Constant-time impl for part (but not all) of PosInt::invert_vartime

[brian-pane]

No description provided.

[codspeed-hq]

Merging this PR will not alter performance

✅ 155 untouched benchmarks

---

_{Comparing brian-pane:vartime (9ab190f) with main (3353ba5)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.76%. Comparing base (3353ba5) to head (9ab190f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #153      +/-   ##
==========================================
- Coverage   99.76%   99.76%   -0.01%     
==========================================
  Files         184      184              
  Lines       50832    50843      +11     
==========================================
+ Hits        50714    50723       +9     
- Misses        118      120       +2     

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[brian-pane]

The performance regression reported by CodSpeed seems to be unrelated. It happens only in the libcrux ML-KEM test, and based on my reading of the code and CodSpeed's flamegraphs, that test doesn't appear to have any dependency on PosInt::invert_vartime.

[brian-pane]

The performance regression reported by CodSpeed seems to be unrelated. It happens only in the libcrux ML-KEM test, and based on my reading of the code and CodSpeed's flamegraphs, that test doesn't appear to have any dependency on PosInt::invert_vartime.

I just rebased, and the CodSpeed test is passing now.

[brian-pane]

On x86_64, rustc+llvm generates a cmov operation for the saturating_sub.
On Aarch64, it generates a csel operation. (Compiler Explorer)

As far as I can tell, both those instructions should run in constant time, independent of their inputs. But if there's a better idiom for "sign-extend this bit to fill 64 bits," I'm happy to switch to that.