movq %rdi, %rcx 6. andl $1, %ecx 7. addq %rax, %rax 8. orq %rcx, %rax 9. shrq %rdi 10.Subq $1, %rdx 11.jne .Loop 12.rep; ret Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 30 Carnegie Mellon Summarizing ! C Control ! if-then-else ! do-while ! while, for ! Assembler Control ! Condi;onal jump ! Condi;onal move ! Indirect jump (via jump tables) ! Compiler generates code sequence to implement more complex control ! Standard Techniques ! Loops converted to do-while or jump-to-middle form
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