Re: [Boost-users] boost::lockfree::queue not working with -fgnu-tm

On Thursday, July 17, 2014 11:08 PM, Angelo Mondaini wrote:

2014-07-16 12:46 GMT-03:00 Ben Pope mailto:benpope81@gmail.com>:

According to the docs:

-O2 is "full optimisation" -O3 is "full optimisation with more aggressive inlining and vectorisation"

That's a strange description that's possibly no longer true.

I don't think either of them optimise for memory or binary size, more for speed (specifically at the expense of binary and memory size).

I also don't think either of them "fail by its own", modulo compiler bugs.

This idea that high optimisation levels produce incorrect code is strange to me. From what I've seen, aggressive optimisations are more likely to highlight undefined behaviour, but code that relies on undefined behaviour is broken code. The compiler doesn't break correct code, it trusts what you tell it.

So please, can we stop the FUD and just get on with a fix; either in the library or the compiler, for technical or practical reasons.

See this link, the "Known problems" section may help:

I only took a quick look, that article is around 2 years old, does gcc-4.9 suffer the same problem?

The thing about "aggressive optimisations are more likely to highlight undefined behaviour" is true if your compiler is perfect, and this may be almost true for well established compiler features, I don't believe this is the case of "-fgnu-tm", "-O3" or even "-std=c++11". That is why this is not enabled by default.

Well, I did say modulo bugs. Of course, there might be bugs in the C++03 code or unoptimised code generation paths of a particular compiler.

Look what Gentoo's wiki says about "-O3": "-O3: This is the highest level of optimization possible. It enables optimizations that are expensive in terms of compile time and memory usage. Compiling with -O3 is not a guaranteed way to improve performance, and in fact in many cases can slow down a system due to larger binaries and increased memory usage. -O3 is also known to break several packages. Therefore, using -O3 is not recommended."

Breaking a package probably means the package invokes UB. Given that those packages haven't been fixed, -O3 is unwise for a system-wide compile flag, but that shouldn't scare you into not using it for code that doesn't invoke UB or assuming the compiler just generates garbage.

Because of this, I am not sure the code is the problem or the only problem.

Right, so a technical fix of the code, or a practical workaround of a popular but broken compiler are useful. Ben