[compiler-rt] [CompilerRT] Add numerical sanitizer (PR #94322)

[Vitaly Buka via llvm-commits]

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@@ -0,0 +1,135 @@
+//===------------------------ nsan_platform.h -------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Platform specific information for NSan.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef NSAN_PLATFORM_H
+#define NSAN_PLATFORM_H
+
+namespace __nsan {
+
+// NSan uses two regions of memory to store information:
+// - 'shadow memory' stores the shadow copies of numerical values stored in
+//   application memory.
+// - 'shadow types' is used to determine which value type each byte of memory
+//   belongs to. This makes sure that we always know whether a shadow value is
+//   valid. Shadow values may be tampered with using access through other
+//   pointer types (type punning). Each byte stores:
+//     - bit 1-0: whether the corresponding value is of unknown (00),
+//       float (01), double (10), or long double (11) type.
+//     - bit 5-2: the index of this byte in the value, or 0000 if type is
+//       unknown.
+//       This allows handling unaligned loat load/stores by checking that a load
+//       with a given alignment corresponds to the alignment of the store.
+//       Any store of a non-floating point type invalidates the corresponding
+//       bytes, so that subsequent overlapping loads (aligned or not) know that
+//       the corresponding shadow value is no longer valid.
+
+// On Linux/x86_64, memory is laid out as follows:
+//
+// +--------------------+ 0x800000000000 (top of memory)
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vitalybuka wrote:

Maybe @thurstond can provide a feedback for these mappings?

https://github.com/llvm/llvm-project/pull/94322