core.checkedint.addu: Faster (like subu: only one compare-and-branch).

[DoctorNoobingstoneIPresume]

Explanation of why a single compare-and-branch is enough for addu:

For unsigned integrals, if we add r = x + y:

• if there is no carry, then r >= x and r >= y;
• if there is carry, then r < x and r < y.

Also, let us look at the implementation of subu. One compare-and-branch suffices for subu. It should suffice for addu as well. (-:

Testing information 1 (correctness):

$ cd src/core/
$ rdmd -unittest --main checkedint.d
1 modules passed unittests

Testing information 2 (improvement of generated code):
Comparison of generated machine code (DMD for Win32):

$ dmd -O -release -unittest checkedint.d -of=checkedint.d.obj

For uint overload: Before: 2 cmp instructions, 2 conditional-jump instructions. After: 1 cmp instruction, 1 conditional-jump instruction.

For ulong overload: Before: 4 cmp instructions, 4 conditional-jump instructions. After: 2 cmp instructions, 2 conditional-jump instructions.

Thank you for considering this.

[dlang-bot]

Thanks for your pull request and interest in making D better, @DoctorNoobingstoneIPresume! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + druntime#3496"

[DoctorNoobingstoneIPresume]

I wish to modify the pull request like this:

https://github.com/dlang/druntime/compare/master...DoctorNoobingstoneIPresume:210614_core_checkedint_addu_03h?expand=1

May I ask what is the best/preferred way to do this, please ?

E.g.:

(1) New PR (old PR closed with comment pointing to new PR) ? => Single commit, but comments/conversations are lost (or more difficult to reach) ?

(2) New commit on the current PR ? But then how to merge as single commit ?

(3) Some other suggestion ?

[thewilsonator]

If HEAD is currently at 5b62e56 and the rest of changes in 7c4ce67 are uncommitted then do:
$ git reset HEAD~1 followed by a git commit, followed by git push -f

If HEAD is currently at 7c4ce67 and the previous commit is 5b62e56 then do $ git reset HEAD~2 followed by a git commit, followed by git push -f

[DoctorNoobingstoneIPresume]

@thewilsonator Thank you for the assistance ! I hope I have got it right.