Changes/Update i686 architectural baseline to include SSE2

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: 2018-06-04
• 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.

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=x86-64 -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 is 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 -mstackrealign, there is substantial experience with this configuration downstream.

• Other developers: Other developers can enable SSE2 optimization in their packages if they want, where this has been a compile-time option only.

• 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.