Use dec/jnz instead of loop in bn_add_words and bn_sub_words.

Imported from upstream's a78324d95bd4568ce2c3b34bfa1d6f14cddf92ef. I
think the "regression" part of that change is some tweak to BN_usub and
I guess the bn_*_words was to compensate for it, but we may as well
import it. Apparently the loop instruction is terrible.

Did 39871000 bn_add_words operations in 1000002us (39870920.3 ops/sec)
Did 38621750 bn_sub_words operations in 1000001us (38621711.4 ops/sec)

Did 64012000 bn_add_words operations in 1000007us (64011551.9 ops/sec)
Did 81792250 bn_sub_words operations in 1000002us (81792086.4 ops/sec)

loop sets no flags (even doing the comparison to zero without ZF) while
dec sets all flags but CF, so Andres and I are assuming that because
this prevents Intel from microcoding it to dec/jnz, they otherwise can't
be bothered to add more circuitry since every compiler has internalized
by now to never use loop.

Change-Id: I3927cd1c7b707841bbe9963e3d4afd7ba9bd9b36
Reviewed-by: Adam Langley <>
1 file changed
tree: 70a0725a61b4781ac38e901483ad21973912b838
  1. .clang-format
  2. .github/
  3. .gitignore
  6. CMakeLists.txt
  14. codereview.settings
  15. crypto/
  16. decrepit/
  17. fipstools/
  18. fuzz/
  19. include/
  20. infra/
  21. sources.cmake
  22. ssl/
  23. third_party/
  24. tool/
  25. util/


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