Advent of Compiler Optimisations 2025
xania.org336 points by vismit2000 15 hours ago
336 points by vismit2000 15 hours ago
Matt is amazing. After checking out his compiler optimizations, maybe check out the recent interview I did with him.
What I’ve come to believe is this: you should work at a level of abstraction you’re comfortable with, but you should also understand the layer beneath it.
If you’re a C programmer, you should have some idea of how the C runtime works, and how it interacts with the operating system. You don’t need every detail, but you need enough to know what’s going on when something breaks. Because one day printf won’t work, and if the layer below is a total mystery, you won’t even know where to start looking.
So: know one layer well, have working knowledge of the layer under it, and, most importantly, be aware of the shape of the layer below that.
Also this article in acmqueue by Matt is not new at all, but super great introduction to these types of optimizations.
The “understand one layer below where you work” is something my professors at uni told us 10+ years ago. Not sure where that originated from, but I really think that benefited me in my career. I.e understanding the JVM when dealing with Java helped optimize code in a relatively heavyweight medical software package.
And also, it’s just fun to understand the lower layers.
I really appreciate that despite being an obvious domain expert, he’s starting with the simple stuff and not jumping straight into crazy obscure parts of the x86 instruction set
Matt Godbolt is an absolute gem for the C & C++ community.
Many thanks to him for that.
Between that and compiler explorer, it is fair to say he made the world a better place for many of us, developers.
Advent of Computer Science Advent Calendars, Day 2
I am personally interested in the code amalgamation technique that SQLite uses[0]. It seems like a free 5-10% performance improvement as is claimed by SQLite folks. Be nice if he addresses it some in one of the sessions.
This is a pretty standard topic, and not really a compiler optimization. It's usually called a unity build.
Unity builds have been largely supplanted by LTO. They still have uses for build time improvements in one-off builds, as LTO on a non-incremental build is usually slower than the equivalent unity build.
At my company, we have not seen any performance benefits from LTO on a GCC cross-compiled Qt application.
GCC version: 11.3 target: Cortex-A9 Qt version: 5.15
I think we tested single core and quad core, also possibly a newer GCC version, but I'm not sure. Just wanted to add my two cents.
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.
After 25-years of software development, I still wonder whether I’m using the best possible compiler flags.
What I've learned is that the fewer flags is the best path for any long lived project.
-O2 is basically all you usually need. As you update your compiler, it'll end up tweaking exactly what that general optimization does based on what they know today.
Because that's the thing about these flags, you'll generally set them once at the beginning of a project. Compiler authors will reevaluate them way more than you will.
Also, a trap I've observed is setting flags based on bad benchmarks. This applies more to the JVM than a C++ compiler, but never the less, a system's current state is somewhat random. 1->2% fluctuations in performance for even the same app is normal. A lot of people won't realize that and ultimately add flags based on those fluctuations.
But further, how code is currently layed out can affect performance. You may see a speed boost not because you tweaked the loop unrolling variable, but rather your tweak may have relocated a hot path to be slightly more cache friendly. A change in the code structure can eliminate that benefit.
I'd say -O2 -march=native -mtune=native is good enough, you get (some) AVX without the O3 weirdness.
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.
Doesn't -O2 still exclude any CPU features from the past ~15 years (like AVX).
If you know the architecture and oldest CPU model, we're better served with added a bunch more flags, no?
I wish I could compile my server code to target CPU released on/after a particular date like:
-O2 -cpu-newer-than=2019You 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.
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.
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.
For x86 it's a pretty good guarantee.
I don't understand if your comment is ironic. Intel is notorious for equipping different processors produced in the same period with different features. Sometimes even among different cores on the same chip. Sometimes later products have less features enabled (see e.g. AVX512 for Alder Lake).
You should at a minimum add flags to enable dead object collection (-fdata-sections and -ffunction-sections for compilation and -Wl,--gc-sections for the linker).
What's your reason for -O2 over -O3?
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.
-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.
Oh, there are also plenty of bugs. And Clang still does not implement the aliasing model of C. For C, I would definitely recommend -O2 -fno-strict-aliasing
Exactly. A lot of people didn’t understand the contract between the programmer and the compiler that is required to use -O3.
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.
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).
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.
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 say there is a fair share of cases where programmers were told it is UB when it actually was a compiler bug - or non-conformance.
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.
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.
Would you say that bug estimate is when using -O2 or -O3?
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.
I'm looking forward to the remaining posts. The first thing I did this AM was teach SBCL how to optimize `(+ base (* index scale))` and `(+ base (ash index n))` patterns into single LEA instructions based on the day 2 learnings.
You can never have too much Godbolt!
I hope he ends up covering integer division by constants. The chapter on this in Hacker's Delight is really good but a little dense for casual readers.
Advent of Code for compiler nerds. Love this format - daily bite-sized optimization lessons build intuition far better than dense textbooks. Understanding what compilers do and why they do it makes you a better programmer in any language.
Is there a PDF somewhere? I'm not really able to follow YT videos.
Thanks for sharing, I've always found optimizing a really interesting field, I will keep a close eye!
This is really cool. Congrats on the quality of the work!
I don't understand
where is the problem to be solved?
The problem is “to add two numbers”. The meta-problem is “to learn how computers work”.
I think they're expecting a daily problem set like Advent of Code. This is not a set of problems to solve, it's a series with one release per day in December, similar to an Advent calendar.