Advent of Compiler Optimisations 2025

I used godbolt.org dozens of times, and I never bothered to look at "about".

D'Oh.

Sponsoring him on Github right now...

That's great if you're compiling for use on the same machine or those exactly like it. If you're compiling binaries for wider distribution it will generate code that some machines can't run and won't take advantage of features in others.

To be able to support multiple arch levels in the same binary I think you still need to do manual work of annotating specific functions where several versions should be generated and dispatched at runtime.

It's not an -O2 thing. Rather it's a -march thing.

-O2 in gcc has vectorization flags set which will use avx if the target CPU supports it. It is less aggressive on vectorization than -O3.

You can use x86_64-v2 or x86_64-v3. Dates are tricky since cpu features aren't included on all SKUs from all manufacturers on a certain date.

A CPU produced after a certain date is not guaranteed to have the every ISA extension, e.g. SVE for Arm chips. Hence things like the microarchitecure levels for x86-64.

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.

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.

You have to profile for your specific use case. Some programs run slower under O3 because it inlines/unrolls more aggressively, increasing code size (which can be cache-unfriendly).

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.

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 would expect a little benefit from devirt (but maybe in-TU optimizations are getting that already?), but if a program is pessimized enough, LTO's improvements won't be measurable.

And programs full of pointer-chasing are quite pessimized; highly-OO code is a common example, which includes almost all GUIs, even in C++.

Do you link against a version of the Qt library that provides IR objects?

In any case even with whole program optimization, O would expect that effectively devirtualizing an heavily object oriented application to be very hard.