direct_mapping: fix iter_memory_pair_chunks in reverse mode

[alessandrod]

iter_memory_pair_chunks was iterating regions in reverse, but not memory within regions in reverse, which broke some overlapping memmoves.

This commit fixes the issue and simplifies the implementation by removing nested loops which made control flow hard to reason about.

I've added (a lot of) tests that compare libc's memmove against our memmove.

[codecov]

Codecov Report

Merging #34204 (bb9aa00) into master (57ec207) will increase coverage by 0.0%.
The diff coverage is 98.7%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #34204   +/-   ##
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  Coverage    81.9%    81.9%           
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  Files         819      819           
  Lines      219302   219320   +18     
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+ Hits       179619   179708   +89     
+ Misses      39683    39612   -71     

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.

[BoredPerson]

Looks good to me! There are still some weird edge cases with copies going over the gaps in the stack but that's a more general issue with the gap implementation and not this code.

[alessandrod]

Looks good to me! There are still some weird edge cases with copies going over the gaps in the stack but that's a more general issue with the gap implementation and not this code.

Thanks for reviewing! Yes we're aware of those, the plan is to disable gaps when we enable direct mapping.

[stegaBOB]

I can't really talk on the actual code changes here, but just chiming in to confirm that this does resolve the memmove issue we were running into with direct mapping on 1.16.x and all existing 1.17.x versions. Here's the minimal example program we used to test it: https://beta.solpg.io/65626e1afb53fa325bfd0c34. The "Can do overlapping memmove" test was failing before this PR.

[Lichtso]

Some tests and variables should be renamed, as discussed on Slack. Can happen in a separate PR.