Outline. Review: Assembly/Machine Code View. Processor State (x86-64, Par2al) Condi2on Codes (Explicit Se^ng: Compare) Condi2on Codes (Implicit Se^ng)

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1 Outline Machine- Level Representa2on: Control CSCI 2021: Machine Architecture and Organiza2on Pen- Chung Yew Department Computer Science and Engineering University of Minnesota Control: Condi2on codes Condi2onal branches Loops Switch Statements With Slides from Bryant, O Hallaron 2 Review: Assembly/Machine Code View Processor State (x86-64, Par2al) CPU (Central Processing Unit) PC Programmer- Visible State PC: Program counter Address of next instruc2on Called %rip in x86-64 file Heavily used program data Condi2on codes s Condi2on Codes Store status informa2on about most recent arithme?c or logical opera?on Used for condi?onal branching Addresses Data Instruc2ons Memory Memory Code Data Stack Byte addressable array Code and user data Stack to support procedures 3 Informa?on about currently execu?ng program Temporary data (, ) Loca?on of run?me stack ( %rsp ) Loca?on of current code control point ( %rip, ) Status of recent tests ( CF, ZF, SF, OF ) Current stack top s %r8 %rbx %r9 %rcx %r10 %r11 %r12 %r13 %rsp %r14 %rbp %r15 %rip Instruc2on pointer CF ZF SF OF Condi2on codes 4 Condi2on Codes (Implicit Se^ng) Condi2on Codes (Explicit Se^ng: Compare) Single bit registers CF Carry Flag (for unsigned) SF Sign Flag (for signed) ZF Zero Flag OF Overflow Flag (for signed) 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 5 Explicit Se^ng by Compare Instruc2on cmpq Src2, Src1 cmpq b,a like compu?ng a-b without sepng 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) 6 1

2 Condi2on Codes (Explicit Se^ng: Test) Reading Condi2on Codes Explicit Se^ng by Test instruc2on testq Src2, Src1 testq b,a like compu?ng a&b without sepng des?na?on SetX Instruc2ons Set low- order byte of des?na?on to 0 or 1 based on combina?ons of condi?on codes Does not alter remaining 7 bytes 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 7 SetX Condi2on Descrip2on sete ZF Equal / Zero setne ~ZF Not Equal / Not Zero sets SF Nega2ve setns ~SF Nonnega2ve 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) 8 x86-64 Integer s %rbx %rcx %rsp %rbp %al %bl %cl %dl %sil %dil %spl %bpl Can reference low- order byte %r8 %r9 %r10 %r11 %r12 %r13 %r14 %r15 %r8b %r9b %r10b %r11b %r12b %r13b %r14b %r15b 9 Reading Condi2on Codes (Cont.) SetX Instruc2ons: Set single byte based on combina?on of condi?on codes One of addressable byte registers Does not alter remaining bytes Typically use movzbl to finish job 32- bit instruc?ons also set upper 32 bits to 0 int gt (long x, long y) urn x > y; cmpq, # Compare x:y setg %al # Set when > movzbl %al, %eax # Zero rest of 10 Announcement 2/12/2016 Outline Homework assignment #2 will be issued today, due Wednesday 2/24/2016 before the class Bomb Lab has been issued on Wednesday 2/10/2016, due 11:55pm Friday 2/26/2016 Control: Condi2on codes Condi2onal branches Loops Switch Statements

3 Jumping jx Instruc2ons Jump to different part of code depending on condi?on codes jx Condi2on Descrip2on jmp 1 Uncondi2onal je ZF Equal / Zero jne ~ZF Not Equal / Not Zero js SF Nega2ve jns ~SF Nonnega2ve 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) 13 Condi2onal Branch Example (Old Style) Genera?on gcc Og -S fno-if-conversion control.c control.c long absdiff (long x, long y) long result; if (x > y) result = x-y; else result = y-x; absdiff: cmpq, # x:y jle.l4 #L4 is a label movq, subq, #urn.l4: # x <= y movq, subq, #urn 14 Expressing with Goto Code C allows goto statement Jump to posi?on designated by a label long absdiff (long x, long y) long result; if (x > y) result = x-y; else result = y-x; 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: 15 General Condi2onal Expression Transla2on (Using Branches) val = Test? Then_Expr : Else_Expr; val = x>y? x-y : y-x; ntest =!Test; if (ntest) goto Else; val = Then_Expr; goto Done; Else: val = Else_Expr; Done: Create separate code regions for then & else expressions Execute appropriate one 16 Using Condi2onal Moves result = Then_Expr; eval = Else_Expr; nt =!Test; if (nt) result = eval; Condi2onal Move Instruc?ons Instruc?on supports: val = Test? Then_Expr : Else_Expr; if (Test) Dest! Src Supported in post x86 processors GCC tries to use them But, only when known to be safe Why? Branches are very disrup?ve to instruc?on flow through pipelines Condi?onal moves do not require control transfer 17 Condi2onal Move Example long absdiff (long x, long y) long result; if (x > y) result = x-y; else result = y-x; absdiff: movq, # x subq, # result = x-y movq, subq, # eval = y-x cmpq, # x:y cmovle, # if <=, result = eval 3

4 Bad Cases for Condi2onal Move Outline Expensive Computa2ons val = Test(x)? Hard1(x) : Hard2(x); Control: Condi2on codes Both values get computed Only makes sense when computa?ons are very simple Risky Computa2ons val = p? *p : 0; Condi2onal branches Loops Switch Statements Both values get computed May have undesirable effects Computa2ons with side effects val = x > 0? x*=7 : x+=3; Both values get computed Must be side- effect free Do- While Loop Example Do- While Loop Compila2on long pcount_do (unsigned long x) do while (x); Count number of 1 s in argument x long pcount_goto (unsigned long x) if(x) goto loop; Use condi?onal branch to either con?nue looping or to exit loop long pcount_goto (unsigned long x) if(x) goto loop; result movl $0, %eax # result = 0.L2: # movq, andl $1, %edx # t = x & 0x1 addq, # result += t shrq # x >>= 1 jne.l2 # if (x) goto loop 22 General Do- While Transla2on General While Transla2on #1 do while (Test); : Statement 1 ; Statement 2 ; Statement n ; if (Test) goto loop Jump- to- middle transla?on Used with -Og While version while (Test) goto test; test: if (Test) goto loop; done:

5 While Loop Example #1 General While Transla2on #2 long pcount_while (unsigned long x) while (x) Compare to do- while version of func?on Jump to Middle long Version pcount_goto_jtm (unsigned long x) goto test; test: if(x) goto loop; Ini?al goto starts loop at test 25 While version while (Test) Do- While Version if (!Test) goto done; do while(test); done: Do- while conversion Used with O1 if (!Test) goto done; if (Test) goto loop; done: While Loop Example #2 long pcount_while (unsigned long x) while (x) Compare to do- while version of func?on Do- While Version long pcount_goto_dw (unsigned long x) if (!x) goto done; if(x) goto loop; done: Ini?al condi?onal guards entrance to loop 27 For Loop Form General Form for (Init; Test; Update ) #define WSIZE 8*sizeof(int) long pcount_for (unsigned long x) size_t i; for (i = 0; i < WSIZE; i++) unsigned bit = (x >> i) & 0x1; result += bit; Init i = 0 Test i < WSIZE Update i++ unsigned bit = (x >> i) & 0x1; result += bit; For Loop " While Loop For- While Conversion For Version for (Init; Test; Update ) Init; while (Test ) Update; While Version 29 Init i = 0 Test i < WSIZE Update i++ unsigned bit = (x >> i) & 0x1; result += bit; long pcount_for_while (unsigned long x) size_t i; i = 0; while (i < WSIZE) unsigned bit = (x >> i) & 0x1; result += bit; i++; 5

6 Outline Control: Condi2on codes Condi2onal branches Loops Switch Statements 32 long switch_eg (long x, long y, long z) switch(x) case 1: w = y*z; case 2: /* Fall Through */ case 3: case 5: case 6: w -= z; default: w = 2; urn w; Switch Statement - Example Mul?ple case labels Here: 5 & 6 Fall through cases Here: 2 Missing cases Here: 4 33 Jump Table Structure Switch Statement Example Switch Form switch(x) case val_0: Block 0 case val_1: Block 1 case val_n-1: Block n 1 Transla2on (Extended C) goto *JTab[x]; jtab: Jump Table Targ0 Targ1 Targ2 Targn-1 Jump Targets Targ0: Code Block 0 Targ1: Targ2: Targn-1: Code Block 1 Code Block 2 Code Block n 1 long switch_eg(long x, long y, long z) switch(x) urn w; Setup: switch_eg: movq, %rcx cmpq $6, # x:6 ja.l8 jmp *.L4(,,8) What range of values takes default? Note that w not ini?alized here Argument z Switch Statement Example Assembly Setup Explana2on long switch_eg(long x, long y, long z) switch(x) urn w; Setup: Indirect jump switch_eg: movq, %rcx cmpq $6, # x:6 ja.l8 # Use default jmp *.L4(,,8) # goto *JTab[x] Jump table.section.rodata.align 8.L4:.quad.L8 # x = 0.quad.L3 # x = 1.quad.L5 # x = 2.quad.L9 # x = 3.quad.L8 # x = 4.quad.L7 # x = 5.quad.L7 # x = 6 36 Table Structure Each target requires 8 bytes Base address at.l4 Jumping Direct: jmp.l8 Jump target is denoted by label.l8 Jump table Indirect: jmp *.L4(,,8) Start of jump table:.l4 Must scale by factor of 8 (addresses are 8 bytes) Fetch target from effec?ve Address.L4 + x*8 Only for 0 x 6.section.rodata.align 8.L4:.quad.L8 # x = 0.quad.L3 # x = 1.quad.L5 # x = 2.quad.L9 # x = 3.quad.L8 # x = 4.quad.L7 # x = 5.quad.L7 # x =

7 Jump Table Jump table switch(x) case 1: //.L3.section.rodata.align 8.L4:.quad.L8 # x = 0.quad.L3 # x = 1.quad.L5 # x = 2.quad.L9 # x = 3.quad.L8 # x = 4.quad.L7 # x = 5.quad.L7 # x = 6 w = y*z; case 2: //.L5 /* Fall Through */ case 3: //.L9 case 5: case 6: //.L7 w -= z; default: //.L8 w = 2; Code Blocks (x == 1) switch(x) case 1: //.L3 w = y*z; Argument z.l3: movq imulq, # y, # y*z 39 Handling Fall- Through Code Blocks (x == 2, x == 3) switch(x) case 2: /* Fall Through */ case 3: case 2: goto merge; case 3: w = 1; merge: 40 switch(x) case 2: /* Fall Through */ case 3:.L5: # Case 2 movq, cqto #convert to oct word idivq %rcx # y/z jmp.l6 # goto merge.l9: # Case 3 movl $1, %eax # w = 1.L6: # merge: addq %rcx, # w += z Argument z 41 Code Blocks (x == 5, x == 6, default) switch(x) case 5: //.L7 case 6: //.L7 w -= z; default: //.L8 w = 2;.L7: # Case 5,6 movl $1, %eax # w = 1 subq, # w -= z.l8: # Default: movl $2, %eax # 2 Argument z 42 Summarizing C Control if- then- else do- while while, for switch 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 Large switch statements use jump tables Sparse switch statements may use decision trees (if- elseif- elseif- else) 43 7

8 Overview What We Have Learned Control: Condi?on codes Condi?onal branches & condi?onal moves Loops Switch statements What Comes Next Stack Call / urn Procedure call discipline 44 8