[lldb-dev] LLDB_PYTHON_HOME

Thu Feb 27 11:33:54 PST 2020

>  This was all fixed in CMake 3.12.

For some definitions of "all fixed." ;-)

It seems weird to me that FindPython3 found the VS-distributed Python,
which isn't mentioned anywhere in the environment block, and that it chose
that one over the Python 3 installation that was in the path and that
included the interpreters and all of the corresponding libraries.

>  With FindPython{2,3} you know you'll have a matching interpreter and
library.

The build failure was that the Python distributed in VS does not have a
debug version of the library (python37_d.lib), so you don't actually get a
matching interpreter and library for debug builds.

It's also not clear whether LLVM was consistently using the Python found
the old way.  My generated build.ninja file seemed to be using both
versions inconsistently to run lit tests and the like.

Anyway, thanks for the explanations.  I've LGTMed your patch, which should
eliminate future surprises when something else smuggles yet another version
of Python onto a machine.

On Thu, Feb 27, 2020 at 11:14 AM Jonas Devlieghere <jonas at devlieghere.com>
wrote:

> So to make my previous explanation more concrete:
>
> On Thu, Feb 27, 2020 at 11:05 AM Jonas Devlieghere <jonas at devlieghere.com>
> wrote:
>
>>
>>
>> On Thu, Feb 27, 2020 at 10:53 AM Adrian McCarthy <amccarth at google.com>
>> wrote:
>>
>>> Thanks for the info.  Setting Python3_ROOT_DIR solves the problem.
>>>
>>> Looking at the cmake output from before setting Python3_ROOT_DIR, cmake
>>> looks for Python twice and finds it at the two different locations.
>>>
>>> Early on:
>>>
>>> -- Found PythonInterp: C:/Python36/python.exe (found version "3.6.8")
>>>
>>
> ^ This is using the "old" (CMake < 3.12) way of finding the Python
> interpreter.
>
>
>>
>>> Which looks good (modulo the incorrect slash direction).  But later:
>>>
>>> -- Found Python3: C:/Program Files (x86)/Microsoft Visual
>>> Studio/Shared/Python37_64/python.exe (found version "3.7.5") found
>>> components:  Interpreter Development
>>> -- Found PythonInterpAndLibs: C:/Program Files (x86)/Microsoft Visual
>>> Studio/Shared/Python37_64/libs/python37.lib
>>>
>>
> ^ This is using the "new" (CMake > 3.12) way of finding the Python
> interpreter and libraries.
>
>
>>
>>> Which is where the discrepancy comes in.  Note that only C:\Python36 is
>>> in my PATH.
>>>
>>> It's frustrating that this keeps breaking.  Last time, I had to purge
>>> all but one Python installation from my machine to get it to make a
>>> consistent choice.  But I just upgraded to VS 2019, and it smuggled in its
>>> own version.
>>>
>>> So why are there two searches anyway?  And why do they have different
>>> algorithms that lead to different results?  (I'm not sure _how_ it ever
>>> found the Microsoft copy, since there's nothing in the process environment
>>> that points that way.)
>>>
>>
>> The reason there's two searches is because LLVM and LLDB have different
>> requirements. LLVM just needs a python interpreter to run some scripts.
>> LLDB on the other hand needs an interpreter and a matching Python library
>> to link against. Before CMake 3.12, finding the interpreter and the
>> libraries are two separate calls to find with no guarantees that they
>> match. This lead to all kinds of issues, where you're linking against one
>> version of Python and then trying to run the test suite with a totally
>> different interpreter. There were other problems on Windows, which meant
>> that we had our own hand-rolled implementation to find Python.
>>
>> This was all fixed in CMake 3.12. With FindPython{2,3} you know you'll
>> have a matching interpreter and library. It also fixed all the problems we
>> had to work around for Windows. Unfortunately, LLVM's minimum CMake version
>> is 3.4, so we can't use it yet. For LLDB on Windows we agreed that the
>> benefits of using FindPython3 are worth bumping the minimum required CMake
>> version (see lldb/CMakeLists.txt, line 2-4). Once LLVM moves to CMake 3.12
>> or later, all these problems should be fixed. We can then call FindPython3
>> once and rely on everything being consistent.
>>
>>
>>>
>>> On Thu, Feb 27, 2020 at 10:23 AM Jonas Devlieghere <
>>> jonas at devlieghere.com> wrote:
>>>
>>>> Hey Adrian,
>>>>
>>>> Config.h gets generated by expanding the corresponding CMake variables.
>>>> If you look at LLDBConfig.cmake, you can see that LLDB_PYTHON_HOME is
>>>> computed from PYTHON_EXECUTABLE. The problem appears that somehow CMake
>>>> ignored your specified PYTHON_HOME and decided to pick a different Python.
>>>> I'm not sure why though, because I use a similar CMake invocation on
>>>> Windows.
>>>>
>>>> > cmake ..\llvm-project\llvm -G Ninja -DCMAKE_BUILD_TYPE=RelWithDebInfo
>>>> -DLLVM_ENABLE_PROJECTS="llvm;clang;lldb;lld" -DLLVM_ENABLE_ASSERTIONS=OFF
>>>> -DLLVM_ENABLE_ZLIB=FALSE -DLLDB_ENABLE_PYTHON=TRUE
>>>> -DPYTHON_HOME="C:/Program Files/Python36/"
>>>>
>>>> According to FindPython3 (
>>>> https://cmake.org/cmake/help/v3.12/module/FindPython3.html), you can
>>>> set Python3_ROOT_DIR as a hint. Can you give that a try? If that works we
>>>> should populate that variable from PYTHON_HOME in
>>>> FindPythonInterpAndLibs.cmake.
>>>>
>>>> Cheers,
>>>> Jonas
>>>>
>>>> On Thu, Feb 27, 2020 at 10:10 AM Adrian McCarthy via lldb-dev <
>>>> lldb-dev at lists.llvm.org> wrote:
>>>>
>>>>> Is there documentation on how lldb\include\lldb\host\config.h is
>>>>> generated?  I'm again having the problem of the config trying to point to
>>>>> the wrong Python installation.
>>>>>
>>>>> When I run cmake, I explicitly point PYTHON_HOME to C:\Python36 like
>>>>> this:
>>>>>
>>>>> cmake -GNinja -DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN=ON
>>>>> -DCMAKE_BUILD_TYPE=Debug -DLLDB_TEST_DEBUG_TEST_CRASHES=1
>>>>> -DPYTHON_HOME=C:\Python36
>>>>> -DLLDB_TEST_COMPILER=D:\src\llvm\build\ninja\bin\clang.exe
>>>>> ..\..\llvm-project\llvm -DLLVM_ENABLE_ZLIB=OFF
>>>>> -DLLVM_ENABLE_PROJECTS="clang;lld;lldb"
>>>>>
>>>>> But the generated Config.h contains:
>>>>>
>>>>> #define LLDB_PYTHON_HOME "C:/Program Files (x86)/Microsoft Visual
>>>>> Studio/Shared/Python37_64"
>>>>>
>>>>>
>>>>> And the mismatch causes my build to fail because it goes looking for
>>>>> python37_d.dll, which is apparently not part of the Microsoft distribution.
>>>>> _______________________________________________
>>>>> lldb-dev mailing list
>>>>> lldb-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
>>>>
>>>>
> I've put up a patch to use PYTHON_HOME as the hint:
> https://reviews.llvm.org/D75275
>
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