Setting & Examining Condition Codes. int gt(long x, long y) { return x > y; } int has_nonzero_masked(long x, long mask) { return!!

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1 Setting & Examining Condition Codes int has_nonzero_masked(long x, long mask) urn!!(x & mask); # x in %rdi, y in %rsi, urn value in %eax has_nonzero_masked: xorl %eax, %eax testq %rsi, %rdi setne %al int has_nonzero_omasked(long x, long mask) urn!!(x mask); # x in %rdi, y in %rsi, urn value in %eax has_nonzero_omasked: orq %rsi, %rdi setne %al movzbl %al, %eax int gt(long x, long y) urn x > y; # x in %rdi, y in %rsi, urn value in %eax gt: xorl %eax, %eax cmpq %rsi, %rdi setg %al Conditional moves long max(long x, long y) urn (x > y)? x : y; # x in %rdi, y in %rsi, urn value in %rax max: cmpq %rdi, %rsi cmovl %rdi, %rsi movq %rsi, %rax

2 Branching code void smax(long x, long *vp) long v = *vp; if (v < x) *vp = x; # x in %rdi, vp in %rsi smax: cmpq %rdi, (%rsi) jge.l6 movq %rdi, (%rsi).l6: rep ; int winx = 0; long maxw(long x, long y) if (x > y) winx++; urn x; else urn y; Do-while loop template long fib_dw(int n) long fprev = 1L; long fcurr = 1L; do long fnext = fprev+fcurr; fprev = fcurr; fcurr = fnext; n--; while (n > 2); urn fcurr; # n in %rdi, urn value in %rax fib_dw: movl $1, %ecx movl $1, %edx.l13: leaq (%rcx,%rdx), %rax decl %edi movq %rdx, %rcx cmpl $2, %edi movq %rax, %rdx jg.l13 rep ; # x in %rdi, y in %rsi, urn value in %rax maxw: cmpq %rsi, %rdi jle.l9 incl winx(%rip) movq %rdi, %rsi.l9: movq %rsi, %rax

3 Jump into middle loop template long fib_while(int n) long fprev = 1L; long fcurr = 1L; while (n > 2) long fnext = fprev+fcurr; fprev = fcurr; fcurr = fnext; n--; urn fcurr; # n in %rdi, urn value in %rax fib_while: movl $1, %ecx movl $1, %edx jmp.l22.l23: leaq (%rcx,%rdx), %rax decl %edi movq %rdx, %rcx movq %rax, %rdx.l22: cmpl $2, %edi jg.l23 movq %rdx, %rax While compiled using do-while template long fib_while2(int n) long fprev = 1L; long fcurr = 1L; int i = 2; while (i < n) long fnext = fprev+fcurr; fprev = fcurr; fcurr = fnext; i++; urn fcurr; # n in %rdi, urn value in %rax fib_while2: cmpl $2, %edi movl $1, %esi movl $1, %ecx jle.l29 leal -2(%rdi), %edx.l27: leaq (%rsi,%rcx), %rax decl %edx movq %rcx, %rsi movq %rax, %rcx jne.l27.l29: movq %rcx, %rax

4 For loop long fib_for(long n) long fprev = 1L; long fcurr = 1L; int i; for (i = 2; i < n; i++) long fnext = fprev + fcurr; fprev = fcurr; fcurr = fnext; urn fcurr; # n in %rdi, urn value in %rax fib_for: cmpq $2, %rdi movl $1, %esi movl $1, %ecx movl $2, %edx jle.l36.l34: leaq (%rsi,%rcx), %rax incl %edx movq %rcx, %rsi movq %rax, %rcx movslq %edx,%rax cmpq %rdi, %rax jl.l34.l36: movq %rcx, %rax Switch statement example long switch_eg(long x, long y, long z) long w = 1; switch(x) case 1: w = y*z; case 2: w = y/z; /* Fall Through */ case 3: w += z; case 5: case 6: w -= z; default: w = 2; urn w;

5 Switch Code # x in %rdi, y in %rsi, z in %rdx # urn value in %rax switch_eg: cmpq $6, %rdi movq %rdx, %rcx movl $1, %r8d ja.l44 jmp *.L45(,%rdi,8).L44: movl $2, %r8d.l40: movq %rsi, %rax cqto idivq %rcx movq %rax, %r8.l41: addq %rcx, %r8.l43: subq %rdx, %r8.l39: movq %rsi, %r8 imulq %rdx, %r8.section.rodata.l45:.quad.l44.quad.l39.quad.l40.quad.l41.quad.l44.quad.l43.quad.l43

6 Binary view of switch code (gdb) disassemble switch_eg Dump of assembler code for function switch_eg: 0x <+0>: cmp $0x6,%rdi 0x <+4>: mov %rdx,%rcx 0x <+7>: mov $0x1,%r8d 0x040068d <+13>: ja 0x <+22> 0x040068f <+15>: jmpq *0x400910(,%rdi,8) 0x <+22>: mov $0x2,%r8d 0x040069c <+28>: mov %r8,%rax 0x040069f <+31>: q 0x04006a0 <+32>: mov %rsi,%rax 0x04006a3 <+35>: cqto 0x04006a5 <+37>: idiv %rcx 0x04006a8 <+40>: mov %rax,%r8 0x04006ab <+43>: add %rcx,%r8 0x04006ae <+46>: mov %r8,%rax 0x04006b1 <+49>: q 0x04006b2 <+50>: sub %rdx,%r8 0x04006b5 <+53>: mov %r8,%rax 0x04006b8 <+56>: q 0x04006b9 <+57>: mov %rsi,%r8 0x04006bc <+60>: imul %rdx,%r8 0x04006c0 <+64>: mov %r8,%rax 0x04006c3 <+67>: q Implementation of jump table (gdb)x/7a 0x x400910: 0x <+22> 0x4006b9 <+57> 0x400920: 0x4006a0 <+32> 0x4006ab <+43> 0x400930: 0x <+22> 0x4006b2 <+50> 0x400940: 0x4006b2 <+50> unix>objdump asm-cntl.64x -s --section=.rodata Contents of section.rodata: b a ab b b

7 Alternate compilation of switch long sparse_switch(long x, long y, long z) long w = 1; switch(x) case 100: w = y*z; case 200: w = y/z; /* Fall Through */ case 300: w += z; case 500: case 600: w -= z; default: w = 2; urn w; # x in %rdi, y in %rsi, z in %rdx # urn value in %rax sparse_switch: cmpq $300, %rdi movq %rdx, %rcx movl $1, %r8d je.l50 jg.l54 cmpq $100, %rdi je.l48 cmpq $200, %rdi je.l49.l53: movl $2, %r8d.l54: cmpq $500, %rdi je.l52 cmpq $600, %rdi jne.l53.l52: subq %rcx, %r8.l49: movq %rsi, %rax cqto idivq %rcx movq %rax, %r8.l50: addq %rcx, %r8.l48: movq %rsi, %r8 imulq %rdx, %r8

8 In-class puzzle long puzzle_loop(long x, long y, long z) long t1 = ; long t2 = ; long i; for (i = ; i > ; i += ) long temp = t1; t1 = t2; t2 = temp; urn t1; # x in %rdi, y in %rsi, z in %rdx # urn value in %rax puzzle_loop: leaq (%rdi,%rsi), %rcx imulq %rsi, %rdi movl $456, %esi jmp.l62.l63: movq %rcx, %rax movq %rdi, %rcx addq %rcx, %rsi movq %rax, %rdi.l62: cmpq %rdx, %rsi jg.l63 movq %rcx, %rax