Hi all
I have the feeling that the msp430-gcc has a bug in its optimization
step when it comes to shift operations on uint32_t. If I write
uint32_t tmp = 0x12345678;
tmp >>= 18;
if(tmp == 0) {
call Leds.led0Toggle();
}
The Led toggles (which it should not to my understanding).
The problem does not occur if:
- tmp is not defined in the local scope of the function
- tmp is defined volatile
- the shift result is assigned to another variable, e.g. the following
is fine:
uint32_t tmp = 0x12345678;
uint32_t tmp2 = tmp >> 18;
// now, tmp2 != 0
For shifts (>>) of 0..16, everything works fine. For shifts (>>) 17..31,
the result is zero in the above case. In fact, the compiler does not
emit any code to determine the result of the shift, but already decides
at compile time that the result is zero, even if the value to be shifted
is unknown. E.g. the following code toggles Led0.
event void Timer.fired() {
uint32_t tmp = call Random.rand32(); // unknown value at compile time
if(tmp < 0x100000) {
call Leds.led2Toggle(); // don't indicate false alarms
}
tmp >>= 18; // or: tmp = tmp >> 18;
if(0 == tmp) {
call Leds.led0Toggle();
}
}
However, if I write the following, everything is OK again. (The
following does not blink Led 0.)
event void Timer.fired() {
uint32_t tmp = call Random.rand32(); // unknown value at compile time
if(tmp < 0x100000) {
call Leds.led2Toggle(); // don't indicate false alarms
}
if(0 == (tmp >> 18)) {
call Leds.led0Toggle();
}
}
Also, if the amount to be shifted is given implicitly, seems to work OK.
(The following does not blink Led 0.)
event void Timer.fired() {
uint32_t tmp = call Random.rand32(); // unknown value at compile time
uint8_t i = 18;
if(tmp < 0x100000) {
call Leds.led2Toggle(); // don't indicate false alarms
}
tmp >>= i;
if(0 == tmp) {
call Leds.led0Toggle();
}
}
Q1: Is this a known problem or am I doing something terribly wrong?
Q2: If it is already known, is there a known workaround (other than
modifying the code?)
Tested on latest TinyOS CVS checkout and 1-week old installation of the
entire tinyos environment under Ubuntu 9.10. (msp toolchain installed
with the apt-get tinyos-2.1.0.)
Also, I've downloaded & installed the latest version of the msp430-gcc
toolchain (using the instructions from
http://sourceforge.net/apps/mediawiki/mspgcc/index.php?title=Linux_installation).
But the problem persists with this compiler.
Kind Regards
Roland
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