Stack Frames. September 2, Indiana University. Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 15

Transcription

1 Stack Frames Geoffrey Brown Bryce Himebaugh Indiana University September 2, 2016 Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 15

2 Outline Preserving Registers Saving and Restoring Registers ABI Rules for Caller and Callee Saved Registers Stack Frames Access Within the Stack Parameter Passing Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 15

3 Recursive Procedure Example As an example, consider a simple recursive function that counts the number of 1 s in an unsigned integer parameter: int ones( unsigned int i) { // if operand is zero, return 0 if (i) { // bit 0 plus recursive call return (i & 1) + ones(i >> 1); } else { return 0; } } This procedure needs the value of i both before and after the recursive call, therefore, it must move i to a callee saved register (eg r4) Geoffrey Brown, Bryce Himebaugh 2015 Preserving Registers September 2, / 15

4 Preserving r4 and lr Figure 91 The ones implementation uses r4 to store i lr contains the return address Both must be pushed onto the stack before they are modified Stack before calling function lr sp r4 Stack grows down Geoffrey Brown, Bryce Himebaugh 2015 Preserving Registers September 2, / 15

5 Push ex:push The push operation can write one or more registers to the stack push write lr and r4 to the stack PRE r4 = 0x lr = 0x sp = 0x Address Data (PRE) Data (POST) 0x x x (pre) 0x x x x80010 (empty) 0x (post) 0x8000C (empty) 0x POST r4 = 0x lr = 0x sp = 0x c Geoffrey Brown, Bryce Himebaugh 2015 Preserving Registers September 2, / 15

6 Pop Example: 92 The pop operation can read one or more registers from the stack stack pop {r4,pc} PRE r4 = 0x pc = 0x sp = 0x C Address Data 0x x (post) 0x x x x (pre) 0x8000C read pc and r4 from the stack POST r4 = 0x pc = 0x sp = 0x Geoffrey Brown, Bryce Himebaugh 2015 Preserving Registers September 2, / 15

7 Skeleton for ones procedure ones: push {r4, save registers mov r4, move argument to callee saved register bl recursive call pop {r4, restore r4 and return Geoffrey Brown, Bryce Himebaugh 2015 Preserving Registers September 2, / 15

8 Core Registers and AAPCS Usage Table 91 Register Special Role in Procedure Call Standard r15 pc The Program Counter r14 lr The Link Register r13 sp The Stack Register r12 ip The Intra-Procedure-call scratch register r11 Variable-register 8 r10 Variable-register 7 r9 Platform register (Variable-register 6) r8 Variable-register 5 r7 Variable-register 4 r6 Variable-register 3 r5 Variable-register 4 r4 Variable-register 1 r3 Argument/scratch register 4 r2 Argument/scratch register 3 r1 Argument/scratch register 2 r0 Argument/scratch register 1 Geoffrey Brown, Bryce Himebaugh 2015 Preserving Registers September 2, / 15

9 Roles of Registers The first four registers r0-r3 are used to pass arguments into a subroutine and return a result from a function They may also be used to hold intermediate values (between subroutine calls) Register r12 (ip) is used by the linker to implement long function calls, it may also be used to hold intermediate values Registers r4-r8 and r10-r11 may be used by a subroutine to hold its local values r9 may have a platform specific use, otherwise it may be used to hold local values A subroutine (callee) must preserve the contents of registers r4-r8, r9, and r10-r11 Thus, the caller saved registers are r0-r3 and r12 All the other registers below r13 are callee saved The roles of r13-r15 are defined Geoffrey Brown, Bryce Himebaugh 2015 Preserving Registers September 2, / 15

10 Stack Layout Figure 92 Stack before calling function Stack after calling function Parameter area Caller Parameter area Caller SP Stack grows down Saved link register(lr) Callee saved registers Local storage Callee SP Stack grows down Geoffrey Brown, Bryce Himebaugh 2015 Stack Frames September 2, / 15

11 Stack Frame Example Example 93 extern foo( int *); int alloc( int i) { int local[15]; return i + foo( local); } prolog push {r4, save sub sp, allocate body movs r4, save i add r0, sp, compute pointer bl foo adds r0, epilog add sp, deallocate array pop {r4, restore Geoffrey Brown, Bryce Himebaugh 2015 Stack Frames September 2, / 15

12 Accessing Words in the Stack ldr rt, [sp, rt = *( sp+( imm8 <<2)) str rn, [sp, *( sp+( imm8 <<2)) imm8 is an 8-bit unsigned constant which must be multiple of 4 the assembler accepts values 0,4,1024 and stores the appropriately shifted value in the instruction word Because the stack groes downward, all active stack storage is at a positive offset from the stack pointer Notice that ldr reads from the pointer computed by adding an offset to sp and str writes from a pointer Geoffrey Brown, Bryce Himebaugh 2015 Access Within the Stack September 2, / 15

13 Accessing Local Variables Example 94 extern void foo( int *); int local( int i) { int l = 3; foo(&l); return i + l; } local: push {r4, save sub sp, allocate space movs r4, r4 = i add r0, sp, r0 = &l movs r3, r3 = 3 str r3, *(sp + 4) = r3 bl foo ldr r3, r3 = *( sp + 4) adds r0, r4, retval = i + l add sp, deallocate space pop {r4, restore Geoffrey Brown, Bryce Himebaugh 2015 Access Within the Stack September 2, / 15

14 Parameter Stack Figure 93 Stack after calling function Slot 5 Parameter area Caller Slot 4 r3 Slot 3 Saved link register(lr) r2 r1 Slot 2 Slot 1 Callee saved registers Local storage Callee r0 Slot 0 sp Stack grows down Geoffrey Brown, Bryce Himebaugh 2015 Parameter Passing September 2, / 15

15 Accessing Parameters in the Stack Example 95 extern int foo(int,int,int, int); int call( int a, int b, int c, int d, int e) { return a + e + foo(a,b,c,d); } call: push {r4, save ldr r4, [sp, r4 = e adds r4, r4 += a bl foo adds r0, r4 pop {r4, restore Geoffrey Brown, Bryce Himebaugh 2015 Parameter Passing September 2, / 15