Fix bug with BigInteger.TrailingZeroCount

[dakersnar]

Fixes #77720

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Issue Details

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Fixes #77720

Author:	dakersnar
Assignees:	-
Labels:	area-System.Numerics
Milestone:	-

[tkekalainen]

This doesn't seem to fix the negative number path that also has the extra increment? Also, the two new tests check the same value. Did you mean -BigInteger.Pow(2, 1000) instead of BigInteger.Pow(-2, 1000)?

[tfenise]

Why separate paths for positive and negative numbers in the first place? uint.TrailingZeroCount(x) and uint.TrailingZeroCount(~x + 1) are equal.

[dakersnar]

@tfenise Yep, I see what you mean after looking closer at your explanation in the bug report. Thanks for bringing this to our attention!

[tkekalainen]

@dakersnar

@tfenise Yep, I see what you mean after looking closer at your explanation in the bug report. Thanks for bringing this to our attention!

Well, it was I who wrote the bug report and noted the unneeded path, but I'm glad @tfenise brought it up again, because I probably wouldn't have had the courage to do so. 🙂

[dakersnar]

@tkekalainen Sounds like I need more coffee too. Thanks 😄.

[tannergooding]

/backport to release/7.0

[github-actions]

Started backporting to release/7.0: https://github.com/dotnet/runtime/actions/runs/3438765946

[github-actions]

@tannergooding backporting to release/7.0 failed, the patch most likely resulted in conflicts:

$ git am --3way --ignore-whitespace --keep-non-patch changes.patch

Applying: Fix bug with BigInteger.TrailingZeroCount
Applying: Update src/libraries/System.Runtime.Numerics/tests/BigIntegerTests.GenericMath.cs
Applying: Update negative code path
Applying: Simplify logic for handling negative values
Applying: Update src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs
Applying: Update comment
error: sha1 information is lacking or useless (src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs).
error: could not build fake ancestor
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0006 Update comment
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!

[github-actions]

@tannergooding an error occurred while backporting to release/7.0, please check the run log for details!

Error: git am failed, most likely due to a merge conflict.