MJIT MSVC: Use /Z7 to avoid PDB write race #5058
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With MSVC, MJIT uses the /Fd option on an installed PDB file when
compiling. Combined with the /Zi option, this causes the PDB file to be
modified every time MJIT compiles. Concurrent modifications to the same
PDB file is known to cause problems. MSVC even has an option, /FS to
deal with it. When running MJIT tests in parallel, sometimes this leads
to corrupting the PDB file, breaking subsequent compilations. On CI,
we get messages like these:
rb_mjit_header-3.1.0.pdb is not the pdb file that was used when this precompiled header was created, recreate the precompiled header.
To avoid this race, use the /Z7 option when building precompiled header,
which asks the compiler to put debug info into the .obj file,
eliminating the need for pointing the compiler to the PDB file for the
precompiled header.
The /Fd option is changed to use a unique path based on the name of the
dll output. Because of the /debug linker flag, we generate a PDB file
at runtime even though we use /Z7.
There are a couple things missing from this change:
- Because MJIT uses the interpreter's CFLAGS build option and that
contains /Zi, putting /Z7 at the end leads to a build warning
- With /Z7 no PDB file is built anymore, so the code for installing
the PDB file can be removed
There might also be other problems with this change I haven't noticed
while developing this change using Github Actions. I don't have a
Windows dev environment with Visual Studio so I can't finish this
change easily. Please feel free to complete this change if it makes
sense.
Note:
- On master, you can see the PDB file changing with llvm-pdbutil or a
simple checksum. There is an age field in the file that is bumped
- I'm not sure if users can specify compile flags on MSVC. If they
couldn't, maybe it's easier to change MJIT's compile options to
use /Z7 when building the precompile header.
- MJIT could pass different options at runtime to generate fewer
files. Right now it inherits the /DEBUG linker flag which causes
a PDB file to be generated at runtime even though /Z7 is used.
Relevant MSVC docs:
- [/Zi,/Z7](https://docs.microsoft.com/en-us/cpp/build/reference/z7-zi-zi-debug-information-format?view=msvc-160)
- [/DEBUG](https://docs.microsoft.com/en-us/cpp/build/reference/debug-generate-debug-info?view=msvc-160)
- [/FS](https://docs.microsoft.com/en-us/cpp/build/reference/fs-force-synchronous-pdb-writes?view=msvc-160)
k0kubun
approved these changes
Oct 29, 2021
With /Z7, no .pdb file is generated, so trying to link it during build fails on my machine even though it's okay on CI. By the way, in my local testing, no .pdb is generated in cwd at runtime even without the /Fd option. I guess we can pass it just in case.
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With MSVC, MJIT uses the /Fd option on an installed PDB file when
compiling. Combined with the /Zi option, this causes the PDB file to be
modified every time MJIT compiles. Concurrent modifications to the same
PDB file is known to cause problems. MSVC even has an option, /FS to
deal with it. When running MJIT tests in parallel, sometimes this leads
to corrupting the PDB file, breaking subsequent compilations. On CI,
we get messages like these:
To avoid this race, use the /Z7 option when building precompiled header,
which asks the compiler to put debug info into the .obj file,
eliminating the need for pointing the compiler to the PDB file for the
precompiled header.
The /Fd option is changed to use a unique path based on the name of the
dll output. Because of the /debug linker flag, we generate a PDB file
at runtime even though we use /Z7.
There are a couple things missing from this change:
contains /Zi, putting /Z7 at the end leads to a build warning
With /Z7 no PDB file is built anymore, so the code for installingaddressed in 28b528fthe PDB file can be removed
There might also be other problems with this change I haven't noticed
while developing this change using Github Actions. I don't have a
Windows dev environment with Visual Studio so I can't finish this
change easily. Please feel free to complete this change if it makes
sense.
Note:
simple checksum. There is an age field in the file that is bumped
couldn't, maybe it's easier to change MJIT's compile options to
use /Z7 when building the precompile header.
files. Right now it inherits the /DEBUG linker flag which causes
a PDB file to be generated at runtime even though /Z7 is used.
Relevant MSVC docs:
@k0kubun