Stack Frame

1
Stack Frame

• Stack Frame can be used to store local variables

For parmeters that I pass to other functions
Parent stack frame
For parmeters that are passed to me
Parameter area
2
Code generated by GCC

• Introduction
• Code generation examples that we have looked at
  so far have been Ideal
• Going to look at code generated by the GCC - MIPs
  cross compiler mcompile -S test.c
• mcompile invokes the gcc MIPs compiler
• -S option says stop at assembly stage
• No optimisation specified so none performed
• All C statements considered independently
• All variables stored in memory
• May appear really Stupid

3
Code generated by GCC

• Introduction
• Code generated adheres to a set of conventions
  set down by the MIPs corporation
• Code generated a function at time
• Two different types functions
• non leaf (function calls another)
• leaf (function doesnt call another)
• Main difference is in the way they allocate space
  on the stack

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Non Leaf functions

• Functions that call other functions
• Have to follow caller and callee save conventions
• code generated for function contains sections
• to setup the stack (known as a frame) for the
  function at the start
• to execute the function
• to reset the stack at the end

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Non Leaf functions(2)

• As it is possible in some cases for the stack
  pointer to alter during execution of a function
  another register call a frame pointer (30) is
  used as a common point of reference
• e.g. lw 2, 12(fp) will always load the same
  word

6
Non Leaf Functions(3)

• Assume
• No Function has more than 4 arguments
• All local variables are integers
• All local variables are stored in memory
• No need to worry about caller and callee saved
  regs apart from RA and FP
• All arguments are passed in registers but will be
  copied to the stack on subroutine entry
• All results returned in registers

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Non Leaf functions(4)

• Frame components
• arguments (to functions it calls)
• although parameters (1 -4) passed in registers
  (4 - 7) space is still created on the stack
• never less than 16 (4 words) bytes allocated
• Locals
• space for the for a functions local variables
• Reg Save
• space to store caller and callee saved registers
  including the stack and frame pointer
• It is a convention that the frame pointer are 8
  byte aligned so it may be necessary to add 4
  bytes padding

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Stack frame for non-leaf functions
SP FP (after entry)
Arguments for functions called (max 4)
size_params 4 no_params 16
All local variables
size_locals no_locals 4
Padding
Padding (if (size_params size_locals)mod 8
then 4
else 0)
Saved RA FP
SP FP (before entry)
size_saved 8
Arguments to called function
9
Stack frame for non-leaf functions(2)

• The size of the stack frame is calculated as
  follows
• size_proc_frame size_params size_locals
  padding 8
• The memory locations of the various components
  calculated as
• parami FP size_proc_frame (i 4)
• locali FP size_params (i 4)
• old FP FP size_params size_locals padding
• old RA FP size_params size_lacals padding
  4

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Assembler Directives

• .text
• Defines the section where the instructions are to
  be placed
• .sdata
• Defines the section where static data is to be
  placed
• .align
• specifies an alignment greater than would
  normally be required for the next data directive
  (specified as a power of 2)
• .ent .end
• Mark the start and end of a function (often used
  by a debugger)

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Assembler Directives

• .frame .mask .fmask
• Used by a debugger to obtain details about the
  current stack frame

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Example
int absolute(int c) if (c lt 0)
return(-c) else return(c) int
greater(int a, int b) int c if (a gt b)
c absolute(a) else c absolute(b)
return c
main() int x,y,z z greater(x,y)
writeint(z)
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main()
L7 move sp,fp lw 31,36(sp) lw fp,32(s
p) addu sp,sp,40 j 31 .end main
.text .align 2 .globl main .ent main main .fr
ame fp,40,31 .mask 0xc0000000,-4 .fmask 0x0000
0000,0 subu sp,sp,40 sw 31,36(sp) sw fp,32
(sp) move fp,sp jal __main lw 4,16(fp) lw
5,20(fp) jal greater sw 2,24(fp) lw 4,24(
fp) jal writeint

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int greater(int a, int b)
.align 2 .globl greater .ent greater greater
.frame fp,32,31 .mask 0xc0000000,-4 .fmask 0x0
0000000,0 subu sp,sp,32 sw 31,28(sp) sw fp
,24(sp) move fp,sp sw 4,32(fp) sw 5,36(f
p) lw 2,32(fp) lw 3,36(fp) slt 2,3,2 be
q 2,0,L5 lw 4,32(fp) jal absolute sw 2,16
(fp) j L6
L5 lw 4,36(fp) jal absolute sw 2,16(fp)
L6 lw 2,16(fp) j L4 L4 move sp,fp lw
31,28(sp) lw fp,24(sp) addu sp,sp,32 j 31
.end greater
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Stack frame for leaf functions
SP FP (after entry)
All local variables
size_locals no_locals 4
Padding
Padding (if (size_locals mod 8)
then 4
else 0)
Saved RA FP
SP FP (before entry)
size_saved 8
Arguments to called function
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Stack frame for non-leaf functions(2)

• The size of the stack frame is calculated as
  follows
• size_proc_frame size_locals padding
  4
• The memory locations of the various components
  calculated as
• parami FP size_proc_frame (i 4)
• locali FP (i 4)

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int absolute(int c)
.align 2 .globl absolute .ent absolute absolute
.frame fp,8,31 .mask 0x40000000,-8 .fmask 0
x00000000,0 subu sp,sp,8 sw fp,0(sp) move
fp,sp sw 4,8(fp) lw 2,8(fp) bgez 2,L2 l
w 2,8(fp) subu 3,0,2 move 2,3 j L1 j
L3
L2 lw 2,8(fp) j L1 L3 L1 move sp,fp
lw fp,0(sp) addu sp,sp,8 j 31 .end absolu
te