Machine-Level Programming III: Procedures
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1 Machine-Level Programming III: Procedures CSE 238/2038/2138: Systems Programming Instructor: Fatma CORUT ERGİN Slides adapted from Bryant & O Hallaron s slides
2 Mechanisms in Procedures Passing control To beginning of procedure code Back to return point Passing data Procedure arguments Return value Memory management Allocate during procedure execution Deallocate upon return Mechanisms all implemented with machine instructions x86-64 implementation of a procedure uses only those mechanisms required P( ) { y = Q(x); print(y) int Q(int i) { int t = 3*i; int v[10]; return v[t]; 2
3 Mechanisms in Procedures Passing control To beginning of procedure code Back to return point Passing data Procedure arguments Return value Memory management Allocate during procedure execution Deallocate upon return Mechanisms all implemented with machine instructions x86-64 implementation of a procedure uses only those mechanisms required P( ) { y = Q(x); print(y) int Q(int i) { int t = 3*i; int v[10]; return v[t]; 3
4 Mechanisms in Procedures Passing control To beginning of procedure code Back to return point Passing data Procedure arguments Return value Memory management Allocate during procedure execution Deallocate upon return Mechanisms all implemented with machine instructions x86-64 implementation of a procedure uses only those mechanisms required P( ) { y = Q(x); print(y) int Q(int i) { int t = 3*i; int v[10]; return v[t]; 4
5 Mechanisms in Procedures Passing control To beginning of procedure code Back to return point Passing data Procedure arguments Return value Memory management Allocate during procedure execution Deallocate upon return Mechanisms all implemented with machine instructions x86-64 implementation of a procedure uses only those mechanisms required P( ) { y = Q(x); print(y) int Q(int i) { int t = 3*i; int v[10]; return v[t]; 5
6 Today Procedures Stack Structure Calling Conventions Passing control Passing data Managing local data Illustration of Recursion 6
7 x86-64 Stack Region of memory managed with stack discipline Memory viewed as array of bytes. Different regions have different purposes. 7
8 x86-64 Stack Region of memory managed with stack discipline Stack Bottom Stack Pointer: %rsp Stack Top 8
9 x86-64 Stack Region of memory managed with stack discipline Grows toward lower addresses Stack Bottom Increasing Addresses Register %rsp contains lowest stack address address of top element Stack Pointer: %rsp Stack Grows Down Stack Top 9
10 x86-64 Stack: Push pushq Src Fetch operand at Src val Decrement %rsp by 8 Write operand at address given by %rsp Stack Bottom Increasing Addresses Stack Pointer: %rsp Stack Grows Down Stack Top 10
11 x86-64 Stack: Push pushq Src Fetch operand at Src val Decrement %rsp by 8 Write operand at address given by %rsp Stack Bottom Increasing Addresses Stack Pointer: %rsp -8 Stack Grows Down Stack Top 11
12 x86-64 Stack: Push popq Dest Read value at address given by %rsp Increment %rsp by 8 Store value at Dest (usually a register) Stack Bottom Increasing Addresses Stack Pointer: %rsp val Stack Grows Down Stack Top 12
13 x86-64 Stack: Pop popq Dest Read value at address given by %rsp Increment %rsp by 8 Store value at Dest (usually a register) Stack Bottom Increasing Addresses Stack Pointer: %rsp val Stack Grows Down Stack Top 13
14 x86-64 Stack: Pop popq Dest Read value at address given by %rsp Increment %rsp by 8 Store value at Dest (usually a register) Stack Bottom Increasing Addresses val Stack Pointer: %rsp +8 val Stack Grows Down Stack Top 14
15 x86-64 Stack: Pop popq Dest Read value at address given by %rsp Increment %rsp by 8 Store value at Dest (usually a register) Stack Bottom Increasing Addresses Stack Pointer: %rsp +8 val Stack Grows Down Stack Top 15
16 Today Procedures Stack Structure Calling Conventions Passing control Passing data Managing local data Illustration of Recursion 16
17 Code Examples void multstore(long x, long y, long *dest) { long t = mult2(x, y); *dest = t; <multstore>: : push %rbx # Save %rbx : mov %rdx,%rbx # Save dest : callq <mult2> # mult2(x,y) : mov %rax,(%rbx) # Save at dest 40054c: pop %rbx # Restore %rbx 40054d: retq # Return long mult2(long a, long b) { long s = a * b; return s; <mult2>: : mov %rdi,%rax # a : imul %rsi,%rax # a * b : retq # Return 17
18 Procedure Control Flow Use stack to support procedure call and return Procedure call: call label Push return address on stack Jump to label Return address: Address of the next instruction right after call Example from disassembly Procedure return: ret Pop address from stack Jump to address 18
19 Control Flow Example # <multstore>: : callq <mult2> : mov %rax,(%rbx) <mult2>: : mov %rdi,%rax : retq 0x130 0x128 0x120 %rsp %rip 0x120 0x
20 Control Flow Example # <multstore>: : callq <mult2> : mov %rax,(%rbx) <mult2>: : mov %rdi,%rax : retq 0x130 0x128 0x120 0x118 %rsp %rip 0x x118 0x
21 Control Flow Example # <multstore>: : callq <mult2> : mov %rax,(%rbx) <mult2>: : mov %rdi,%rax : retq 0x130 0x128 0x120 0x118 %rsp %rip 0x x118 0x
22 Control Flow Example # <multstore>: : callq <mult2> : mov %rax,(%rbx) <mult2>: : mov %rdi,%rax : retq 0x130 0x128 0x120 0x118 %rsp %rip 0x x120 0x
23 Today Procedures Stack Structure Calling Conventions Passing control Passing data Managing local data Illustrations of Recursion & Pointers 23
24 Procedure Data Flow Registers First 6 arguments %rdi %rsi %rdx %rcx %r8 %r9 Return value Stack Arg n Arg 8 Arg 7 %rax Only allocate stack space when needed 24
25 Data Flow Examples void multstore(long x, long y, long *dest) { long t = mult2(x, y); *dest = t; <multstore>: # x in %rdi, y in %rsi, dest in %rdx : mov %rdx,%rbx # Save dest : callq <mult2> # mult2(x,y) # t in %rax : mov %rax,(%rbx) # Save at dest long mult2(long a, long b) { long s = a * b; return s; <mult2>: # a in %rdi, b in %rsi : mov %rdi,%rax # a : imul %rsi,%rax # a * b # s in %rax : retq # Return 25
26 Today Procedures Stack Structure Calling Conventions Passing control Passing data Managing local data Illustration of Recursion 26
27 Stack-Based Languages Languages that support recursion e.g., C, Pascal, Java Code must be Reentrant Multiple simultaneous instantiations of single procedure Need some place to store state of each instantiation Arguments Local variables Return pointer Stack discipline State for given procedure needed for limited time From when called to when return Callee returns before caller does Stack allocated in Frames state for single procedure instantiation 27
28 Call Chain Example yoo( ) { who(); who( ) { (); (); ( ) { (); Example Call Chain yoo who Procedure () is recursive 28
29 Stack Frames Contents Return information Local storage (if needed) Temporary space (if needed) Frame Pointer: %rbp (Optional) x Previous Frame Frame for proc Management Space allocated when enter procedure Set-up code Includes push by call instruction Deallocated when return Finish code Includes pop by ret instruction Stack Pointer: %rsp Stack Top 29
30 Example Stack yoo( ) { who(); yoo who %rbp %rsp yoo yoo 30
31 Example Stack yoo( ) { who( ) { who(); (); (); yoo who %rbp %rsp yoo yoo who 31
32 Example Stack yoo( ) { who( ) { ( ) who(); (); { (); (); yoo who %rbp %rsp yoo yoo who 32
33 Example Stack yoo( ) { who( ) { ( ) who(); (); { ( ) (); { (); (); yoo who %rbp %rsp yoo yoo who 33
34 Example Stack yoo( ) { who( ) { ( ) who(); (); { ( ) (); { (); ( ) { (); (); yoo who %rbp %rsp yoo yoo who 34
35 Example Stack yoo( ) { who( ) { ( ) who(); (); { ( ) (); { (); (); yoo who %rbp %rsp yoo yoo who 35
36 Example Stack yoo( ) { who( ) { ( ) who(); (); { (); (); yoo who %rbp %rsp yoo yoo who 36
37 Example Stack yoo( ) { who( ) { who(); (); (); yoo who %rbp %rsp yoo yoo who 37
38 Example Stack yoo( ) { who( ) { ( ) who(); (); { (); (); yoo who %rbp %rsp yoo yoo who 38
39 Example Stack yoo( ) { who( ) { who(); (); (); yoo who %rbp %rsp yoo yoo who 39
40 Example Stack yoo( ) { who(); yoo who %rbp %rsp yoo yoo 40
41 x86-64/linux Stack Frame Current Stack Frame ( Top to Bottom) Argument build: Parameters for function about to call Local variables If can t keep in registers Saved register context Old frame pointer (optional) Caller Stack Frame Return address Pushed by call instruction Arguments for this call Frame pointer %rbp (Optional) Stack pointer %rsp Caller Frame Arguments 7+ Return Addr Old %rbp Saved Registers + Local Variables Argument Build (Optional) 41
42 Example: incr long incr(long *p, long val) { long x = *p; long y = x + val; *p = y; return x; incr: movq addq movq ret (%rdi), %rax %rax, %rsi %rsi, (%rdi) Register %rdi %rsi %rax Use(s) Argument p Argument val, y x, Return value 42
43 Example: Calling incr #1 long call_incr() { long v1 = 15213; long v2 = incr(&v1, 3000); return v1+v2;... Rtn address Initial Stack Structure %rsp call_incr: subq $16, %rsp movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq 8(%rsp), %rax addq $16, %rsp ret Resulting Stack Structure... Rtn address %rsp+8 Unused %rsp 43
44 Example: Calling incr #2 long call_incr() { long v1 = 15213; long v2 = incr(&v1, 3000); return v1+v2; call_incr: subq $16, %rsp movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq 8(%rsp), %rax addq $16, %rsp ret... Rtn address Unused Register %rdi Stack Structure Use(s) &v1 %rsi 3000 %rsp+8 %rsp 44
45 Example: Calling incr #3 long call_incr() { long v1 = 15213; long v2 = incr(&v1, 3000); return v1+v2; call_incr: subq $16, %rsp movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq 8(%rsp), %rax addq $16, %rsp ret... Rtn address Unused Register %rdi Stack Structure Use(s) &v1 %rsi 3000 %rsp+8 %rsp 45
46 Example: Calling incr #4 Stack Structure long call_incr() { long v1 = 15213; long v2 = incr(&v1, 3000); return v1+v2;... Rtn address Unused %rsp+8 %rsp call_incr: subq $16, %rsp movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq 8(%rsp), %rax addq $16, %rsp ret Register Use(s) %rax Return value Updated Stack Structure... Rtn address %rsp 46
47 Example: Calling incr #5 long call_incr() { long v1 = 15213; long v2 = incr(&v1, 3000); return v1+v2; Updated Stack Structure... Rtn address %rsp call_incr: subq $16, %rsp movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq 8(%rsp), %rax addq $16, %rsp ret Register Use(s) %rax Return value Final Stack Structure... %rsp 47
48 Register Saving Conventions When procedure yoo calls who: yoo is the caller who is the callee Can register be used for temporary storage? yoo: movq $15213, %rdx call who addq %rdx, %rax ret who: subq $18213, %rdx ret Contents of register %rdx overwritten by who This could be trouble something should be done! Need some coordination 48
49 Register Saving Conventions When procedure yoo calls who: yoo is the caller who is the callee Can register be used for temporary storage? Conventions Caller Saved Caller saves temporary values in its frame before the call Callee Saved Callee saves temporary values in its frame before using Callee restores them before returning to caller 49
50 x86-64 Linux Register Usage #1 %rax Return value Also caller-saved Can be modified by procedure %rdi,..., %r9 Arguments Also caller-saved Can be modified by procedure %r10, %r11 Caller-saved Can be modified by procedure Return value Arguments Caller-saved temporaries %rax %rdi %rsi %rdx %rcx %r8 %r9 %r10 %r11 50
51 x86-64 Linux Register Usage #2 %rbx, %r12, %r13, %r14 Callee-saved Callee must save & restore %rbp Callee-saved Callee must save & restore May be used as frame pointer Can mix & match %rsp Special form of callee save Restored to original value upon exit from procedure Callee-saved Temporaries Special %rbx %r12 %r13 %r14 %rbp %rsp 51
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54 Callee-Saved Example #1 long call_incr2(long x) { long v1 = 15213; long v2 = incr(&v1, 3000); return x+v2; Initial Stack Structure... Rtn address %rsp x comes in register %rdi. We need %rdi for the call to incr. Where should be put x, so we can use it after the call to incr? 54
55 Callee-Saved Example #2 long call_incr2(long x) { long v1 = 15213; long v2 = incr(&v1, 3000); return x+v2; call_incr2: pushq %rbx subq $16, %rsp movq %rdi, %rbx movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq %rbx, %rax addq $16, %rsp popq %rbx ret Initial Stack Structure... Rtn address %rsp Resulting Stack Structure... Rtn address Saved %rbx %rsp 55
56 Callee-Saved Example #3 long call_incr2(long x) { long v1 = 15213; long v2 = incr(&v1, 3000); return x+v2; call_incr2: pushq %rbx subq $16, %rsp movq %rdi, %rbx movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq %rbx, %rax addq $16, %rsp popq %rbx ret Initial Stack Structure... Rtn address Saved %rbx %rsp Resulting Stack Structure... Rtn address Saved %rbx %rsp+8 %rsp 56
57 Callee-Saved Example #4 long call_incr2(long x) { long v1 = 15213; long v2 = incr(&v1, 3000); return x+v2; call_incr2: pushq %rbx subq $16, %rsp movq %rdi, %rbx movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq %rbx, %rax addq $16, %rsp popq %rbx ret... Rtn address Saved %rbx Stack Structure x saved in %rbx %rsp+8 %rsp A callee saved register 57
58 Callee-Saved Example #5 long call_incr2(long x) { long v1 = 15213; long v2 = incr(&v1, 3000); return x+v2; call_incr2: pushq %rbx subq $16, %rsp movq %rdi, %rbx movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq %rbx, %rax addq $16, %rsp popq %rbx ret... Rtn address Saved %rbx Unused Stack Structure x saved in %rbx %rsp+8 %rsp A callee saved register 58
59 Callee-Saved Example #6 Stack Structure long call_incr2(long x) { long v1 = 15213; long v2 = incr(&v1, 3000); return x+v2; call_incr2: pushq %rbx subq $16, %rsp movq %rdi, %rbx movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq %rbx, %rax addq $16, %rsp popq %rbx ret... Rtn address Saved %rbx %rsp+8 Unused %rsp x is safe in %rbx Return result in %rax 59
60 Callee-Saved Example #7 Stack Structure long call_incr2(long x) { long v1 = 15213; long v2 = incr(&v1, 3000); return x+v2; call_incr2: pushq %rbx subq $16, %rsp movq %rdi, %rbx movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq %rbx, %rax addq $16, %rsp popq %rbx ret... Rtn address Saved %rbx %rsp Unused Return result in %rax 60
61 Callee-Saved Example #8 long call_incr2(long x) { long v1 = 15213; long v2 = incr(&v1, 3000); return x+v2; call_incr2: pushq %rbx subq $16, %rsp movq %rdi, %rbx movq $15213, 8(%rsp) movl $3000, %esi leaq 8(%rsp), %rdi call incr addq %rbx, %rax addq $16, %rsp popq %rbx ret... Rtn address Saved %rbx Unused... Rtn address Saved %rbx Unused Initial Stack Structure %rsp Final Stack Structure %rsp 61
62 Today Procedures Stack Structure Calling Conventions Passing control Passing data Managing local data Illustration of Recursion 62
63 Recursive Function /* Recursive popcount */ long pcount_r(unsigned long x) { if (x == 0) return 0; else return (x & 1) + pcount_r(x >> 1); pcount_r: movl $0, %eax testq %rdi, %rdi je.l6 pushq %rbx movq %rdi, %rbx andl $1, %ebx shrq %rdi call pcount_r addq %rbx, %rax popq %rbx.l6: rep; ret 63
64 Recursive Function Terminal Case /* Recursive popcount */ long pcount_r(unsigned long x) { if (x == 0) return 0; else return (x & 1) + pcount_r(x >> 1); Register Use(s) Type %rdi x Argument %rax Return value Return value pcount_r: movl $0, %eax testq %rdi, %rdi je.l6 pushq %rbx movq %rdi, %rbx andl $1, %ebx shrq %rdi call pcount_r addq %rbx, %rax popq %rbx.l6: rep; ret 64
65 Recursive Function Register Save /* Recursive popcount */ long pcount_r(unsigned long x) { if (x == 0) return 0; else return (x & 1) + pcount_r(x >> 1); pcount_r: movl $0, %eax testq %rdi, %rdi je.l6 pushq %rbx movq %rdi, %rbx andl $1, %ebx shrq %rdi call pcount_r addq %rbx, %rax popq %rbx.l6: rep; ret Register Use(s) Type %rdi x Argument... Rtn address Saved %rbx %rsp 65
66 Recursive Function Call Setup /* Recursive popcount */ long pcount_r(unsigned long x) { if (x == 0) return 0; else return (x & 1) + pcount_r(x >> 1); Register Use(s) Type %rdi x >> 1 Rec. argument %rbx x & 1 Callee-saved pcount_r: movl $0, %eax testq %rdi, %rdi je.l6 pushq %rbx movq %rdi, %rbx andl $1, %ebx shrq %rdi call pcount_r addq %rbx, %rax popq %rbx.l6: rep; ret 66
67 Recursive Function Call /* Recursive popcount */ long pcount_r(unsigned long x) { if (x == 0) return 0; else return (x & 1) + pcount_r(x >> 1); Register Use(s) Type %rbx x & 1 Callee-saved %rax Recursive call return value pcount_r: movl $0, %eax testq %rdi, %rdi je.l6 pushq %rbx movq %rdi, %rbx andl $1, %ebx shrq %rdi call pcount_r addq %rbx, %rax popq %rbx.l6: rep; ret 67
68 Recursive Function Result /* Recursive popcount */ long pcount_r(unsigned long x) { if (x == 0) return 0; else return (x & 1) + pcount_r(x >> 1); Register Use(s) Type %rbx x & 1 Callee-saved %rax Return value pcount_r: movl $0, %eax testq %rdi, %rdi je.l6 pushq %rbx movq %rdi, %rbx andl $1, %ebx shrq %rdi call pcount_r addq %rbx, %rax popq %rbx.l6: rep; ret 68
69 Recursive Function Completion /* Recursive popcount */ long pcount_r(unsigned long x) { if (x == 0) return 0; else return (x & 1) + pcount_r(x >> 1); pcount_r: movl $0, %eax testq %rdi, %rdi je.l6 pushq %rbx movq %rdi, %rbx andl $1, %ebx shrq %rdi call pcount_r addq %rbx, %rax popq %rbx.l6: rep; ret Register Use(s) Type %rax Return value Return value... %rsp 69
70 Observations About Recursion Handled Without Special Consideration Stack frames mean that each function call has private storage Saved registers & local variables Saved return pointer Register saving conventions prevent one function call from corrupting another s data Unless the C code explicitly does so (e.g., buffer overflow in Lecture 9) Stack discipline follows call / return pattern If P calls Q, then Q returns before P Last-In, First-Out Also works for mutual recursion P calls Q; Q calls P 70
71 x86-64 Procedure Summary Important Points Stack is the right data structure for procedure call / return If P calls Q, then Q returns before P Recursion (& mutual recursion) handled by normal calling conventions Can safely store values in local stack frame and in callee-saved registers Put function arguments at top of stack Result return in %rax Pointers are addresses of values On stack or global Caller Frame %rbp (Optional) %rsp Arguments 7+ Return Addr Old %rbp Saved Registers + Local Variables Argument Build 71
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