bpo-40791: Make compare_digest more constant-time. #20444
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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 python#1 from https://bugs.python.org/issue40791 .)
gpshead
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May 27, 2020
gpshead
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needs backport to 3.7
needs backport to 3.9
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Can you add a NEWS entry (https://blurb-it.herokuapp.com/ can do it for you)? |
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OK, wasn't sure if this would deserve a news entry. Used the blurb tool -- really nice! Thanks for the recommendation. :) (Feel free to tweak the exact wording of the news entry, committers should have edit access to my PR if I understand it right.) |
miss-islington
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Nov 21, 2020
* 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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GH-23436 is a backport of this pull request to the 3.9 branch. |
miss-islington
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Nov 21, 2020
* 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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GH-23437 is a backport of this pull request to the 3.8 branch. |
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GH-23438 is a backport of this pull request to the 3.7 branch. |
miss-islington
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to miss-islington/cpython
that referenced
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Nov 21, 2020
* 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>
miss-islington
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Nov 21, 2020
* 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>
miss-islington
added a commit
that referenced
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Nov 21, 2020
* 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>
adorilson
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Mar 13, 2021
* 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 .)
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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 1 from https://bugs.python.org/issue40791 .)
https://bugs.python.org/issue40791