Comment by 201984
Comment by 201984 a day ago
What's your reason for -O2 over -O3?
Comment by 201984 a day ago
What's your reason for -O2 over -O3?
-O3 gained a reputation of being more likely to "break" code, but in reality it was almost always "breaking" code that was invalid to start with (invoked undefined behavior). The problem is C and C++ have so many UB edge cases that a large volume of existing code may invoke UB in certain situations. So -O2 thus had a reputation of being more reliable. If you're sure your code doesn't invoke undefined behavior, though, then -O3 should be fine on a modern compiler.
That's a little vague, I'd put that more pointedly: they don't understand how the C and C++ languages are defined, have a poor grasp of undefined behaviour in particular, and mistakenly believe their defective code to be correct.
Of course, even with a solid grasp of the language(s), it's still by no means easy to write correct C or C++ code, but if your plan it to go with this seems to work, you're setting yourself up for trouble.
Compiler speed matters. I will confess to not as much practical knowledge of -O3, but -O2 is usually reasonable fast to compile.
For cases where -O2 is too slow to compile, dropping a single nasty TU down to -O1 is often beneficial. -O0 is usually not useful - while faster for tiny TUs, -O1 is still pretty fast for them, and for anything larger, the increased binary size bloat of -O0 is likely to kill your link time compared to -O1's slimness.
Also debuggability matters. GCC's `-O2` is quite debuggable once you learn how to work past the possibility of hitting an <optimized out> (going up a frame or dereferencing a casted register is often all you need); this is unlike Clang, which every time I check still gives up entirely.
The real argument is -O1 vs -O2 (since -O1 is a major improvement over -O0 and -O3 is a negligible improvement over -O2) ... I suppose originally I defaulted to -O2 because that's what's generally used by distributions, which compile rarely but run the code often. This differs from development ... but does mean you're staying on the best-tested path (hitting an ICE is pretty common as it is); also, defaulting to -O2 means you know when one of your TUs hits the nasty slowness.
While mostly obsolete now, I have also heard of cases where 32-bit x86 inline asm has difficulty fulfilling constraints under register pressure at low optimization levels.
Yeah, -O3 generally performs well in small benchmarks because of aggressive loop unrolling and inlining. But in large programs that face icache pressure, it can end up being slower. Sometimes -Os is even better for the same reason, but -O2 is usually a better default.
Most people use -O2 and so if you use -O3 you risk some bug in the optimizer that nobody else noticed yet. -O2 is less likely to have problems.
In my experience a team of 200 developers will see 1 compiler bug affect them every 10 years. This isn't scientific, but it is a good rule of thumb and may put the above in perspective.
The estimate includes visual studio, and other compilers that are not open source for whatever optimization options we were using at the time. As such your question doesn't make sense (not that it is bad, but it doesn't make sense).
In the case of open source compilers the bug was generally fixed upstream and we just needed to get on a newer release.
People keep saying "O3 has bugs," but that's not true. At least no more bugs than O2. It did and does more aggressively expose UB code, but that isn't why people avoid O3.
You generally avoid O3 because it's slower. Slower to compile, and slower to run. Aggressively unrolling loops and larger inlining windows bloat code size to the degree it impacts icache.
The optimization levels aren't "how fast do you want to code to go", they're "how aggressive do you want the optimizer to be." The most aggressive optimizations are largely unproven and left in O3 until they are generally useful, at which point they move to O2.
I am not sure. I saw quite a few of these bugs where programmers were told it is UB but it isn't.
For example, people showed me
extern void g(int x);
int f(int a, int b)
{
g(b ? 42 : 43);
return a / b;
}
https://developercommunity.visualstudio.com/t/Invalid-optimi...
Other compilers have similar issues.
More aggressive optimization is necessarily going to be more error prone. In particular, the fact that -O3 is "the path less traveled" means that a higher number of latent bugs exist. That said, if code breaks under -O3, then either it needs to be fixed or a bug report needs to be filed.
Historically, -O3 has been a bit less stable (producing incorrect code) and more experimental (doesn't always make things faster).
Flags from -O3 often flow down into -O2 as they are proven generally beneficial.
That said, I don't think -O3 has the problems it once did.