Authoritative answer wanted: "-g -O1" vs. "-O1"

[Gene Smith]

If I compile my embedded program with options "-g -O1" I obtain an elf 
file with debug information. I can objcopy the elf file to a binary or 
hex file that can be loaded to flash, effectively striping out the debug 
information leaving only the optimized code in ROM.

But if I re-build with options the same except omit the -g option, 
obviously I will have no symbols in the elf file making debugging 
impossible or at least more difficult. However, when I object copy this 
elf to a binary or hex file they are different somewhat than the binary 
or hex produced with options -g present. At least with 4.7.3 the main 
difference, as seen with objdump, is in the prologue to certain function 
calls with only a few bytes different in total code length on a fairly 
large embedded application (arm). So -g has some effect on the actual 
code produced it appears.

Is this difference expected? Should -g cause changes in the actual code 
generated and not just add debug symbols to the elf? Possibly it is 
related to the optimization level? I have not checked to see if the 
results differ with higher or lower levels than -O1.

I have seen several opinions regarding this but no authoritative answer. 
The gcc manual also does not really answer this.


Thanks,
-gene