crypto: Share the seed addchain between the point and Fq12 powers - #1640
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chfast wants to merge 1 commit into
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crypto: Share the seed addchain between the point and Fq12 powers#1640chfast wants to merge 1 commit into
chfast wants to merge 1 commit into
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mul_by_X and cyclotomic_pow_to_X were the same 30-step addchain for the curve seed X written twice, once over points and once over Fq12, with identical temporaries and schedule — checked to agree step for step including operand order. Express it once as pow_by_X parameterised by the squaring and the multiplication, with n_sqr replacing the parallel n_dbl and n_cyclotomic_square. Costs about 0.02% in instructions, which is the price of the indirection.
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #1640 +/- ##
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- Coverage 97.71% 97.71% -0.01%
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Lines 15607 15578 -29
Branches 3610 3609 -1
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- Hits 15251 15222 -29
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Partials 87 87
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mul_by_X and cyclotomic_pow_to_X were the same 30-step addchain for the
curve seed X written twice, once over points and once over Fq12, with
identical temporaries and schedule. Express it once as pow_by_X,
parameterised by the squaring and the multiplication, with n_sqr replacing
the parallel n_dbl and n_cyclotomic_square.
The two chains agree step for step including operand order, and the shared
version was generated from mul_by_X's body so the steps could not drift.
Draft because it costs about 0.02%, the price of the indirection. The case
for taking it anyway is that an addchain is where a transcription slip is
invisible, and this code has already had one: #1636 fixed a term that #1542
had substituted in the sibling function and missed here. The case against
is trading concrete code for a generic template, where the names mul_by_X
and cyclotomic_pow_to_X document intent better than pow_by_X and lambdas.
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