[CMake] modifying cmake_build_type cflags per target

[Eric Doenges]

Am 19.11.19 um 22:51 schrieb Craig Scott:
>
>
> On Tue, Nov 19, 2019 at 10:36 PM Eric Doenges <doenges at mvtec.com 
> <mailto:doenges at mvtec.com>> wrote:
>
>     Am 19.11.19 um 12:09 schrieb Stéphane Ancelot:
>>
>>     Hi,
>>
>>     I have a particular target (using swig / jni) that must not have
>>     -O3 -NDEBUG flags from Relase build type
>>
>>     How can I overload this target flags ?
>>
>     Since CMAKE_<lang>_FLAGS_RELEASE does not map to a user-visible
>     target property, you cannot override it directly (anything added
>     to the COMPILE_FLAGS property is appended to the command line and
>     does not replace any of the compiler flags CMake sets by itself).
>     However, you can redefine this variable before you create your
>     special target(s), and then restore it so that other targets are
>     not affected:
>
>     set(_CMAKE_CXX_FLAGS_RELEASE_SAVE ${CMAKE_CXX_FLAGS_RELEASE})
>     set(CMAKE_CXX_FLAGS_RELEASE)
>     < add your target(s) here >
>     set(CMAKE_CXX_FLAGS_RELEASE ${_CMAKE_CXX_FLAGS_RELEASE_SAVE})
>
>     This assumes your target is C++; if it is C, simply replace the
>     _CXX_ with _C_.
>
>
> Actually, no, that's not how it works. This is actually a great 
> example of why projects shouldn't generally try to avoid manipulating 
> CMAKE_CXX_FLAGS and should instead prefer to modify target or 
> directory properties instead (not possible here for the original 
> problem, but worth highlighting nonetheless). The (often surprising) 
> behavior at play in the proposed example above is that it is not the 
> value of the CMAKE_CXX_FLAGS_RELEASE variable at the time the target 
> is created that matters, it's the variable's value /at the end of the 
> directory scope/. You can change the variable's value as much as you 
> like along the way before or after creating targets, but only the 
> final value at the end of the scope will actually be used in the build 
> command lines for targets created in that directory scope. This is 
> rarely what developers expect, but that's how it works, for better or 
> worse. When you start adding in calls to add_subdirectory(), it can 
> get really confusing what value is getting used where, so take great 
> care if your project really must modify these variables.

Very interesting, thanks for pointing that out. In our case things work 
because all the actual targets are in fact created in subdirectories. I 
think CMake really needs some straightforward way to allow you to 
override flags on a per-target and per-file basis (in our case, we need 
to be able to reduce the optimization level for individual source files 
to work around bugs in the Intel compiler's optimizer).

-- 

*Dr. Eric Dönges*
Senior Software Engineer

MVTec Software GmbH | Arnulfstr. 205 | 80634 Munich | Germany
doenges at mvtec.com <mailto:mustermann at mvtec.com> | Tel: +49 89 457 695-0 
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