Re: [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 > 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__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
doen...@mvtec.com  | Tel: +49 89 457 695-0 
| www.mvtec.com 


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Re: [CMake] modifying cmake_build_type cflags per target

[Craig Scott]

On Tue, Nov 19, 2019 at 10:36 PM Eric Doenges  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__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.

-- 
Craig Scott
Melbourne, Australia
https://crascit.com

Get the hand-book for every CMake user: Professional CMake: A Practical
Guide 
Consulting services (CMake, C++, build/release processes):
https://crascit.com/services
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Re: [CMake] modifying cmake_build_type cflags per target

[Eric Doenges]

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__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_.


--

*Dr. Eric Dönges*
Senior Software Engineer

MVTec Software GmbH | Arnulfstr. 205 | 80634 Munich | Germany
doen...@mvtec.com  | Tel: +49 89 457 695-0 
| www.mvtec.com 


Find our privacy policy here .

Sign up  for our MVTec Newsletter!

Geschäftsführer: Dr. Wolfgang Eckstein, Dr. Olaf Munkelt
Amtsgericht München HRB 114695

MVTec Software GmbH Logo
-- 

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[CMake] modifying cmake_build_type cflags per target

[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 ?

Regards,

S.Ancelot

-- 

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