Interprocedural optimization in GCC | Hacker News Reader

Interprocedural optimization in GCC

kristerw.blogspot.com

84 pointsmatt_d8 comments

Tangential Q: someone mentioned that GCC is right up there in terms of most complex piece of software ever written. People with enough know-how would vouch for this? Are compilers the most complex piece of the puzzle? Or, are there other type of wares that dwarf GCC or compilers, in general, in terms of complexity (like, say, JIT interpreters, or DB Query Execution Engines, or HDLs).

Web browsers are astoundingly complicated. They generally contain multiple JIT compilers, a database engine, sophisticated rendering code for 2d and 3d graphics and fonts, video and audio codecs, support for a variety of network protocols, graphical debuggers and development tools... the list goes on. And they expose all this to arbitrary applications downloaded from the internet while attempting (and mostly succeeding) to maintain your security and privacy and remain backwards compatible with 20+ years of legacy content. At a target frame rate of 60 fps.

Browsers are, when you think about it, pretty damn amazing.

Taken as a whole, sure, but all those things you refer to in a browser are components, probably developed by different teams or even reused by entirely separate projects. By that measure, a modern Linux distribution is far more complicated than a web browser, since it includes a kernel, compilers, multiple web browsers, codecs, etc. The Firefox browser, for instance, I believe uses the SQLite database, so you can't claim that the Mozilla devs created that complexity, they just included it.

I think a compiler is different because you can't break it apart into separate projects like that. All the parts that go into a compiler are really only useful for that compiler, not as common pieces of "infrastructure" that can be used by many disparate larger projects.

Depending on whether they qualify as "software" (IMO, yes) my guess is the HDL[1] specifications for modern CPUs are tops in complexity, in lines of code if not man-hours (I imagine they're mostly procedurally-generated).

[1] https://en.wikipedia.org/wiki/Hardware_description_language

Identical code folding is slick. It's the opposite of using macros, and is harder to fold code than to generate it.

devirtualization sounds like something that could significantly improve performance in large C++ projects.

Hope to see some real-world benchmarks soon!

Hi, I just created an account to give you a bit of feedback.

When g++4.9 went out, our old mathematical/financial library was finally able to be built with LTO.

See, to be effective, devirtualisation requires LTO, it also requires profile-guided analysis to detect the right use-cases to devirtualise, and it requires that your functions can in effect be "hidden" from the outside, so that the compiler can tune the inlined functions.

As our codebase was also portable to Windows (and so, had all the right declspec (dllexport) lying around), we were able to use the -fvisibility=hidden flag. We just had to make sure that everything was within one big fat shared-lib.

On old CPUs (e.g: Nehalem class), our code ran 2 times faster than the same code compiled with -O3. On Haswell-E, it ran 50% faster.

There's also a very cheap way to force devirtualisation: use the C++11 "final" keyword on your methods. G++ can optimise and inline where applicable.

Thank you!

I was hoping this could automatically improve performance for existing projects but it seems it is a bit more complicated than that. Either way, 50-100% improvements sounds very nice.

Reply on news.ycombinator.com