Question

I’m debugging an ANSI C program run on 64-bit Linux CentOS 5.7 using gcc44 and gdb. I have the following loop in the program:

for (ii = 1; ii < 10001; ii++) {
    time_sec[ii] = ( 10326 ) * dt - UI0_offset;  /* in seconds */ 
    printf("\ntime_sec[%d] = %16.15e, dt = %16.15e, UI0_offset = %26.25e\n", 
           ii, time_sec[ii], dt, UI0_offset);
}

where time_sec, dt, and UI0_offset are doubles. The relevant gdb session is:

(gdb) p time_sec[1]
$2 = 2.9874137906250006e-15
(gdb) p ( 10326 ) * dt - UI0_offset
$3 = 2.9874137906120759e-15

Why are $2 and $3 different numbers? The $2=time_sec[1] is computed by the c program, whereas $3 is the same equation, but computed in gdb.

I’m porting a Matlab algorithm to C and Matlab (run on a different machine) matches the gdb number $3 exactly, and I need this precision. Anyone know what could be going on here, and how to resolve?

UPDATE: After some debugging, it seems the difference is in the value of UI0_offset. I probed gdb to reveal a few extra digits for this variable (note: anyone know a better way to see more digits in gdb? I tried an sprintf statement but couldn’t get it to work):

(gdb) p UI0_offset -1e-10
$5 = 3.2570125862093849e-12

I then inserted the printf() code in the loop shown in the original posting above, and when it runs in gdb it shows:

time_sec[1] = 2.987413790625001e-15, dt = 1.000000000000000e-14, 
UI0_offset = 1.0325701258620937565691357e-10

Thus, to summarize:

1.032570125862093849e-10 (from gdb command line, the correct value)
1.0325701258620937565691357e-10 (from program's printf statement, NOT correct value)

Any theories why the value for UI0_offset is different between gdb command line and the program running in gdb (and, how to make the program agree with gdb command line)?

Answer 1

I’m not sure if the x64 architecture contains the same 80-bit (long double) FP registers as the x86 does, but oftentimes results like these in the x86 world arise when intermediate results (i.e. the first multiplication) remain in the 80-bit registers rather than getting flushed back to cache/RAM. Effectively part of your calculation is done at higher precision, thus the differing results.

GCC has an option (-ffloat-store if my memory serves) that will cause intermediate results to be flushed back to 64-bit precision. Try to enable that and see if you match the GDB/Matlab result.