re: Different object code generated each time file is compiled

[Duane Ellis <duane-gcc-help at duaneellis dot com>]

>> They pre-processor output from each run is the same.  So it appears 
for these two source files that the issue with the object code being 
generated differing comes from the compiler.  It is the object files 
before linking that are different (as well as the resulting final 
binaries but we have tracked it back to the object files emitted by the 
compiler as the first point where the files differ.

You are missing a step. You are thinking "visual studio" type approach 
which creates a .O file directly. GCC does it the traditional way - it 
generates assembler output.

The next step is the *ASSEMBLER* output, the "-S" output, the 
pre-processor output is from -E.

Is the "-S" output same or different?

If same - Then the problem is in GAS.... (the assembler)

If it is in GAS,  you can use "objdump" for your target, ie:  
mips32-<SOMETHING>-objdump, and dump the elf  <dot><oh> file, and see 
what is different.

Often, you find something dumb - like a date/time stamp of the input 
file, you forgot about.

ie: I have been burned by this construct buried deep within layers of 
macro expansion.

   printf("This file was built on: %s at %s\n", __DATE__ , __TIME__ );

I had our "identical binary image" rule amended to read something like this:

   Resulting binary must be identical after using the command line tool 
FOOBAR that "back date/time stamps" the image header and re-calculates 
the image checksum.  We also wrote a tool, which did a 'binary image 
compare' and knew about the location of our "image build date/time 
stamp" and could ignore it.

-Duane.