tree 0596a25a57302928a432ff81662d42141bfbc391
parent aa3a3521cef3998d4f9e7f7ff721163b6e3e5f39
author Devin Jeanpierre <jeanpierreda@google.com> 1605948923 -0700
committer GitHub <noreply@github.com> 1605948923 -0800
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bpo-40791: Make compare_digest more constant-time. (GH-20444)

* 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 #1 from https://bugs.python.org/issue40791 .)