1 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Carnegie Mellon Machine-Level Programming IV Control Comp 21000: Introduction to Computer Organization & Systems March 2016 Systems book chapter 3* Ithaca College

2 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Ithaca College Today Control: Condition codes Conditional branches Loops Switch Statements

3 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Processor State (x86-64, Partial) Information about currently executing program  Temporary data ( %rax, … )  Location of runtime stack ( %rsp )  Location of current code control point ( %rip, … )  Status of recent tests ( CF, ZF, SF, OF ) %rip Registers Current stack top Instruction pointer CFZFSFOF Condition codes %rsp %r8 %r9 %r10 %r11 %r12 %r13 %r14 %r15 %rax %rbx %rcx %rdx %rsi %rdi %rbp Ithaca College

4 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Condition Codes (Implicit Setting) Single bit registers  CF Carry Flag (for unsigned)SF Sign Flag (for signed)  ZF Zero FlagOF Overflow Flag (for signed) Implicitly set (think of it as side effect) by arithmetic operations 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) Not set by leaq instruction Ithaca College

5 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Condition Codes Examples: assume %rax contains 20 addl $10, %raxCF  0 ZF  0 SF  0 OF  0 subl $50, %raxCF  0 ZF  0 SF  1 OF  0 Note that these flags are set just like we talked about in chapter 2!

6 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Condition Codes Single Bit Registers CF Carry Flag SF Sign Flag ZF Zero Flag OF Overflow Flag Also implicitly Set By logical Operations xorl Src,Dest C analog: t = a ^ b(a = Src, b = Dest)  CF set to 0  ZF set if t == 0  SF set if t < 0  OF set to 0  For shift operations:  CF set to last bit shifted out  OF set to 0

7 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Condition Codes (Implicit Setting) There are no Boolean types in assembly language. There is no concept of Boolean operators in assembly language. So what is there?  Bits  All we can do is look at bits. What those bits mean is determined by the context of the program.  Instructions set flags.  Other instructions take action based on the bits in those flags.  Don’t think about it as comparison as in the C language sense.

8 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Condition Codes (Explicit Setting: Compare) Explicit Setting by Compare Instruction  cmpq Src2, Src1  cmpq b,a like computing a-b without setting destination  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) Note: notice that the operands are in reverse order; b is the first operand, but we compute a – b! Note 2: there are also cmpw and cmpb instructions. Ithaca College

9 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Example cmpl %rax, %rcx do %rcx - %rax then set condition codes NO other action taken! %rax 0000 %rcx %rdx 0000 %rbx 0000 %rsi %rdi %rsp %rbp CFZFSFOF Condition codes ??

10 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Example cmpl %rax, %rcx do %rcx - %rax then set condition codes NO other action taken! %rax 0000 %rcx %rdx 0000 %rbx 0000 %rsi %rdi %rsp %rbp CFZFSFOF Condition codes 0000

11 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Condition Codes (Explicit Setting: Test) Explicit Setting by Test instruction  testq Src2, Src1  testq b,a like computing a&b without setting destination  Sets condition codes based on value of Src1 & Src2  Useful to have one of the operands be a mask  CF set to 0  ZF set when a&b == 0  SF set when a&b < 0  OF set to 0 Note: typically the same operand is repeated to test whether it is negative, zero, or positive: testl %rax, %rax sets the condition codes depending on the value in %rax Note 2: there are also testl, testw and testb instructions. Ithaca College

12 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Reading Condition Codes SetX Instructions  Set low-order byte of destination to 0 or 1 based on combinations of condition codes  Does not alter remaining 7 bytes SetXConditionDescription seteZF Equal / Zero setne~ZF Not Equal / Not Zero setsSF Negative setns~SF Nonnegative 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) setbCF Below (unsigned) Ithaca College

13 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition %rsp x86-64 Integer Registers  Can reference low-order byte %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 %al

14 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Reading condition codes Consider: setl D (SF^OF)Less (Signed <)D  (SF^OF ) First compare two numbers, a and b where both are in 2’s complement form using an arithmetic, logical, test or cmp instruction Then use setX to set a register with the result of the test cmpq%rax, %rdx setl %al puts the result in byte register %al

15 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Reading condition codes (cont) Assume a is in %rax and b in %rdx cmpq%rax, %rdx setl %al puts the result in byte register %al First instruction sets the condition code.  cmpq computes b – a  If b < a then b – a < 0 If there is no overflow, this is indicated by SF  If there is positive overflow (b – a is large), we have b – a < 0 but OF is set  If there is negative overflow (b – a is very small), we have b – a > 0 but OF is set  In either case the sign flag will indicate the opposite of the sign of the true difference. Hence, use exclusive-or of the OF and SF Second instruction sets the %al register to or depending on the value of (SF^OF) setl D ; D  (SF^OF )

16 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Reading condition codes (cont) Example: assume %rax holds 20 and %rdx holds 50 cmpq%rax, %rdx 50 – 20, SF  0, OF  0 setl %al %al  0 ^ 0 = Example: assume %rax holds 0x and %rdx holds 20 cmpq%rax, %rdx 20 – ( ), SF  1, OF  1 setl %al %al  1 ^ 1 = setq gives false; 20 is not less than setl D ; D  (SF^OF )

17 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Reading condition codes (cont) Example: assume %rax holds 20 and %rdx holds 0x cmpq%rax, %rdx ( ) - 20, SF  0, OF  1 0x xFFFFFFEC = 0x7FFFFFED (note that 7 = 0111) setl %al %al  0 ^ 1 = setq gives true; is less than 20

18 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Setg Greater (Signed) ~(SF^OF)&~ZF CFZFSFOF Condition codes ~SF&~OF If this is zero, can’t be > If the overflow flag is set, can’t be > if we did a cmpl or testl as the previous instruction. Why? First do an arith, logical, cmp or test. Then use the flags

19 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition cmpq %rsi, %rdi # Compare x:y setg %al # Set when > movzbl %al, %rax # Zero rest of %rax ret Reading Condition Codes (Cont.) SetX Instructions:  Set single byte based on combination of condition codes One of addressable byte registers  Does not alter remaining bytes  Typically use movzbl to finish job  32-bit instructions also set upper 32 bits to 0 int gt (long x, long y) { return x > y; } int gt (long x, long y) { return x > y; } RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value Ithaca College

20 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition cmpq %rsi, %rdi # Compare x:y setg %al # Set when > movzbl %al, %rax # Zero rest of %rax ret Reading Condition Codes (Cont.) int gt (long x, long y) { return x > y; } int gt (long x, long y) { return x > y; } RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value %rax %rdx %rcx %rbx %rsi %rdi %rsp %rbp %al%ah %dl%dh %cl%ch %bl%bh All 0’s: Either or Ithaca College

21 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Today Control: Condition codes Conditional branches Loops Switch Statements Ithaca College

22 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Jumping A jump instruction causes execution to jump to a new address  Really just puts the argument of the jump into the EIP  Jump destinations are usually indicated by a label  A label marks a place in code 1xorq%rax, %rax 2jmp.L1 3movq(%rax), %rdx 4.L1: popq %rdx  The instruction jmp.L1 causes program to skip line 3  jmp is an unconditional jump instruction  Note that we wouldn’t really use the code this way!

23 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Jumping A jump instruction has two types of arguments  Direct jumps. Target is part of the instruction  Use a label  Indirect jumps. Target read from a register or memory.  Use a ‘*’ followed by an operand specifier jmp *%eax or jmp *(%eax)  Any addressing mode can be used  Note that jmp is an unconditional jump instruction

24 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Jumping Other jump instructions are conditional  Either jump or continue executing at next instruction in the code  Depends on some combination of the condition codes  Names and conditions match the set instructions.  Conditional jumps can only be direct Assembler changes labels into actual memory addresses  See section of the book  Not very important now; will be very important in chapter 7

25 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Jumping jX Instructions  Jump to different part of code depending on condition codes jXConditionDescription jmp1 Unconditional jeZF Equal / Zero jne~ZF Not Equal / Not Zero jsSF Negative jns~SF Nonnegative 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) jbCF Below (unsigned) Ithaca College

26 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition jg Greater (Signed) ~(SF^OF)&~ZF CFZFSFOF Condition codes ~SF&~OF If this is zero, can’t be > If the overflow flag is set, can’t be > if we did a cmpq or testq as the previous instruction. Why? First do an arith, logical, cmp or test. Then use the flags

27 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Conditional Branch Example (Old Style) long absdiff (long x, long y) { long result; if (x > y) result = x-y; else result = y-x; return result; } long absdiff (long x, long y) { long result; if (x > y) result = x-y; else result = y-x; return result; } absdiff: cmpq %rsi, %rdi # x:y jle.L4 movq %rdi, %rax subq %rsi, %rax ret.L4: # x <= y movq %rsi, %rax subq %rdi, %rax ret Generation nori> gcc –Og -S –fno-if-conversion control.c RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value Ithaca College

28 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Expressing with Goto Code long absdiff (long x, long y) { long result; if (x > y) result = x-y; else result = y-x; return result; } 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 position designated by label long absdiff_j (long x, long y) { long result; int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } long absdiff_j (long x, long y) { long result; int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } Ithaca College Generally considered bad coding style

29 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Expressing with Goto Code C allows goto statement Jump to position designated by label long absdiff_j (long x, long y) { long result; int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } long absdiff_j (long x, long y) { long result; int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } Ithaca College absdiff: cmpq %rsi, %rdi # x:y jle.L4 movq %rdi, %rax subq %rsi, %rax ret.L4: # x <= y movq %rsi, %rax subq %rdi, %rax ret RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value

30 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Expressing with Goto Code C allows goto statement Jump to position designated by label long absdiff_j (long x, long y) { long result; int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } long absdiff_j (long x, long y) { long result; int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } Ithaca College absdiff: cmpq %rsi, %rdi # x:y jle.L4 movq %rdi, %rax subq %rsi, %rax ret.L4: # x <= y movq %rsi, %rax subq %rdi, %rax ret RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value

31 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Expressing with Goto Code C allows goto statement Jump to position designated by label long absdiff_j (long x, long y) { long result; int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } long absdiff_j (long x, long y) { long result; int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } Ithaca College absdiff: cmpq %rsi, %rdi # x:y jle.L4 movq %rdi, %rax subq %rsi, %rax ret.L4: # x <= y movq %rsi, %rax subq %rdi, %rax ret RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value

32 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Expressing with Goto Code C allows goto statement Jump to position designated by label long absdiff_j (long x, long y) { long result; int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } long absdiff_j (long x, long y) { long result; int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } Ithaca College absdiff: cmpq %rsi, %rdi # x:y jle.L4 movq %rdi, %rax subq %rsi, %rax ret.L4: # x <= y movq %rsi, %rax subq %rdi, %rax ret RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value

33 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Example 2 Download ifStmt.c gcc –O0 –o ifStmt –fno-if-conversion ifStmt.c gdb ifStmt In gdb: disass testIf1 int testIf1(int x, int y) { int result; if (x < y) result = x * 4; else result = y * 4; return result; }

34 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Example 2 continued… pushq %rbp movq %rsp, %rbp movq %rdi, -8(%rbp) movq %rsi, -16(%rbp) movq -8(%rbp), %rsi cmpq -16(%rbp), %rsi jge LBB0_2 movq -8(%rbp), %rax shlq $2, %rax movq %rax, -24(%rbp) jmp LBB0_3 LBB0_2: movq -16(%rbp), %rax shlq $2, %rax movq %rax, -24(%rbp) LBB0_3: movq -24(%rbp), %rax popq %rbp retq Setting up the runtime stack if x - y >= 0 jump if x - y >= 0 jump else calculate x=x*4 and jump else calculate x=x*4 and jump Calculate y=y*4 Set up the return value RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value int testIf1(int x, int y) { int result; if (x < y) result = x * 4; else result = y * 4; return result; }

35 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition C Code val = Test ? Then_Expr : Else_Expr ; Goto Version ntest = ! Test ; if (ntest) goto Else; val = Then_Expr ; goto Done; Else: val = Else_Expr ; Done:... ntest = ! Test ; if (ntest) goto Else; val = Then_Expr ; goto Done; Else: val = Else_Expr ; Done:... General Conditional Expression Translation (Using Branches)  Test Is an expression returning an integer  = 0 interpreted as false  ≠ 0 interpreted as true  Create separate code regions for then & else expressions  Execute appropriate one val = x>y ? x-y : y-x; Ithaca College

36 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition C Code val = Test ? Then_Expr : Else_Expr ; val = Test ? Then_Expr : Else_Expr ; Goto Version result = Then_Expr ; eval = Else_Expr ; nt = ! Test ; if (nt) result = eval; return result; result = Then_Expr ; eval = Else_Expr ; nt = ! Test ; if (nt) result = eval; return result; Using Conditional Moves Conditional Move Instructions  Instruction supports: if (Test) Dest  Src  Supported in post-1995 x86 processors  GCC tries to use them  But, only when known to be safe Why?  Branches are very disruptive to instruction flow through pipelines  Conditional moves do not require control transfer Ithaca College

37 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Conditional Move cmovX Instructions  Jump to different part of code depending on condition codes jXSynonymConditionDescription cmove S*,R*cmovzZF Equal / Zero cmovne S,Rcmovnz~ZF Not Equal / Not Zero cmovs S,RSF Negative cmovns S,R~SF Nonnegative cmovg S,Rcmovnle~(SF^OF)&~ZF Greater (Signed) cmovge S,Rcmovnl~(SF^OF) Greater or Equal (Signed) cmovl S,Rcmovnge(SF^OF) Less (Signed) cmovle S,Rcmovng(SF^OF)|ZF Less or Equal (Signed) cmova S,Rcmovnbe~CF&~ZF Above (unsigned) cmovae S,Rcmovnb~CF Above or equal (unsigned) cmovb S,RcmovnaeCF Below (unsigned) cmovbe S,RcmovnaCF | ZF Below or equal (unsigned) Ithaca College * source/destination may be 16, 32, or 64 bits (not 8). No size suffix: assembler infers the operand length based on destination register

38 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Conditional 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 # 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; } long absdiff (long x, long y) { long result; if (x > y) result = x-y; else result = y-x; return result; } RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value Ithaca College

39 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Expensive Computations Bad Cases for Conditional Move Both values get computed Only makes sense when computations are very simple val = Test(x) ? Hard1(x) : Hard2(x); Risky Computations Both values get computed May have undesirable effects (always dereference p but it may be null!) val = p ? *p : 0; Computations with side effects Both values get computed Must be side-effect free val = x > 0 ? x*=7 : x+=3; Ithaca College In general, gcc only uses conditional moves when the two expressions can be computed very easily, e.g., single instructions.

40 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Practice Problem long test (long x, long y) { long val = _________; if (_________){ if (__________){ val = ________; else val = ________; } else if (________) val = ________; return val; } long test (long x, long y) { long val = _________; if (_________){ if (__________){ val = ________; else val = ________; } else if (________) val = ________; return val; } test: leaq0(,%rdi,8), %rax testq%rsi, %rsi jle.L4 movq %rsi, %rax subq %rdi, %rax movq%rdi, %rdx andq %rsi, %rdx cmpq %rsi, %rdi cmovge%rdx, %rax ret.L4: # x <= y addq%rsi, %rdi cmpq%-2, %rsi cmovle%rdi, %rax ret Generation RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value Ithaca College

41 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Practice Problem long test (long x, long y) { long val = 8*x; if (y > 0){ if (x < y){ val = y - x; else val = x & y; } else if (y <= -2) val = x + y; return val; } long test (long x, long y) { long val = 8*x; if (y > 0){ if (x < y){ val = y - x; else val = x & y; } else if (y <= -2) val = x + y; return val; } test: leaq0(,%rdi,8), %rax testq%rsi, %rsi jle.L4 movq %rsi, %rax subq %rdi, %rax movq%rdi, %rdx andq %rsi, %rdx cmpq %rsi, %rdi cmovge%rdx, %rax ret.L4: # x <= y addq%rsi, %rdi cmpq%-2, %rsi cmovle%rdi, %rax ret Generation RegisterUse(s) %rdi Argument x %rsi Argument y %rax Return value Ithaca College

42 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Summarizing C Control  if-then-else  do-while  while, for  switch Assembler Control  Conditional jump  Conditional 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  Large switch statements use jump tables  Sparse switch statements may use decision trees (if-elseif-elseif-else) Ithaca College

43 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Summary Today  Control: Condition codes  Conditional branches & conditional moves Next Time  Loops  Switch statements  Stack  Call / return  Procedure call discipline Ithaca College