c++ - Are there any drawbacks to using -O3 in GCC?

Question

I've been a Software Engineer for 13 years in various languages, though I'm just now making my way into C and later C++. As I'm learning C, I'm using the GCC compiler to compile my programs, and I'm wondering if there are any gotchas to using -O3 or other optimization flags. Is there a chance that my software will break in ways that I won't be able to catch without testing the compiled code, or perhaps during cross-compilation, I might inadvertently mess something up for a different platform.

Before I blindly turn those options on, I'd like to know what I can expect. Also, as -Ofast turns on non-standards-compliant flags, I'm leaning towards not using that. Am I correct in my assumptions that -Ofast will most likely have "side-effects?"

I've glanced over https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html before I posted this question.

Answer 1

The only drawback of -O3 should be the inability to follow the code in a debugger.

The use of -Ofast can affect some of your floating point operations causing rounding errors, but unless you are running specifically long chains of floating point calculations you are unlikely to ever notice.

Broken code (code with bad pointers or statements with undefined behavior) is likely to behave differently at different level of optimization -- the first reaction of many programmers is to blame the compiler -- enabling all warnings and fixing them usually helps.