Commits on Oct 22, 2022

1. Fix mod_inv termination for the last iteration …

   On usize=u64 platforms, the 4th iteration would overflow the `mod_gate`
   back to 0. Similarly for usize=u32 platforms, the 3rd iteration would
   overflow much the same way.
   
   I tested various approaches to resolving this, including approaches with
   `saturating_mul` and `widening_mul` to a double usize. Turns out LLVM
   likes `mul_with_overflow` the best. In fact now, that LLVM can see the
   iteration count is limited, it will happily unroll the loop into a nice
   linear sequence.
   
   You will also notice that the code around the loop got simplified
   somewhat. Now that LLVM is handling the loop nicely, there isn’t any
   more reasons to manually unroll the first iteration out of the loop
   (though looking at the code today I’m not sure all that complexity was
   necessary in the first place).
   
   Fixes rust-lang#103361

   

   nagisa committed Oct 22, 2022
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