Re: [avr-libc-dev] [bugs #11396] Large number jumps to hyperspace

[Paul Schlie]

> It's a total pain trying to find tool bugs by looking at the
> asm output in a simulator. I've been looking at:
> 
> #include<inttypes.h>
> 
> int
> main(void)
> {
>      uint16_t baud=9600;
>      uint16_t reg=16000000/(16*baud) - 1;
>      return 0;
> }
> 
> using gcc flags to put out rtl code (it'll take me a while
> to understand it).

If it's of any help, the code as written shouldn't actually do anything,
as it will be optimized away. If you want to force an expression to be
evaluated as written although it's assigned value may not be used, designate
all interesting variables as volatile for testing purposes.  Also as a heads
up, if baud has a known value, reg's will also be statically computed (i.e.
at compile time), so don't expect to see anything other than it's
pre-computed value being assigned.  Lastly, which is likely your problem, if
baud is not known statically, reg will computed dynamically (assuming it's
value is used), but 16000000 is a ~34 bit number, resulting in a 64-bit (DI
mode) division if designated as a long long computation, but it's not clear
to me that 64-bit arithmetic has ever properly worked for the avr target; or
you should have seen a warning that value was too large an int literal,
where the value would have been truncated to a 16-bit int value = 0x2400 =
9216, which divided by (16*baud), would result in 0 - 1, resulting in -1,
which may have affected a conditional expression or hw timer somewhere else
in your code or system, or something like that.