From fcdfd5b0b9efcf7797b0f1bf7e7ed47ff99de198 Mon Sep 17 00:00:00 2001
From: Federico Stra <stra.federico@gmail.com>
Date: Thu, 28 Sep 2023 13:59:19 +0200
Subject: [PATCH] isqrt: `assume` that `isqrt` takes half as many bits

https://github.com/rust-lang/rust/issues/89273#issuecomment-970581089
---
 library/core/src/num/uint_macros.rs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/library/core/src/num/uint_macros.rs b/library/core/src/num/uint_macros.rs
index 565fc1930ea4f..f2190efa4d3c9 100644
--- a/library/core/src/num/uint_macros.rs
+++ b/library/core/src/num/uint_macros.rs
@@ -2017,6 +2017,13 @@ macro_rules! uint_impl {
                 one >>= 2;
             }
 
+            // SAFETY: the result is positive and fits in an integer with half as many bits.
+            // Inform the optimizer about it.
+            unsafe {
+                intrinsics::assume(0 < res);
+                intrinsics::assume(res < 1 << (Self::BITS / 2));
+            }
+
             res
         }