On 4/23/2021 9:57 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 ?
I'd think most packages would be consistent, but I don't think it
necessarily needs to be policy. If a package maintainer has a good
reason to use gcc for part of the package and llvm/clang for another,
then they should be able to do that.
== 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.
Note the lack of stack-clash on aarch64 is a transient issue. It is
being worked on, but just didn't make it into llvm12, I would expect it
to land for llvm13 (Tom's a better resource on that going forward, so if
he's got newer information than mine, believe his :-)
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.
This is a great example where trying to force consistency doesn't make
sense and the maintainers can/should do what is best for the package and
Fedora.
jeff
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