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SubjectRe: Very slow clang kernel config ..
On 2021-05-03, Tom Stellard wrote:
>On 5/1/21 10:19 PM, Dan Aloni wrote:
>>On Fri, Apr 30, 2021 at 06:48:11PM -0700, Nick Desaulniers wrote:
>>>On Fri, Apr 30, 2021 at 6:22 PM Linus Torvalds
>>><> wrote:
>>>> 0.92% llvm::StringMapImpl::LookupBucketFor
>>>^ wait a minute; notice how in your profile the `Shared Object` is
>>>attributed to `` while mine is `clang-13`? Clang can be
>>>built as either having libllvm statically linked or dynamically; see
>>>the cmake variables
>>>I think those are frowned upon; useful for cutting down on developers
>>>iteration speed due to not having to relink llvm when developing
>>>clang. But shipping that in production? I just checked and it doesn't
>>>look like we do that for AOSP's build of LLVM.
>>There's also `-DLLVM_ENABLE_LTO=Thin` that enables LTO for building LLVM
>>and Clang themselves, considered they can be bootstrapped like this
>>using a previous version of Clang. Combining that with a non-shared
>>library build mode for both Clang and LLVM, the result is possibly the
>>fastest and most optimized build that is achievable. Unfortunately I
>>see distributions neglecting to enable this in packaging this as well.
>>On a side note, I'm also a Fedora user and agree with Linus about this.
>>I'd like to see an opt-in bypass of the shared library policy via
>>something like `dnf install clang-optimized` that would install the
>>fastest and most optimized Clang build regardless of RPM install size.
>I have experimented with creating a static version of clang in the past,
>but never really found a solution I liked enough to upstream into Fedora.
>e.g. This solution[1] that we're using to bootstrap clang in our internal
>clang-as-the-default-cc Fedora buildroots that we use for testing.
>If someone could file a bug[2] against the clang package in Fedora (or RHEL even)
>with some data or other information that shows the downsides of the shared
>build of of clang, that would be really helpful.

I have filed with
information from my previous reply

-fpic (.so) -fno-semantic-interposition -Wl,-Bsymbolic-functions is very
close to -fpic/-fpie (.a) in terms of performance.

If Fedora is willing to use -fprofile-use (profile guided optimization)
or ThinLTO, that's great as well, with the cost of much longer build time.)

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