Msg/ClobberedRegs - Revision history

Henry at 03:52, 17 March 2019

2019-03-17T03:52:27Z

@@ Line 25: / Line 25: @@
 	addi r16, r16, 1			# Modify a callee-saved register
 	ret
 </syntaxhighlight>

Henry: /* Examples */

2019-03-10T07:11:26Z

Examples

← Older revision		Revision as of 07:11, 10 March 2019
Line 8:		Line 8:
	_start:		_start:
	bl Func		bl Func
	nop		nop

	Func:		Func:
	add r4, #1 // Modify a callee-saved register		add r4, #1 // Modify a callee-saved register
	bx lr		bx lr
	</syntaxhighlight>		</syntaxhighlight>

			==== Nios II ====
			<syntaxhighlight lang="Asm" line highlight="8">
			.global _start
			_start:
			call Func
			nop

			Func:
			addi r16, r16, 1 # Modify a callee-saved register
			ret
			</syntaxhighlight>

	=== Debugging ===		=== Debugging ===

Henry at 05:04, 10 March 2019

2019-03-10T05:04:28Z

← Older revision		Revision as of 05:04, 10 March 2019
Line 24:		Line 24:

	{{DisableMsg\|Function clobbered callee-saved register}}		{{DisableMsg\|Function clobbered callee-saved register}}
			The main reason you might consider disabling this warning is if you are intentionally using a non-standard calling convention.

Henry: Created page with "A calling convention defines which registers a function is allowed to modify (caller-saved), and which registers a function should preserve (callee-saved). Clobbering a regist..."

2019-03-10T05:02:57Z

Created page with "A calling convention defines which registers a function is allowed to modify (caller-saved), and which registers a function should preserve (callee-saved). Clobbering a regist..."

New page

A calling convention defines which registers a function is allowed to modify (caller-saved), and which registers a function should preserve (callee-saved). Clobbering a register means overwriting its value. This message tells you that there were callee-saved registers (those a function should not modify) that were changed inside the current function.

=== Examples ===
==== ARMv7 ====

<syntaxhighlight lang="Asm" line highlight="8">
.global _start
_start:
bl Func
nop

Func:
add r4, #1 // Modify a callee-saved register
bx lr
</syntaxhighlight>

=== Debugging ===
* This message is complaining that one or more registers differ between the start of the function and at the function return. Use breakpoints and make a note of the value of those registers at both the function entry and return. Are they the same?
* If your function wants to (temporarily) modify callee-saved registers, it should preserve its value first, usually by storing the old value onto the stack and restoring it just before leaving the function. Saving and restoring (also called spilling and filling) can go wrong in many ways, such placing the stack at a location that doesn't contain memory.

=== Implementation ===
The simulator identifies idiomatic call and return instructions executed at runtime. It records the values of registers when executing call instructions, and verifies that callee-saved registers haven't changed when executing the matching function return. This warning is generated at the return instruction.

{{DisableMsg|Function clobbered callee-saved register}}