[SOLVED] gcc inline asm clobber list question

[dchapiesky]

for the code...

Code: Select all

unsigned long b_input(unsigned char *str, unsigned long count)
{
	unsigned long len;
	asm volatile ("call *0x0000000000100020" : "=c" (len) : "c"(count), "D"(str));
	return len;
}

Should "mem" be added in the clobber list? as in:

Code: Select all

unsigned long b_input(unsigned char *str, unsigned long count)
{
	unsigned long len;
	asm volatile ("call *0x0000000000100020" : "=c" (len) : "c"(count), "D"(str) : "mem");
	return len;
}

Since the call to 100020 modifies memory pointed to by str

Just trying to tidy up code and your input will help.

cheers

[Kevin]

Yes, you should add "memory" to the clobber list then.

[dchapiesky]

Thanks... I thought so but inline asm isn't my strong suite

[Octocontrabass]

You should also add "cc" to the clobber list if that function modifies any flags. Everyone seems to forget that one.

[dchapiesky]

'"cc" like carry flag? If you can clarify I need this....

[Icee]

'"cc" like carry flag? If you can clarify I need this....

CC is for all condition codes, i.e. AF, PF, CF, OF, SF, ZF. Otherwise GCC is free to assume that your inline assembly snippet preserves them and might try to use previous values after the call in generated code.

[dchapiesky]

Thanks

Final code:

Code: Select all

unsigned long b_input(unsigned char *str, unsigned long count)
{
	unsigned long len;
	asm volatile ("call *0x0000000000100020" : "=c" (len) : "c"(count), "D"(str) : "cc", "memory");
	return len;
}

edited for typo

[Icee]

Thanks

Also note that when calling functions from inline assembly many of the usual call convention related aspects may not hold. For instance, the direction flag (DF) may be set and the stack may be misaligned.

[dchapiesky]

For instance, the direction flag (DF) may be set and the stack may be misaligned.

Yah.. ran into the misaligned stack on a different call from this example.

I am going to open a separate thread for another inline asm question that has been bugging me...

Thank you all for your responses