Machine-Level Programming II: Control FlowCondition CodesSetting Condition Codes (cont.)Slide 4Reading Condition CodesReading Condition Codes (Cont.)JumpingConditional Branch ExampleConditional Branch Ex. (Cont.)“Do-While” Loop Example“Do-While” Loop CompilationGeneral “Do-While” Translation“While” Loop Example #1Actual “While” Loop TranslationGeneral “While” Translation“For” Loop Exampleipwr ComputationSlide 18“For” “While”“For” Loop CompilationSwitch StatementsJump Table StructureSwitch Statement ExampleAssembly Setup ExplanationJump TableSwitch Statement CompletionObject CodeObject Code (cont.)Extracting Jump Table from BinaryDisassembled TargetsMatching Disassembled TargetsSparse Switch ExampleSparse Switch CodeSparse Switch Code StructureSummarizingMachine-Level Programming II:Control FlowMachine-Level Programming II:Control FlowTopicsTopicsCondition CodesSettingTestingControl FlowIf-then-elseVarieties of LoopsSwitch StatementsX86.2.pptCS 105“Tour of the Black Holes of Computing”– 2 –CS 105Condition CodesCondition CodesSingle-Bit RegistersSingle-Bit RegistersCF Carry Flag SF Sign FlagZF Zero Flag OF Overflow FlagImplicitly Set By Arithmetic OperationsImplicitly Set By Arithmetic Operationsaddl Src,DestC analog: t = a + b; b = t;CF set if carry out from most significant bitDetects unsigned overflow; also used for multiprecision arithmeticZF set if t == 0SF set if t < 0OF set if two’s complement overflow(a>0 && b>0 && t<0) || (a<0 && b<0 && t>=0)NotNot Set by Set by lealleal instruction instruction– 3 –CS 105Setting Condition Codes (cont.)Setting Condition Codes (cont.)Explicit Setting by Compare InstructionExplicit Setting by Compare Instructioncmpl Src2,Src1 cmpl b,a like computing a-b without setting destinationCF set if carry out from most significant bitUsed for unsigned comparisonsZF set if a == bSF set if (a-b) < 0OF set if two’s complement overflow(a>0 && b<0 && (a-b)<0) || (a<0 && b>0 && (a-b)>0)– 4 –CS 105Setting Condition Codes (cont.)Setting Condition Codes (cont.)Explicit Setting by Test instructionExplicit Setting by Test instructiontestl Src2,Src1Sets condition codes based on value of Src1 & Src2Useful to have one of the operands be a mask testl b,a like computing a&b without setting destination ZF set when a&b == 0SF set when a&b < 0– 5 –CS 105Reading Condition CodesReading Condition CodesSetX Condition Descriptionsete ZFEqual / Zerosetne ~ZFNot Equal / Not Zerosets SFNegativesetns ~SFNonnegativesetg ~(SF^OF)&~ZFGreater (Signed)setge ~(SF^OF)Greater or Equal (Signed)setl (SF^OF)Less (Signed)setle (SF^OF)|ZFLess or Equal (Signed)seta ~CF&~ZFAbove (unsigned)setb CFBelow (unsigned)SetX InstructionsSetX InstructionsSet single byte based on combinations of condition codes– 6 –CS 105Reading Condition Codes (Cont.)Reading Condition Codes (Cont.)SetX InstructionsSetX InstructionsSet single byte based on combinations of condition codesOne of 8 addressable byte registersEmbedded within first 4 integer registersDoes not alter remaining 3 bytesTypically use movzbl to finish job%eax%edx%ecx%ebx%esi%edi%esp%ebp%al%ah%dl%dh%cl%ch%bl%bhint gt (int x, int y){ return x > y;}movl 12(%ebp),%eax # eax = ycmpl %eax,8(%ebp) # Compare x : ysetg %al # al = x > ymovzbl %al,%eax # Zero rest of %eaxNote inverted ordering!Body– 7 –CS 105JumpingJumpingjX Condition Descriptionjmp 1Unconditionalje ZFEqual / Zerojne ~ZFNot Equal / Not Zerojs SFNegativejns ~SFNonnegativejg ~(SF^OF)&~ZFGreater (Signed)jge ~(SF^OF)Greater or Equal (Signed)jl (SF^OF)Less (Signed)jle (SF^OF)|ZFLess or Equal (Signed)ja ~CF&~ZFAbove (unsigned)jb CFBelow (unsigned)jX InstructionsjX InstructionsJump to different part of code depending on condition codes– 8 –CS 105Conditional Branch ExampleConditional Branch Exampleint max(int x, int y){ if (x > y) return x; else return y;}_max:pushl %ebpmovl %esp,%ebpmovl 8(%ebp),%edxmovl 12(%ebp),%eaxcmpl %eax,%edxjle L9movl %edx,%eaxL9:movl %ebp,%esppopl %ebpretBodySetUpFinish– 9 –CS 105Conditional Branch Ex. (Cont.)Conditional Branch Ex. (Cont.)movl 8(%ebp),%edx # edx = xmovl 12(%ebp),%eax # eax = ycmpl %eax,%edx # x : yjle L9 # if <= goto L9movl %edx,%eax # eax = xL9: # Done:int goto_max(int x, int y){ int rval = y; int ok = (x <= y); if (ok) goto done; rval = x;done: return rval;}Skipped when x yC allows “goto” as means of transferring controlCloser to machine-level programming styleGenerally considered bad coding style– 10 –CS 105C Codeint fact_do (int x){ int result = 1; do { result *= x; x = x-1; } while (x > 1); return result;}Goto Versionint fact_goto(int x){ int result = 1;loop: result *= x; x = x-1; if (x > 1) goto loop; return result;}“Do-While” Loop Example“Do-While” Loop ExampleUse backward branch to continue loopingOnly take branch when “while” condition holds– 11 –CS 105Goto Versionint fact_goto (int x){ int result = 1;loop: result *= x; x = x-1; if (x > 1) goto loop; return result;}“Do-While” Loop Compilation“Do-While” Loop CompilationRegistersRegisters%edx x%eax result_fact_goto:pushl %ebp # Setupmovl %esp,%ebp # Setupmovl $1,%eax # eax = 1movl 8(%ebp),%edx # edx = xL11:imull %edx,%eax # result *= xdecl %edx # x--cmpl $1,%edx # Compare x : 1jg L11 # if > goto loopmovl %ebp,%esp # Finishpopl %ebp # Finishret # FinishAssembly– 12 –CS 105C Codedo Body while (Test);Goto Versionloop: Body if (Test) goto loopGeneral “Do-While” TranslationGeneral “Do-While” TranslationBody can be any C statementTypically compound statement:Test is expression returning integer= 0 interpreted as false 0 interpreted as true{ Statement1; Statement2; … Statementn;}– 13 –CS 105C Codeint fact_while (int x){ int result = 1; while (x > 1) { result *= x; x = x-1; }; return result;}First Goto Versionint fact_while_goto (int x){ int result = 1;loop: if (!(x > 1)) goto done; result *= x; x = x-1; goto loop;done: return result;}“While” Loop Example #1“While” Loop Example #1Is this code equivalent to the do-while version?Must jump out of loop if test fails– 14 –CS 105C Codeint fact_while(int x){ int result = 1; while (x > 1) { result *= x; x = x-1; }; return result;}Second Goto Versionint fact_while_goto2 (int x){ int result = 1; if (!(x > 1)) goto
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