[vcpkg] Correct UInt128 code 😇 (#10583)

* [vcpkg] Correct UInt128 code 😇

`UInt128::operator<<(x, y)` should clear the bottom 64 bits of `x` if
`y >= 64`; however, we don't do this, and so we duplicate `x`'s bottom
bits into `x.top` instead of moving them. Similarly, we have the
opposite problem for `UInt128::operator>>`. This commit fixes these
latent bugs, which we weren't hitting because the thing we use them for
never actually shifts more than 64 bits.

1 files changed, 26 insertions, 0 deletions

diff --git a/toolsrc/include/vcpkg/base/uint128.h b/toolsrc/include/vcpkg/base/uint128.h
new file mode 100644
index 000000000..6b7816760
--- /dev/null
+++ b/toolsrc/include/vcpkg/base/uint128.h
@@ -0,0 +1,26 @@
+#pragma once
+
+#include <stdint.h>
+
+namespace vcpkg {
+
+struct UInt128 {
+    UInt128() = default;
+    UInt128(uint64_t value) : bottom(value), top(0) {}
+
+    UInt128& operator<<=(int by) noexcept;
+    UInt128& operator>>=(int by) noexcept;
+    UInt128& operator+=(uint64_t lhs) noexcept;
+
+    uint64_t bottom_64_bits() const noexcept {
+        return bottom;
+    }
+    uint64_t top_64_bits() const noexcept {
+        return top;
+    }
+private:
+    uint64_t bottom;
+    uint64_t top;
+};
+
+}