Coding Standard: Prefer `int` for regular arithmetic

[mehdi_amini]

Summary

This is a proposal to standardize a preference to use signed integer when possible. See inline diff for more info.

http://lists.llvm.org/pipermail/llvm-dev/2019-June/132890.html

[jdoerfert]

llvm/docs/CodingStandards.rst

1553

I think I lost my elementary school algebra over the years. What I get is:

A + B > C        | - B

A     > C - B

And with A = 2, B = 2, C = 3:

2 + 2 > 3       <=> 4 > 3
2     > 3 - 2   <=> 2 > 1

What am I missing here?

[mehdi_amini]

llvm/docs/CodingStandards.rst

1553

Right, Justin mentioned that A=2, B=3, C=2 are better choices here!

[jdoerfert]

llvm/docs/CodingStandards.rst

1553

Also, fix the inequality ;)

llvm/docs/CodingStandards.rst

     }
   };
 
+Prefer `int` for regular arithmetic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Prefer `int` when possible and use `unsigned` only for bitmask and when you
+intend to rely on wrapping behavior.
+
+It is rare that overflowing (and wrapping) an unsigned integer won't trigger
+a program bug when the overflow was not intentionally handled. Using signed
+arithmetic means that you can actually trap on over/underflow and catch these
+bugs (when using fuzzing for instance).
+
+Unsigned integer also have a discontinuity right to the left of zero. Suppose
+A, B and C are small positive integers close to zero, say all less than a
+hundred or so. Then given `A + B > C` and knowing elementary school algebra,
+one can rewrite that as `A > B - C`. But C might be greater than B, and the
+subtraction would produce some huge number. This happens even when working
+with seemingly harmless numbers like A=2, B=3, and C=2.
+
+For more information, see [unsigned: A Guideline for Better
+Code](https://www.youtube.com/watch?v=wvtFGa6XJDU) and [Garbage In, Garbage
+Out: Arguing about Undefined Behavior...](https://www.youtube.com/watch?v=yG1OZ69H_-o).
+This panel discussion may also be helpful:
+- https://www.youtube.com/watch?v=Puio5dly9N8#t=12m12s
+- https://www.youtube.com/watch?v=Puio5dly9N8#t=42m40s
+
 Microscopic Details
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