Chapter 7Subroutines

Dr. A.P. Preethy

7.1 Introduction

There is frequently a need either to repeat a computation or to repeat the computation with different arguments.

• Subroutines can be used in such situations

• Subroutines may be either open or closed

Open subroutine • is insertion of required code whenever it is needed in the program e.g. macro

• arguments are passed in the registers that are given as arguments to the subroutine.

Closed subroutine • is one in which the code appears only once in the program; whenever it is needed, a jump to the code is executed, and when it completes, a return is made to the instruction occurring after the jump instruction.

• arguments may be placed in registers or on the stack

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• A subroutine also allows you to debug code once and then sure

that all future instantiations of the code will be correct

• Any register that the subroutine uses must first be saved and then restored after the subroutine completes execution

• Arguments to subroutines are normally considered to be local variables of the subroutine,and the subroutine is free to change them

• However, this is not always the case, for e.g., in multiplication, multiplicand is not changed

7.2 Open Subroutines• The cmul macro discussed in Chapter 6 is an open subroutine, and can handle multiplication by constants:

cmul (%r0, 603, %g1, %r1)

• To multiply %r0 by 100,

cmul(%r0, 100, %g1, %r0)

• And the code expands into:

!start open coded multiply for

!%r0= %r0 * 100, using %g1 as temp

sll %r0, 2, %r0

sll %r0, 3, %g1

sub %r0, %g1, %r0

sll %g1, 2, %g1

add %r0, %g1, %r0

! end open coded multiply

• Open Subroutines are very efficient with no wasted instructions

• Open Subroutines are very flexible and can be as general as the program wishes to make them

• Every time open subroutine referenced, the code is expanded, resulting in long code

• So it is better to write code once as a closed subroutine and to branch to the code, whenever needed

7.3 Register Saving

• Almost any computation will involve the use of registers

Usually when subroutines are called, registers are pushed onto the stack and popped from, when it returns

• To avoid the execution time involved, in CISC, sometimes a special register save mask is used, that would indicate, by bits that were set, which registers were to be saved

(contd….)

(Register Saving contd…)

• SPARC architecture provides a register file with a mapping register that indicates the active registers

• It provides 128 registers, with the programmer having access to the eight global registers, and only 24 of the mapped registers at a time

• save instruction changes the register mapping so that new registers are provided

• restore instruction restores the register mapping on subroutine return

• The 32 registers are divided into four groups : in, local, out and general

• The eight general register %g0 to %g8 are not mapped and are global to all subroutines

• “in” & “out” register are used to pass arguments to closed subroutine

• “local” registers are used for subroutine’s local variables

• When save instruction is executed the out register become the in register, and a new set of local and out registers is provided

• The mapping pointer into the register file is changed by 16 registers

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The next two slides show Register Sets

(Figure 7.1)

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Figure 7.1: A Register Set (Figure contd…on next slide)

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Figure 7.1: A Register Set

(Figure contd…from the prev. slide)

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• The current register set is indicated by the current window ptr (cwp).

• The last free register set is marked by the window invalid bit, in the WIM

• After save instruction is executed, the situation in Figure 7.2 results, the prior subroutine’s register contents remain unchanged until a restore instruction is executed, resetting the cwp

(Figure 7.2 follows …)

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Figure 7.2: Register Sets

• If a further five subroutine calls are made without any returns, window overflow will occur (Figure 7.3 follows on the next slide)

• The out registers being used are from the invalid register window marked by the wim bit

• Hardware trap will occur at the time of window overflow

• saves and restores can be made in a range of six without window overflows or underflows (it is expensive if recursive subroutine calls are frequently made)

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Figure 7.3: Windows Overflow

More about Register window mapping…

• Register window mapping explains why the frame pointer (%o6) becomes stack pointer (%i6) after save instruction

Save %sp, -64, %sp

• This will subtract 64 from the current stack pointer, but stores the result into the new stack pointer, leaving the old %sp contents unchanged, which becomes the new %fp

• restore instruction restores the register window set. On doing this, a register window can underflow if the cwp is moved to the wim. When this happens the window trap routine restores the registers from the stack and resets the pointers

• restore is also an add instruction and is used as the final add instruction in a subroutine

7.4 Subroutine Linkage

• The SPARC architecture supports two instructions, call and jmpl, for linking to subroutines

• The address of instruction which called the subroutine is stored in %o7

• The return from subroutine is to %o7 + 8, which is the address of the next instruction to be executed in the main program

• If a save instruction is executed at the beginning of the subroutine, the contents of %o7 will become %i7, and the return will have to be to %i7 + 8

(contd…)

Subroutine Linkage contd…

• call instruction

• If the subroutine name is known at assembly time, the call instruction may be used

• call instruction has a target address label

• It stores %pc contents to %o7

• always followed by a delay slot instruction

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jmpl instruction

• If address of the subroutine is computed, it must be loaded into a register, and then jmpl instruction is used to call the subroutine

• jmpl instruction has two source arguments (two registers or a register and a constant), and a destination register

• subroutine address is the sum of the source arguments, and the address of the jmpl instruction is stored in the destination register

• always followed by a delay slot instruction

• to call a subroutine whose address is in register %o0 and to store the return address into %o7, we would write:

jmpl %o0, %o7

Subroutine Linkage contd…

• The assembler recognizes

call %o0 as

jmpl %o0, %07

• The return from a subroutine also makes use of the jmpl instruction

• We need to return to %i7 + 8

• Assembler recognizes ret for:

jmpl %i7 + 8, %g0

Subroutine Linkage contd…

• The call to subroutine is:

call subr

nop

• And at the entry of the subroutine

subr: save %sp, … %sp

with the return

ret

restore

• The restore instruction is normally used to fill the delay slot of the ret instruction

The ret is expanded to: jmpl %i7 + 8, %g0

restore

7.5 Arguments to Subroutines

• Arguments to subroutines can follow in-line after the call instruction, be on the stack, or located in registers

• If the addresses and the values of the arguments are known at the assembly time (e.g. 3 and 4) then we can write :

call add

nop

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4 (contd… on next slide)

• The following subroutine code results:

add: save %sp, -64, %sp

ld [%i7 + 8], %i0 !first argument

ld [%i7 +12], %i1 !second argument

add %i1, %i0, %i0

jmpl %i7 + 16, %g0 !return address

restore

• This type of argument passing is very efficient, but limited

• Recursive calls are not possible, nor is it possible to compute any of the arguments

(contd.. from the prev. slide)

Using Stack

• Each argument should be stored before the subroutine may be called

• But allows flexibility to compute arguments, pass any number of arguments, and support recursive calls

• Time is wasted to store the arguments on stack and retrieve them at the time of computation

In SPARC:

• allows first six arguments to be placed in %o0-%05, the rest on the stack, however, space is reserved on the stack for the first six also

• One word space reserved for each argument, so bytes must be moved as words

• %o6 is sp and %07 is for return address

•After the execution of a save instruction, the arguments will be in %o0-%05

• The arguments are located on the stack, after the 64 bytes reserved for register window saving

• On the stack, immediately after 64 bytes reserved for register window saving, there is a pointer to where a structure may be returned (discussed in Section 7.7)

• Thus structure return pointer will be at %sp + 64 and the first argument, if it were on the stack, at %sp + 68

• Before arguments may be placed onto the stack, space on the stack must be provided by subtracting the number of bytes required for arguments from the stack pointer

The space is created when we execute the save instruction on subroutine entry

.global subroutine_name

subroutine_name:

save %sp, -(64 + 4 + 24 + local) & -8, %sp

This save instruction will provide:

• Space for saving the register window set, if necessary

• A structure pointer

• A place to save six arguments

• Space for any local variable

(contd…)

• If we had a subroutine vector with local variables

vector()

{

int a,b;

char d;

• Then save instruction would be

save %sp, -(64 + 4 + 24 + 9) & -8, %sp

Resulting in subtraction of 104 bytes (Figure 7.4)

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Figure 7.4: The stack part I (figure contd… on next slide)

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Figure 7.4: The stack part II

(Figure 7.4 contd… from the prev. slide)

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The stack is shown again in the figure below to differentiate the frames referenced by %fp and %sp.

Figure 7.5: The stack showing Two Frames part I (Figure continues on next slide…)

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Figure 7.5: The stack showing Two Frames part II

(Figure contd… from the prev. slide…)

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we might define a subroutine entry macro, begin-fn, to be called after the

definition of local variables with the name of the subroutine as argument:

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7.6 Examples

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The example’s translation into assembly languages is:

(code contd… on next slide)

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Code contd… from the prev. slide

sth %o0, [%o1 + %o2]Id [%fp + x_s] , %o0 ‘!y = x*a’call .mulmov %a_r, %o1st %o0, [%fp + y_s]

ld [%fp + x_s], %o0 ‘!j = x+i’Add %i_r, %o0, %j_r

ld [%fp + x_s], %o0 ‘!return = x+y’

ld [%fp + y_s], %o1retRestore %o0, %o1, %o0

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The code expands into: ‘!a_r in %i0

‘!b_r in %i1

‘!c_r in %i2

!local variables

x_s = -4

y_s = -8

ary_s = -264

‘! i_r in %l0

‘!j_r in %l1

(contd…. on next slide)

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(contd…)

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(contd… from the prev. slide)

7.7 Return Values

• Functions are subroutines which return a value

• In SPARC, the return value is always returned in register %o0, i.e. %i0 of called program

• We have to put the return value in %i0 before executing restore instruction

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The function zero returns a structure.

When call is made to zero, a pointer to where the returned struct is to be stored is passed to the function at %sp + 64.

(contd…on next slide)

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•Thus returning structure in this manner is a little dangerous. (due to insufficient size)

• This type of errors are hard to debug

• The other method is :–The caller, passes a pointer to the beginning of the storage in %sp + struct_s and place number of bytes of storage expected to be received. For example:

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7.8 Subroutines with many arguments

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The stack when foo has been entered is shown in Figure 7.6. Inside foo the arguments may be accessed bydefine(a8-s, arg-d(8))define(a7-s, arg-d(7))

7.9 Leaf Subroutines

• A leaf routine is one that does not call any other routines (e.g. .mul)

• Leaf routine may only use the first six out register and the global register %g0 and %g1

• A leaf subroutine does not execute either call or restore instruction

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%fp ->

Figure 7.6: The Stack with additional Arguments Part I

(Figure contd… on next slide)

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(Figure contd…from the prev. slide)

Figure 7.6: The Stack with additional Arguments Part II

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7.10 Pointers as Arguments to SubroutinesGiven the swap function, arguments must be passed to the function in order for the values to be swapped:

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7.11 Summary

• Subroutines simplify writing code, provide structures, and help to control programming errors.

• In the case of closed subroutines, register-saving mechanism facilitates subroutine linkages.

• Stack frame introduced as storage for registers, arguments, local variables, and the return address.

• Return of scalars and structures, and passing of arguments discussed.