( My apologies to the admins, who have seen this multiple times already. )
Developers,
I believe I've found a bug in msp430-gcc. I have no great interest in
msp430-gcc development, just getting this
particular bug fixed. So I'll likely unsubscribe soon.
Version
======
$ msp430-gcc --version
msp430-gcc.exe (GCC) 3.2.3
Copyright (C) 2002 Free Software Foundation, Inc. ...
This likely came from mspgcc-20081230.exe .
Test program:
==========
int main(unsigned char x[]) {
unsigned int a, b;
a = x[0] * 100;
b = x[1] * 100;
return a;
}
==========
Compile command line (cygwin):
msp430-gcc -mmcu=msp430x247 -Wall -O1 -S test.c
(Bug does not happen with -O0)
Incorrect output (trimmed)
==============
push r2
dint
nop
mov.b @r15, &__MPY
mov.b #llo(100), &__OP2
mov &__RESLO, r14
pop r2
push r2
dint
nop
mov.b 1(r15), &__MPY // NOTE 1
mov &__RESLO, r15
pop r2
mov r14, r15
==================
Note the lack of assignment to __OP2 after NOTE 1 and before the
result is read with __RESLO. It is not necessary to reload one of the
OP1 registers (__MPY is one of these), but it IS necessary to load
__OP2 to trigger the multiply process. The result of the multiply is
not used in this simplified test program, but I assure you that this
bug produces wrong behaviour in real-world code.
It seems that gcc feels it can omit the second assignment to __OP2,
since it already has the required value (immediate value 100).
Somehow, gcc needs to be told that assignment to __OP2 has side
effects, and cannot be optimised out.
it seems that my bug is already in the system with this ID: 2082985
http://sourceforge.net/tracker/index.php?func=detail&aid=2082985&group_id=42303&atid=432701
I can only hope that my simpler example makes it easier to track down.
- Mike
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