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Von: Peter Bigot
An: Conrado P. L. Gouvêa
Gesendet am: 08 Mrz 2012 13:35:56
Betreff: Re: [Mspgcc-users] Incorrect generated code
> Yes, unless it is marked as "hosted" main is handled specially; there
> has been recent discussion about why in a thread about creating
> archives that contain the main routine. See also
> http://www.mail-archive.com/mspgcc-users@lists.sourceforge.net/msg09750.html.
> At a quick glance does appear that the second call is not appropriate,
> though the return value is right (r15 has the correct value when main
> is left). If you'd like this investigated, please file a ticket on
> the tracker. https://sourceforge.net/tracker/?group_id=42303&atid=432701
>> int main(void) {
>> if (dummy() == 1) {
>> foo();
>> return 1;
>> }
>> return 0;
>> }
>> 0000403e <main>:
>> 403e: b0 12 62 40 call #0x4062
>> 4042: 1f 93 cmp #1, r15 ;r3 As==01
>> 4044: 01 24 jz $+4 ;abs 0x4048
>> 4046: 0f 43 clr r15
>> 4048: b0 12 58 40 call #0x4058
>> 404c: 1f 43 mov #1, r15 ;r3 As==01
Teh second call is appropriate. The code does what it shall do.
Except that the source code does something that is not expected for main:
It returns. Where to?
The 'problem' in the above code is that main is not expected to end.
So the return statement does not produce a RET instruction
(which otherwise would be between the "clr r15" and the second call)
main is a naked function, so even if you use a return statement, no
RET instruction is generated. And in this specific case, due to the
optimizing reordering, the 'return 0' falls through to the if block.
Well, I must admit I had expected some jump to a dummy
'end of main' label or such. But thinking over it, it is obvious, that
on a naked function you're responsible for exiting the function properly
A return statement sets the return value, but that's it.
i foyu don't add your own jmp/ret/whatever code after the return statement,
it just continues executing what follows.
Returning something form main (or returning from main in general) doesn't make
any sense at all for a microcontroller program that isn't hosted by an OS. :)
JMGross
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