i686 Is For x86-64
Summary
Fedora builds its i686 packages for use on x86-64 systems as multi-lib RPMs.
Owner
- Name: Florian Weimer
- Email: fweimer@redhat.com
- Release notes owner:
Current status
- Targeted release: Fedora 29
- Last updated: 2018-06-15
- Tracker bug: <will be assigned by the Wrangler>
Detailed Description
Currently, the i686 RPM packages are built in such a way that they are compatible with very old i686 systems, such as the Pentium III. The only addition over the i686/Pentium Pro baseline is a requirement to support long NOPs, for Intel CET. However, the majority of installations of i686 packages is for use on x86_64 systems, as multi-lib RPMs. Furthermore, there are reports that the i686 kernel does not run stable on old hardware which is not x86-64-capable.
This proposal suggests to accept this reality and build the i686 packages in such a way that they require the ISA level of (early) x86-64 CPUs.
The compiled binaries should still remain compatible with the i686 alternative architecture, so they do not need rebuilding. (The instructions generated by GCC are compatible with a pure 32-bit x86 platform.) However, Fedora does not currently perform automated tests to enforce this.
Benefit to Fedora
32-bit Fedora packages will be optimized for use on current x86_64 systems:
- The default compiler flags will be switched to
-march=i686 -msse2 -mtune=generic -mfpmath=sse -mstackrealign. This enables SSE2 support with optimal backwards compatibility due to automatic stack realignment. (16-byte stack alignment was introduced with SSE2 support in the i386 ABI, but old binaries only provide 4-byte stack alignment.) - x86 packages can use SSE2 optimizations unconditionally.
- This increases compatibility with some upstreams which have mostly given up on support for non-SSE2 x86 systems.
- Excess precision problems inherent to the legacy i387 FPU disappear, and math results become more predictable.
The latter two times are primarily a benefit to those developers who work on Fedora itself.
Scope
- Proposal owners: Adjust the redhat-rpm-config, gcc, and glibc packages to switch to the new compiler flags. Except for the interaction
-mstackrealign, there is substantial experience with SSE2-enabled compilation on i686 downstream.
- Other developers:
- Other developers can enable SSE2 optimization in their packages if they want, where this has been a compile-time option only.
- The x86 SIG continues testing the i686 alternative architecture on legacy hardware.
- Release engineering: #7543
- List of deliverables: TBD
- Policies and guidelines: i686 is no longer a primary architecture. The Packaging Guidelines do not currently require support for non-SSE2 x86 systems, so no change is required there.
- Trademark approval: N/A (not needed for this Change)
Upgrade/compatibility impact
32-bit x86 systems which lack SSE2 support (or do not match the x86-64 architecture baseline for other reasons) will not be able to upgrade to Fedora 29.
This only applies to 32-bit x86 systems. For 64-bit x86 systems, SSE2 support is a required support of the baseline ABI and already extensively used by Fedora.
How To Test
Install Fedora 29 after the mass rebuild and test 32-bit (i686) packages for functionality.
Extra tests could involve old 32-bit legacy applications compiled with non-GCC compilers.
User Experience
There is no expected substantial change in user experience, except for a minor performance improvements and potentially some slight reduction in power usage.
Dependencies
We are not aware of any dependencies at this point.
Contingency Plan
- Contingency mechanism: Back out the change.
- Contingency deadline: Before the final mass rebuild.
- Blocks release? No.
- Blocks product? No.
Documentation
Documentation in a Release Note appears sufficient.
Release Notes
- Fedora 29 requires an x86-64 system for installation. This includes the 32-bit x86 (i686) packages built primarily for use x86-64 systems.