[llvm] [APInt] Provide isqrt (floor of square root) instead of sqrt (rounded) (PR #197406)

[Jay Foad via llvm-commits]

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@@ -1809,8 +1809,8 @@ class [[nodiscard]] APInt {
     return logBase2();
   }
 
-  /// Compute the square root.
-  LLVM_ABI APInt sqrt() const;
+  /// Compute the floor of the square root of the unsigned value.
+  LLVM_ABI APInt isqrt() const;
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jayfoad wrote:

I guess any those names would be fine.

I did look at https://gmplib.org/manual/Integer-Roots for inspiration. It just uses the name `mpz_sqrt` for "the truncated integer part of the square root". (This further convinces me that the truncating behaviour is the only behaviour worth implementing.)

https://github.com/llvm/llvm-project/pull/197406