[fps_doug]

We've got a few components written in C that I'm (partially) responsible for. It's mostly maintenance, but for reasons like this I run that code with -O0 in production, and add all those kinds of flags.

I'd be curious to know how much production code today that's written in C is that performance critical, i.e. depends on all those bonkers exploits of UB for optimizations. The Linux kernel seems to do fine without this.

[NohatCoder]

I'm fairly confident in declaring the answer to your question: None.

Most programs rarely issue all the instructions that a CPU can handle simultaneously, they are stuck waiting on memory or linear dependencies. An extra compile-out-able conditional typically doesn't touch memory and is off the linear dependency path, which makes it virtually free.

So the actual real-world overhead ends up at less than 1%, but in most cases something that is indistinguishable from 0.

If you care that much about 1% you are probably already writing the most performance critical parts in Assembly anyway.

[sanxiyn]

> If you care that much about 1% you are probably already writing the most performance critical parts in Assembly anyway.

I call this hotspot fallacy and it is a common one. This assumes there is relatively small performance critical parts that can be rewritten in assembly. Yes, sometimes there is a hotspot, but by no means always. A lot of people caring about 1% is running gigabytes binary on datacenter scale computer without hotspots.