llvm dev - May 2021 - 回复: A Few Questions Regarding The New Pass Manager, LTO, and LLD

Sure, I’ll take a look at the legacy pass manager’s implementation and see if I
can send a patch.

Thanks,
Chibin Zhang
2021.5.12
发件人: Arthur Eubanks<mailto:aeubanks at google.com>
发送时间: 2021年5月11日 3:40
收件人: 张驰斌<mailto:zhangchb1 at shanghaitech.edu.cn>
抄送: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
主题: Re: [llvm-dev] A Few Questions Regarding The New Pass Manager, LTO, and LLD



On Sat, May 8, 2021 at 8:15 AM 张驰斌 via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
LLVM Developers,
       Hi!
       There is a scenario where I want to do full program analysis, ideally
after link time optimization is done and before assembly is emitted, when all
symbols are in LLVM IR form and contained in a single module. But after some
time browsing stackoverflow, lld documentation, and the mailing lists, I still
have little clue how to do it… I am aware of the email sent by Y Liu and titled
“how to add my own passes to LTO pass”. But it doesn’t seem to work for the new
pass manager. A few questions:

1.  The legacy pass manager has an extension point,
`PassManagerBuilder::EP_FullLinkTimeOptimizationLast`. Unfortunately, I can’t
find a alternative in the new pass manager. The `Bye` example seems to suggest
using `PB.registerVectorizerStartEPCallback`. I am not sure whether doing so
will result in my module pass being ran at compile or link time?
Feel free to create a patch to add a PassBuilder equivalent of
EP_FullLinkTimeOptimizationLast.

2.  I wonder how does clang communicates passes that need to be run at lto to
lld. The command line that I attempt to get lto running is `clang
-fexperimental-new-pass-manager -fpass-plugin=libmymodulepass.so -flto
-fuse-ld=lld …`. I straced the command for execv calls, and expected to see
libmymodulepass.so to be passed to ld.lld as an argument, but found out it
wasn’t. Could  you shed some light on how clang and lld cope with each other
during lto?
I don't know pass plugins that well, but maybe llvm/examples/Bye/Bye.cpp and
its surrounding infrastructure may help?

3.  Greping `ld.lld --help` reveals an interesting option:
`--lto-newpm-passes=<value>`. Does the <value> here mean built-in
pass names like `dce`/`mem2reg`, or is it an option to insert our plugin like
libmymodulepass.so?
I’ve looking into this for a while but haven’t made much progress…Any examples /
command line on how to do this are greatly appreciated!

Best Wishes,
Chibin Zhang
2021.5.8
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