# Csci 136 Computer Architecture II MIPS Procedure Call

Csci 136 Computer Architecture II MIPS Procedure Call Handling Xiuzhen Cheng [email protected] Announcement Homework Assignment #3: due Feb. 08, before class Readings: Sections 2.7-2.8, 2.10, 2.13, 2.15, 2.17-2.18 Problems: 2.21-2.24, 2.28, 2.38 Quiz #1 is scheduled on Feb 08. Project #1 is due on Feb 13, Sunday. C functions main() { int i,j,k,m; ... i = mult(j,k); ... m = mult(i,i); ... }

/* really dumb mult function */ int mult (int mcand, int mlier){ int product; product = 0; while (mlier > 0) { product = product + mcand; mlier = mlier -1; } return product; } What information must compiler/programmer keep track of? What instructions can accomplish this? Function Call Bookkeeping Registers play a major role in keeping track of information for function calls. Register conventions:

Return address Arguments Return value Local variables \$ra \$a0, \$a1, \$a2, \$a3 \$v0, \$v1 \$s0, \$s1, , \$s7 The stack is also used; more later. Instruction Support for Functions (1/6) C M I P S ... sum(a,b);... /* a,b:\$s0,\$s1 */

} int sum(int x, int y) { return x+y; } address 1000 In MIPS, all instructions 1004 are 4 bytes, and stored in 1008 memory just like data. So 1012 1016 here we show the 2000 addresses of where the

2004 programs are stored. Instruction Support for Functions (2/6) C M I P S ... sum(a,b);... /* a,b:\$s0,\$s1 */ } int sum(int x, int y) { return x+y; } address 1000 add \$a0,\$s0,\$zero # x = a 1004 add \$a1,\$s1,\$zero # y = b 1008 addi \$ra,\$zero,1016 #\$ra=1016

1012 j sum #jump to sum 1016 ... 2000 sum: add \$v0,\$a0,\$a1 2004 jr \$ra # new instruction Instruction Support for Functions (3/6) C M I P S ... sum(a,b);... /* a,b:\$s0,\$s1 */ } int sum(int x, int y) { return x+y; }

Question: Why use jr here? Why not simply use j? Answer: sum might be called by many functions, so we cant return to a fixed place. The calling proc to sum must be able to say return here somehow. 2000 sum: add \$v0,\$a0,\$a1 2004 jr \$ra # new instruction Instruction Support for Functions (4/6) Single instruction to jump and save return address: jump and link (jal) Before: 1008 addi \$ra,\$zero,1016 #\$ra=1016 1012 j sum #goto sum After: 1008 jal sum # \$ra=1012,goto sum Why have a jal? Make the common case fast: function calls are very common. Also, you dont have to know where the code is loaded into memory with jal.

Only useful if we know exact address to jump to. Very useful for function calls: jal stores return address in register (\$ra) jr \$ra jumps back to that address Nested Procedures (1/2) int sumSquare(int x, int y) { return mult(x,x)+ y; } Something called sumSquare, now sumSquare is calling mult. So theres a value in \$ra that sumSquare wants to jump back to, but this will be overwritten by the call to mult. Need to save sumSquare return address before call to mult. Nested Procedures (2/2) In general, may need to save some other info in addition to \$ra. When a C program is run, there are 3 important

memory areas allocated: Static: Variables declared once per program, cease to exist only after execution completes. E.g., C globals Heap: Variables declared dynamically Stack: Space to be used by procedure during execution; this is where we can save register values Address \$sp stack pointer 0 C memory Allocation review Stack Space for saved

procedure information Heap Explicitly created space, e.g., malloc(); C pointers Static Variables declared once per program Code Program Using the Stack (1/2) So we have a register \$sp which always points to the last used space in the stack. To use stack, we decrement this pointer by the amount of space we need and then fill it with info.

So, how do we compile this? int sumSquare(int x, int y) { return mult(x,x)+ y; } Using the Stack (2/2) Hand-compile int sumSquare(int x, int y) { return mult(x,x)+ y; } sumSquare: addi \$sp,\$sp,-8 # space on stack push sw \$ra, 4(\$sp) # save ret addr sw \$a1, 0(\$sp) # save y add \$a1,\$a0,\$zero # mult(x,x) jal mult # call mult lw \$a1, 0(\$sp) # restore y

add \$v0,\$v0,\$a1 # mult()+y lw \$ra, 4(\$sp) # get ret addr pop addi \$sp,\$sp,8 # restore stack jr \$ra mult: ... Steps for Making a Procedure Call 1) Save necessary values onto stack. 2) Assign argument(s), if any. 3) jal call 4) Do the job 5) Return value 6) Restore values from stack. Rules for Procedures Called with a jal instruction, returns with a jr \$ra Accepts up to 4 arguments in \$a0, \$a1, \$a2 and \$a3 Return value is always in \$v0 (and if necessary in

\$v1) Must follow register conventions (even in functions that only you will call)! So what are they? Basic Structure of a Function Prologue entry_label: addi \$sp,\$sp, -framesize sw \$ra, framesize-4(\$sp) # save \$ra save other regs if need be ra Body ... (call other functions) Epilogue restore other regs if need be

lw \$ra, framesize-4(\$sp) # restore \$ra addi \$sp,\$sp, framesize jr \$ra memory MIPS Registers The constant 0 \$0 \$zero Reserved for Assembler \$1 \$at Return Values \$2-\$3 \$v0-\$v1 Arguments \$4-\$7 \$a0-\$a3 Temporary \$8-\$15

\$t0-\$t7 Saved \$16-\$23 \$s0-\$s7 More Temporary \$24-\$25 \$t8-\$t9 Used by Kernel \$26-27 \$k0-\$k1 Global Pointer \$28 \$gp Stack Pointer \$29 \$sp Frame Pointer \$30 \$fp Return Address \$31

\$ra (From COD 3rd Ed. green insert) Use names for registers -- code is clearer! Reserved/Special Registers \$at: may be used by the assembler at any time; unsafe to use \$k0-\$k1: may be used by the OS at any time; unsafe to use \$gp, \$fp: dont worry about them Note: Feel free to read up on \$gp and \$fp in Appendix A, but you can write perfectly good MIPS code without them. Register Conventions (1/4) CalleR: the calling function CalleE: the function being called When callee returns from executing, the caller needs to know which registers may have changed and which are guaranteed to be unchanged. Register Conventions: A set of generally accepted

rules as to which registers will be unchanged after a procedure call (jal) and which may be changed. Register Conventions (2/4) - saved \$0: No Change. Always 0. \$s0-\$s7: Restore if you change. Very important, thats why theyre called saved registers. If the callee changes these in any way, it must restore the original values before returning. \$sp: Restore if you change. The stack pointer must point to the same place before and after the jal call, or else the caller wont be able to restore values from the stack. HINT -- All saved registers start with S! Register Conventions (3/4) - volatile \$ra: Can Change. The jal call itself will change this register. Caller needs to save on stack if nested call. \$v0-\$v1: Can Change. These will contain the new returned values.

\$a0-\$a3: Can change. These are volatile argument registers. Caller needs to save if theyll need them after the call. \$t0-\$t9: Can change. Thats why theyre called temporary: any procedure may change them at any time. Caller needs to save if theyll need them afterwards. Register Conventions (4/4) What do these conventions mean? If function R calls function E, then function R must save any temporary registers that it may be using onto the stack before making a jal call. Function E must save any S (saved) registers it intends to use before garbling up their values Remember: Caller/callee need to save only temporary/saved registers they are using, not all registers. Parents leaving for weekend analogy (1/5) Parents (main) leaving for weekend They (caller) give keys to the house to kid

(callee) with the rules (calling conventions): You can trash the temporary room(s), like the den and basement (registers) if you want, we dont care about it BUT youd better leave the rooms (registers) that we want to save for the guests untouched. these rooms better look the same when we return! Who hasnt heard this in their life? Parents leaving for weekend analogy (2/5) Kid now owns rooms (registers) Kid wants to use the saved rooms for a wild, wild party (computation) What does kid (callee) do? Kid takes what was in these rooms and puts them in the garage (memory) Kid throws the party, trashes everything (except garage, who goes there?) Kid restores the rooms the parents wanted saved after the party by replacing the items from the garage (memory) back into those saved rooms

Parents leaving for weekend analogy (3/5) Same scenario, except before parents return and kid replaces saved rooms Kid (callee) has left valuable stuff (data) all over. Kids friend (another callee) wants the house for a party when the kid is away Kid knows that friend might trash the place destroying valuable stuff! Kid remembers rule parents taught and now becomes the heavy (caller), instructing friend (callee) on good rules (conventions) of house. Parents leaving for weekend analogy (4/5) If kid had data in temporary rooms (which were going to be trashed), there are three options: Move items directly to garage (memory) Move items to saved rooms whose contents have already been moved to the garage (memory) Optimize lifestyle (code) so that the amount youve got to shlep stuff back and forth from garage (memory) is minimized

Otherwise: Dude, wheres my data?! Parents leaving for weekend analogy (5/5) Friend now owns rooms (registers) Friend wants to use the saved rooms for a wild, wild party (computation) What does friend (callee) do? Friend takes what was in these rooms and puts them in the garage (memory) Friend throws the party, trashes everything (except garage) Friend restores the rooms the kid wanted saved after the party by replacing the items from the garage (memory) back into those saved rooms And In Conclusion Register Conventions: Each register has a purpose and limits to its usage. Learn these and follow them, even if youre writing all the code yourself. For nested calls, we need to save the

argument registers if they will be modified. Save to stacks Save to \$sx registers arguments can be treated as local variables. Summary: Six Steps in a Procedure Call Pass parameters to callee \$a0 -- \$a3, stack Transfer control to the procedure jal procName Allocate storage resources For local variables and saved registers, stack Procedure frame Perform the desired task Return value \$v0, \$v1

Transfer control to the caller jr \$ra Summary: Procedure Call Conventions Register contents across procedure calls: either caller or callee saves A simple convention is used for each processor Name \$zero Register # 0 Usage Preserved on call? Constant value 0 N/A

\$v0-\$v1 2-3 Return values No \$a0-\$a3 4-7 Parameters No \$t0-\$t7 8-15 Temporaries

No \$s0-\$s7 16-23 Saved Yes \$t8-\$t9 24-25 Temporaries No \$gp

28 Global pointer Yes \$sp 29 Stack pointer Yes \$fp 30 Frame pointer Yes

\$ra 31 Return address Yes Remark: They are still general-purpose registers. A procedure needs to save all YES registers when using them. A Simple Example int leaf_example (int g, int h, int i, int j) { int f; f = (g+h) - (i+j); return f; } Given assembly code Behavior of the stack

Example swap Procedure (1/2) swap (int v[], int k) { int temp; temp = v[k]; v[k] = v[k+1]; v[k+1] = temp; } Given assembly code Study stack behavior Example swap Procedure (2/2) Register assignment: \$a0 base address v of the integer array \$a1 index k \$t0 local variable temp No stack operation. swap (int v[], int k) { int temp; temp = v[k];

v[k] = v[k+1]; v[k+1] = temp; } swap: add add add \$t0, \$a1, \$a1 \$t0, \$t0, \$t0 \$t0, \$t0, \$a0 lw lw sw sw \$t1, 0(\$t0) \$t2, 4(\$t0)

\$t2, 0(\$t0) \$t1, 4(\$t0) jr \$ra Example Bubble Sort (1/4) sort (int v[], int n) { int i, j; for (i=1; i=0 && v[j]>v[j+1]; j--) { swap(v,j); } } } Given assembly code Analyze stack behavior very simple Example Bubble Sort (2/4) Register assignment:

\$a0 base address of integer array v \$a1 length n of the array \$s0 local variable i \$s1 local variable j Registers need to be saved: \$s0, \$s1, \$ra, \$a1, more? Why? sort (int v[], int n) { int i, j; for (i=1; i=0 && v[j]>v[j+1]; j--) { swap(v,j); } } } bubbleSort: addi sw

sw sw sw move exit: \$sp, \$sp, -16 \$s0, 0(\$sp) \$s1, 4(\$SP) \$s2, 8(\$sp) \$ra, 12(\$sp) \$s2, \$a1 procedure body lw lw lw lw addi

\$ra, 12(\$sp) \$s2, 12(\$sp) \$s1, 4(\$sp) \$s0, 0(\$sp) \$sp, \$sp, 16 jr \$ra Example Bubble Sort (3/4) The loop for index I for (i=1; i

slt beq \$t0, \$s0, \$s2 \$t0, \$zero, exit the loop indexed by j exit_j: addi j \$s0, \$s0, 1 for_i Example Bubble Sort (4/4) The loop for index j for (j=I-1; j>=0 &&

v[j]>v[j+1]; j--) { swap(v,j); } for_j: addi \$s1, \$s0, -1 slt bne \$t0, \$s1, \$zero \$t0, \$zero, exit_j add add add lw lw

slt beq \$t0, \$s1, \$s1 \$t0, \$t0, \$t0 \$t0, \$t0, \$a0 \$t1, 0(\$t0) \$t2, 4(\$t0) \$t0, \$t2, \$t1 \$t0, \$zero, exit_j move jal \$a1, \$s1 swap addi j \$s1, \$s1, -1

for_j Example Factorial Computation (1/4) Int fact (int n) { If (n<1) return (1); Else return (n*fact(n-1)); } Assembly Code Stack behavior Remark: This is a recursive procedure Example Factorial Computation (2/4) Register Assignment: \$a0 argument n. Since n will be needed after the recursive procedure call, \$a0 needs to be saved. We save \$a0 to \$s0, the first saved

register \$v0 returned value. Other registers need to be saved: \$ra, fact: \$sp, \$sp, -8 \$s0, 0(\$sp) \$ra, 4(\$sp) move \$s0, \$a0 exit: int fact (int n) { if (n<1) return (1); else return (n*fact(n-1)); } addi

sw sw procedure body lw lw addi jr \$ra, 4(\$sp) \$s0, 0(\$sp) \$sp, \$sp, 8 \$ra Example Factorial Computation (3/4) Procedure body Exit condition Recursion slti

beq addi j recursion: int fact (int n) { addi if (n<1) return (1); jal else return (n*fact(n-1)); mul } j \$t0, \$s0, 1 \$t0, \$zero, recursion \$v0, \$zero, 1 exit \$a0, \$s0, -1 fact \$v0, \$v0, \$s0

exit Example Factorial Computation (4/4) Stack contents during the execution of fac(2). \$sp \$ra \$s0 = 0 \$sp \$ra \$s0 = 0 \$ra \$s0 = 2 \$sp \$ra \$s0 = 0 \$ra \$s0 = 2

\$ra \$s0 = 1 \$sp \$ra \$s0 = 0 \$ra \$s0 = 2 \$sp \$ra \$s0 = 0 MIPS Procedure Handling Summary Need jump and return jal ProcAddr # issued by the caller jumps to ProcAddr save the return instruction address (PC+4) in \$31

jr \$31 # last instruction in the callee jump back to the caller procedure Need to pass parameters Registers \$4 -- \$7 (\$a0 -- \$a3) are used to pass first 4 parameters Other parameters are passed through stack. Returned values are in \$2 and \$3 (\$v0 & \$v1) How about nested procedure? Get help from stack! What Happens at a Procedure Call Before jal, caller does the following Put arguments to be passed into \$4 -- \$7, and stack Save any caller-saved registers Adjust \$sp if necessary At the beginning of a procedure, callee does the following Setup new frame pointer

Save callee-saved registers (\$ra, etc.) Setup \$sp Adjust \$sp if necessary Before jr \$ra, callee does the following Put return values in \$2, \$3 Restore any saved registers Adjust \$sp if necessary After jr, caller does the following Restore any saved registers Adjust \$sp Call Frame Summary Which registers will be saved within a procedure? Save callee-saved registers -- always Save \$s0~\$s7 if they will be used within the procedure Save caller-saved registers Proc1 {

Proc2; Proc 3; } Save \$t0~\$t9, \$a0~\$a3, \$v0~\$v1 if their values will be needed after the procedure call. Where to save them? Stack or \$sx registers. Cons and pros? Always save \$ra for non-leaf procedure Proc1s context. Any registers like \$ra, \$t0--\$t9, \$a0--\$a3 needs to be saved before calling Proc2 or Proc3 if it is needed Proc2 after these procedure call. Proc3 If Proc2 or Proc3 uses \$s0--\$s7,

they must be saved within Proc2 or Proc3 if their values need to be preserved after the procedure call in Proc1. To preserve register values, need help from \$sp Summary on Procedure Calls Allocate any needed storage Save callee-saved registers that might be modified Execute the job procedure body To make a procedure call Save caller-saved registers on stack Put arguments in \$a0..\$a3 Jump and link (jal) Restore caller-saved registers from stack Put return value in \$v0 and \$v1 Restore callee-saved registers Jump to the return address (jr \$ra)

Summary on Procedure Calls Callers responsibility Place arguments where procedure can access them (\$a0..\$a3, and the stack just above (modified) \$fp) Transfer control Callees responsibility Do the work, using the arguments in \$a0..\$a3 Put return value where caller can access it (\$v0..\$v1) Return control to the calling procedure Follow the convention!!! Why?

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