mips code from compiler

The Stack for "Code from a real compiler"

Move the mouse down the code to see the stack change




	space for $ra	(unused)


	a	local vars
	b	of fun3


$sp->		space for arguments
		to fun3

fun3:
	sub	$sp, 24		
	sw	$ra, 20($sp)	
	sw	$fp, 16($sp)	
	move	$fp, $sp	
	sw	$a0, 24($fp)	
	sw	$a1, 28($fp)	
	li	$a0, 5
	li	$a1, 7
	jal	fun1
	....
fun1:
	sub	$sp, 16	
	sw	$fp, 8($sp)	
	move	$fp, $sp	
	sw	$a0, 16($fp)	#args to fun1	
	sw	$a1, 20($fp)	
	li	$2, 7		#local i
	sw	$2, 0($fp)	
	li	$2, 9		#local j
	sw	$2, 4($fp)	
	...
# Back from fun1, store result in a	
	sw $v0, 8($fp)

What is going on here?

You have just seen an animation of some MIPS code produced by a "real compiler" from C. This code is found on pp. 100-103 in "Introduction to RISC Assembly Language Programming" by John Waldron, Addison-Wesley, 1999. It shows the use of the stack as each function moves the stack pointer, saves the frame pointer value, and then moves the frame pointer. On return, the saved value of frame pointer is restored. Please note that the functions "don't do anything useful," they just illustrate how the compiler uses the stack to save the registers $ra and $fp, and provide storage for arguments and local variables.

Why? Why this strange pattern? - Answer: Compiler evolution.

Just like our arms evolved from fins, when we swam in water, where they weren't attached to our skeletons, - Compilers evolved in an environment with very few registers, so they evolved a stack structure in which calling a function proceeded as follows:

PUSH arguments onto the stack, from right to left
CALL the function. This instruction does 2 things:
1. PUSH the return address
2. JUMP to the start of function code
PUSH the frame pointer (save the value used by caller).
Set the frame pointer to the stack pointer, it will remain fixed until the function returns.

You will notice that this results in the stack use we see in this example. Being MIPS, the arguments are actually passed in a-registers, so the code of the functions puts them into the stack where they should have been. (And our arms are attached to shoulder blades that sort of float near our ribs, even though they aren't fins any more. -- this was explained on Quirks and Quarks March 16, 2019).

Prepared by Lin Jensen, Bishop's University, 17 March 2019

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