mantissa_util: Implement ResidualErrorOnRightShift

Accurately calculate residual error that is shifted out

src/CMakeLists.txt

@@ -18,6 +18,7 @@ add_library(dynarmic
     common/crc32.h
     common/fp/fpsr.h
     common/fp/info.h
+    common/fp/mantissa_util.h
     common/fp/process_exception.cpp
     common/fp/process_exception.h
     common/fp/rounding_mode.h

src/common/fp/mantissa_util.h

@@ -0,0 +1,48 @@
+/* This file is part of the dynarmic project.
+ * Copyright (c) 2018 MerryMage
+ * This software may be used and distributed according to the terms of the GNU
+ * General Public License version 2 or any later version.
+ */
+
+#pragma once
+
+#include "common/bit_util.h"
+#include "common/common_types.h"
+
+namespace Dynarmic::FP {
+
+enum class ResidualError {
+    Zero,
+    LessThanHalf,
+    Half,
+    GreaterThanHalf,
+};
+
+template<typename MantissaT>
+ResidualError ResidualErrorOnRightShift(MantissaT mantissa, int shift_amount) {
+    if (shift_amount <= 0 || mantissa == 0) {
+        return ResidualError::Zero;
+    }
+
+    if (shift_amount > static_cast<int>(Common::BitSize<MantissaT>())) {
+        return Common::MostSignificantBit(mantissa) ? ResidualError::GreaterThanHalf : ResidualError::LessThanHalf;
+    }
+
+    const size_t half_bit_position = static_cast<size_t>(shift_amount - 1);
+    const MantissaT half = static_cast<MantissaT>(1) << half_bit_position;
+    const MantissaT error_mask = Common::Ones<MantissaT>(static_cast<size_t>(shift_amount));
+    const MantissaT error = mantissa & error_mask;
+
+    if (error == 0) {
+        return ResidualError::Zero;
+    }
+    if (error < half) {
+        return ResidualError::LessThanHalf;
+    }
+    if (error == half) {
+        return ResidualError::Half;
+    }
+    return ResidualError::GreaterThanHalf;
+}
+
+} // namespace Dynarmic::FP