Auto-detect "bad" compiler flags

Usage
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#1

Hi all,

I’ve got an old project that uses the clang /Ox flag for certain configurations on Windows. According to the online clang documentation (Clang Compiler User’s Manual — Clang 11 documentation),
this flag is deprecated. However, clang doesn’t produce any warnings or errors when this flag is used.

I would like to configure CMake to warn me about “bad” compiler flags (on both windows and Linux and with clang and gcc), so /Ox for clang (which is deprecated), /Og on clang (which does nothing), and so on. The only way I can think of to do this is to trawl through the clang and gcc documentation, draw up my own blacklist of “bad” flags, and compare that against CMAKE_CXX_FLAGS and friends. Is there a better way?

Disclaimer: It’s really hard to google for this - mostly I just get advice about how to check for deprecated C++ features, which isn’t quite what I’m interested in.

Thanks,
Tom

#2

Cmake has a CheckCompilerFlag module, but I believe that just checks if enabling a certain flag produces an error.

Really, I think the best answer is to just not deal with specific flags at all. CMake is meant to be a higher level of abstraction for describing your build requirements, and you let the cmake devs worry about the actual low level flags for each compiler.

#3

For the flags CMake has abstractions for, sure. Optimization flags are not abstracted as such in CMake, so anything would involve manual manipulation.

#4

True. I actually have a stock “optimization flags” target that can be linked to: Oranges/OrangesOptimizationFlags.cmake at main · benthevining/Oranges · GitHub

But it’s also arguable that optimization flags are not part of the build requirements, and so should be turned on from outside the project, or controlled with an option