Since testing the proposed changes for i386 may be trickier for kinetic,
I tested it in the following fashion:

- Deployed a Debian stable i386 VM and upgraded it to Debian testing (which 
contains php 8.1);
- Ensured the VM CPU supported the sse2 instruction (which is checked in the 
configuration test to enable the performance boost related to this bug;
- Built and installed Debian's PHP i386 package with the related patch (see 
https://salsa.debian.org/php-team/php/-/merge_requests/12);
- Ran the test script provided by this bug reporter to look for regressions (no 
regressions found, no performance boost observed);
- Changed the VM CPU to support avx2/sse3 instructions and re-ran the test. 
Now, the performance boost was observed;
- Finally, Changed the VM CPU for a pre-pentium 4 micro-architecture, where 
sse2 is not available (this is the most important i386 test here).
- Re-ran the test script. No performance boost observed. No regressions 
observed.

This provides enough evidence that no regressions should be introduced
if the patch is introduced in stable releases for i386 users with
machines that do not support avx2/sse3/sse2 instructions. It is
important to note that this feature will be enabled in the i386 builds
since LP builders do support sse2.

The next steps here are to introduce this change as a delta into kinetic
and SRU it all the way back to focal.

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https://bugs.launchpad.net/bugs/1882279

Title:
  PHP built from source performs much better than the Ubuntu packaged
  version

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