[3.9] bpo-40791: Make compare_digest more constant-time. (GH-20444) #23436
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* bpo-40791: Make compare_digest more constant-time. The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization. (This is change GH-1 from https://bugs.python.org/issue40791 .) (cherry picked from commit 3172936) Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>
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Nov 21, 2020
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The existing volatile
left/rightpointers guarantee that the reads will all occur, but does not guarantee that they will be used. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency betweenresultand the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization.(This is change GH-1 from https://bugs.python.org/issue40791 .)
(cherry picked from commit 3172936)
Co-authored-by: Devin Jeanpierre jeanpierreda@google.com
https://bugs.python.org/issue40791
Automerge-Triggered-By: GH:gpshead