I have recently come across what appears to be an optimization bug. I am
using gcc version 4.3.1 configured for a powerpc 750. I am using -O2
optimization. The bug is related to passing a double defined as a local
variable into a trace function which takes 8 32-bit unsigned integers of
auxiliary data. The trace function is using the c++ default arguments (not
varargs). When I want to trace my local double I am performing the
following cast conversions to map the upper and lower data words into 32-bit
aux data parameters.
double myDouble;
...
Trace((unsigned int)(*(long long int*)&myDouble) >> 32),
// aux data word 1 myDouble
upper word
(unsigned int)(*(long long int*)&myDouble) & 0xffffffff) );
// aux data word 2 myDouble
lower word
The trace method logs the data to a buffer in memory. When I dump the data
logged in my buffer, the values are clearly not 64-bit floating point. They
appear to be 32-bit memory addresses that were residual on the stack. This
only seemed to be a problem with some of the double variables that I passed
into the trace function. For example, in the case below,
double myDouble1 = func1();
...
double myDouble2 = func2();
Trace(((unsigned int)(*(long long int*)&myDouble1) >> 32),
(unsigned int)(*(long long int*)&myDouble1) & 0xffffffff),
(((unsigned int)(*(long long int*)&myDouble2) >> 32),
(unsigned int)(*(long long int*)&myDouble2) & 0xffffffff));
The "myDouble1" value looks correct but the "myDouble2" value is incorrect.
I was able to fix the problem 3 different ways. The first was simply to
remove optimization (i.e -O0). The second was to make the bad double
parameters global as in the example below.
double myDouble1 = func1();
...
static double myDouble2 = func2();
Trace(((unsigned int)(*(long long int*)&myDouble1) >> 32),
(unsigned int)(*(long long int*)&myDouble1) & 0xffffffff),
(((unsigned int)(*(long long int*)&myDouble2) >> 32),
(unsigned int)(*(long long int*)&myDouble2) & 0xffffffff));
Another thing I tried was to insert a nop instruction just before the call
to Trace as in the example below.
double myDouble1 = func1();
...
double myDouble2 = func2();
asm("nop");
Trace(((unsigned int)(*(long long int*)&myDouble1) >> 32),
(unsigned int)(*(long long int*)&myDouble1) & 0xffffffff),
(((unsigned int)(*(long long int*)&myDouble2) >> 32),
(unsigned int)(*(long long int*)&myDouble2) & 0xffffffff));
I am not sure why the last example worked. The only thing I can think of is
that the "asm" call prevents the method from being optimized. Let me know
if this is a known issue or if you need more details.
Thanks,
Kevin
