[WIP] BoxedResidue: fix constant-time TODO in montgomery_mul
#415
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Replaces branching with an arithmetic-based approach. This unfortunately
seems to double the time multiplication takes (and with it, modpow).
Montgomery arithmetic/multiplication, BoxedUint*BoxedUint
time: [10.027 µs 10.048 µs 10.068 µs]
change: [+101.70% +102.44% +103.19%] (p = 0.00 < 0.05)
Performance has regressed.
Montgomery arithmetic/modpow, BoxedUint^BoxedUint
time: [48.200 ms 48.273 ms 48.352 ms]
change: [+97.268% +97.613% +97.972%] (p = 0.00 < 0.05)
Performance has regressed.
tarcieri
commented
Dec 10, 2023
Comment on lines
+269
to
+273
| /// Compare limbs in constant time, returning `Limb::ONE` if the left size is less than the right. | ||
| #[inline(always)] | ||
| fn limb_ct_lt(a1: Limb, b1: Limb, a2: Limb, b2: Limb) -> Limb { | ||
| (a1.sbb(b1, Limb::ZERO).1 | a2.sbb(b2, Limb::ZERO).1) & Limb::ONE | ||
| } |
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This was faster than using the ct_lt approach previously used on L214, but still slower than using branch instructions
Contributor
|
This could benefit from #418 |
Contributor
|
Doesn't seem like it benefits. But judging by Godbolt, using |
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Author
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Closing this for now |
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Replaces branching with an arithmetic-based approach. This unfortunately seems to double the time multiplication takes (and with it, modpow).
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