enlight planned changes to this revision.
enlight added a comment.
I'll submit a revised patch that contains an explanation of the generator
expressions to aid future maintainers/contributors.
================
Comment at: cmake/modules/LLDBConfig.cmake:61
@@ +60,3 @@
+ # below, otherwise CMake will replace the whitespace with a semicolon in
some contexts (which would stuff things up).
+ set (PYTHON_EXECUTABLE
$<$<CONFIG:Debug>:${PYTHON_DEBUG_EXE}>$<$<NOT:$<CONFIG:Debug>>:${PYTHON_RELEASE_EXE}>)
+ set (PYTHON_LIBRARY
$<$<CONFIG:Debug>:${PYTHON_DEBUG_LIB}>$<$<NOT:$<CONFIG:Debug>>:${PYTHON_RELEASE_LIB}>)
----------------
zturner wrote:
> enlight wrote:
> > zturner wrote:
> > > This line is hard to parse mentally, and I'm not sure I've seen this kind
> > > of nested generator expression. I trust you when you say it's right, but
> > > can you explain what this does?
> > Sure thing.
> >
> > # `$<CONFIG:Debug>` evaluates to `1` when the `Debug` configuration is
> > being generated, or `0` in all other cases.
> > # `$<$<CONFIG:Debug>:${PYTHON_DEBUG_EXE}>` expands to
> > `${PYTHON_DEBUG_EXE}` when the `Debug` configuration is being generated, or
> > nothing (literally) in all other cases.
> > # `$<$<NOT:$<CONFIG:Debug>>:${PYTHON_RELEASE_EXE}>` expands to
> > `${PYTHON_RELEASE_EXE}` when any configuration other than `Debug` is being
> > generated, or nothing in all other cases.
> >
> > Since the conditionals in 2 & 3 are mutually exclusive, and a conditional
> > expression that evaluates to `0` yields no value at all, it's possible to
> > concatenate them to obtain a single value. This value will be
> > `${PYTHON_DEBUG_EXE}` when generating the Debug configuration, or
> > `${PYTHON_RELEASE_EXE}` when generating any other configuration.
> Ahh, I understand. Is it equivalent to write this:
>
> if ($<CONFIG:Debug> == 1)
> set(PYTHON_EXECUTABLE ${PYTHON_DEBUG_EXE})
> else()
> set(PYTHON_EXECUTABLE ${PYTHON_RELEASE_EXE})
> endif()
>
> ? If so, I kind of prefer this way, if nothing else so that other people
> will be able to understand it in the future if they go to edit this code.
Unfortunately that's not currently possible because the `if` command doesn't
support generator expressions (not that the docs mention it or anything). So
while CMake won't complain if you did this:
```
if ($<CONFIG:Debug>)
set (PYTHON_EXECUTABLE ${PYTHON_DEBUG_EXE})
else ()
set (PYTHON_EXECUTABLE ${PYTHON_RELEASE_EXE})
endif ()
```
It wouldn't actually work as intended because the `if/else` is only evaluated
once during the configuration step rather than per-build-configuration (during
the generation step).
Repository:
rL LLVM
http://reviews.llvm.org/D13234
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