On 4/23/21 8:57 AM, Daniel P. Berrangé wrote:
On Fri, Apr 23, 2021 at 11:18:59AM -0400, Ben Cotton wrote:
https://fedoraproject.org/wiki/Changes/CompilerPolicy
== Summary ==
Fedora has historically forced packages to build with GCC unless the
upstream project for the package only supported Clang/LLVM. This
change proposal replaces that policy with one where, given a good
technical reason, a packager may:
* Choose to build with their package with clang even if the upstream
project supports gcc.
* Choose to build with gcc even if upstream does not support it.
The goal is to give the packager the ability to use their own
technical judgement to choose the best compiler.
Does the choice have to be consistent per package, or is it within
expectations to use different compilers on different Fedora arches ?
This would be up to the packager to decide.
== Detailed Description ==
It is worth noting that Clang/LLVM's implementation of
-fstack-clash-protection, which is one of Fedora's default compiler
flags, is under development and does not yet have an implementation
for AArch64.
This would suggest need to use GCC on x86 and CLang on aarch64 within
a single package.
Again, the packager would need to consider this when weighing the pros
and cons.
-Tom
== Feedback ==
* The original proposal stated that Fedora packagers should follow
upstream's compiler preferences, but based on feedback, the proposal
was updated to allow a packager to choose the compiler they feel is
best as long as there is a valid technical reason to do so.
== Benefit to Fedora ==
Other packages that may benefit include:
* llvm/clang (currently has to disable LTO due to compilation failures).
* qemu (could take advantage of clang's CFI hardening feature).
Upstream QEMU has been considering whether there's any loss of
features implied by replacing GCC with use of CLang in QEMU builds.
One edge case was discovered for s390x target.
GCC has a min code generation target of z900 (hw release circa 2000),
CLang has a min code genreation target of z10 (hw release circa 2008)
This doesn't matter for compiling QEMU itself, as anyone actually
running QEMU on an s390x is likely to have z10 or newer.
It does cause a problem with firmware builds though. If QEMU wants
to emulate old hardware (qemu-system-s390x -cpu z900 ..args...),
then the firmware blob needs to have been built to target z900
which is not possible with CLang.
So if QEMU is switched to use CLang, then we will possibly need to
use GCC still for QEMU's firmware builds, or only use CLang on
certain Fedora arches.
This is quite a niche problem that's unlikely to cause issues
for most people, but its a illustration that swapping compilers
out can have unexpected consequences/complications.
Regards,
Daniel
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