Comment by nwallin | Hacker News Reader

Comment by nwallin | Hacker News Reader

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It's very dependent on the code.

I'm currently writing a path tracer as a side project, and -Ofast is sickeningly faster than -O2. Like 4-5 times faster. -O3 duplicates all the loops into two versions: one with AVX instructions chunking 8 iterations at a time, and a second scalar version that does the final 1-7 iterations. -Ofast is a little bit faster than -O3 because it generates the approximate rsqrtps and rcpps instructions.

However this is a special case. Most code isn't heavy on crunching massive quantities of floats, and much of the code that does isn't written in a way that gives the compiler the freedom to autovectorize your loops. And a surprisingly large amount of code is still compiled with MSVC which won't vectorize at all.

In gcc, -O2 is generally fastest for general purpose code, both faster than -Os and -O3. As an example of why -O2 is faster than -Os, -O2 will optimize signed integer division by a constant power of two into a handful of bitwise instructions, which are larger than a single (but much, much slower) division instruction. (signed integer division can't be replaced with a single bitshift because negative integers work differently. unsigned integer division can be replaced by a single bitshift.) People think this is a universal optimization, but it's a size tradeoff so -Os specifically eschews it.

You might be misremembering but strength reduction is part of ‘-O1’ [0].

However, my point for a general purpose language system like OpenJDK that favors multithreading or multiprocessing or server workloads, each core/thread and process/thread share the I-Cache/L1 cache. Address lines are still 64 bytes. Code for these systems rarely is expected to run in isolation. I tend to want to be a good neighbor and reduce code size when I can.

[0] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

I'm actually super way too drunk to give you a full rebuttal, so here's a godbolt compilation thing that proves that I'm right and you're wrong:

https://cpp.godbolt.org/z/-zeMWT

(stop reading now if that's enough)

The -Os compilation is some bookkeeping and an idiv instruction, the -O2 compilation is bookkeeping, two shifts, and an and instruction. I don't know what they because I'm drunk sorry not sorry but the one that does the idiv instruction is both slower and smaller and that's on purpose. (you can tell the idiv one is smaller by clicking the 11010 button to display offsets. the starting instructions of both functions have the same offset, but the final instruction of the -Os compilation is substantially smaller than the -O2 compilation.)

The description of -Os which you linked is telling. It says it's -O2 without certain optimizations, and then it also says:

> It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size.

Somewhere buried in that tuning for code size rather than execution speed and further optimizations designed to reduce code size is an optimization that will replace bitwise magic with division statements. -Os does what it says on the tin. It makes your code small. It makes your code slow. It does so on purpose.

People think that -Os is a superset of -O1 and a subset -O2. It's neither. It is neither a superset of -O1 nor -O2, nor is it a subset of -O1 or -O2. There are speed optimizations that -Os adds to -O1 and there are size optimizations that -Os adds to both -O1 and -O2.

The point, I think, of -Os, is for embedded. If you have a size n PROM, and your code compiles to size n+1 with -O2, and if you apply -Os and it compiles to size n, that's a feature. -Os, in my opinion, ought to be uncompromising towards that goal. For better or for worse.

I'll be sober in the morning and can engage with you better then. Sorry.

Just wanted to say, you were surely drunk when writing that, but it was still very fun to read.

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