Merge bitcoin#15391: Add compile time verification of assumptions we'…

…re currently making implicitly/tacitly

7cee858 Add compile time verification of assumptions we're currently making implicitly/tacitly (practicalswift)

Pull request description:

  Add compile time verification of assumptions we're currently making implicitly/tacitly.

  As suggested by @sipa in bitcoin#14239 (comment) and @MarcoFalke in bitcoin#14479 (comment).

Tree-SHA512: e68fe51164dbd3eeb76aa8a7e83dfcd3b4d5a66037c0f1822bbbd189bbe3c280e03b3b10af870880ecc09b612e62fb3d9bcd6cf1e16cb7ba818c257db0712ce4

src/Makefile.am

@@ -118,6 +118,7 @@ BITCOIN_CORE_H = \
   clientversion.h \
   coins.h \
   compat.h \
+  compat/assumptions.h \
   compat/byteswap.h \
   compat/endian.h \
   compat/sanity.h \

src/compat/assumptions.h

@@ -0,0 +1,49 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+// Compile-time verification of assumptions we make.
+
+#ifndef BITCOIN_COMPAT_ASSUMPTIONS_H
+#define BITCOIN_COMPAT_ASSUMPTIONS_H
+
+#include <limits>
+
+// Assumption: We assume that the macro NDEBUG is not defined.
+// Example(s): We use assert(...) extensively with the assumption of it never
+//             being a noop at runtime.
+#if defined(NDEBUG)
+# error "Bitcoin cannot be compiled without assertions."
+#endif
+
+// Assumption: We assume the floating-point types to fulfill the requirements of
+//             IEC 559 (IEEE 754) standard.
+// Example(s): Floating-point division by zero in ConnectBlock, CreateTransaction
+//             and EstimateMedianVal.
+static_assert(std::numeric_limits<float>::is_iec559, "IEEE 754 float assumed");
+static_assert(std::numeric_limits<double>::is_iec559, "IEEE 754 double assumed");
+
+// Assumption: We assume eight bits per byte (obviously, but remember: don't
+//             trust -- verify!).
+// Example(s): Everywhere :-)
+static_assert(std::numeric_limits<unsigned char>::digits == 8, "8-bit byte assumed");
+
+// Assumption: We assume floating-point widths.
+// Example(s): Type punning in serialization code (ser_{float,double}_to_uint{32,64}).
+static_assert(sizeof(float) == 4, "32-bit float assumed");
+static_assert(sizeof(double) == 8, "64-bit double assumed");
+
+// Assumption: We assume integer widths.
+// Example(s): GetSizeOfCompactSize and WriteCompactSize in the serialization
+//             code.
+static_assert(sizeof(short) == 2, "16-bit short assumed");
+static_assert(sizeof(int) == 4, "32-bit int assumed");
+
+// Some important things we are NOT assuming (non-exhaustive list):
+// * We are NOT assuming a specific value for sizeof(std::size_t).
+// * We are NOT assuming a specific value for std::endian::native.
+// * We are NOT assuming a specific value for std::locale("").name().
+// * We are NOT assuming a specific value for std::numeric_limits<char>::is_signed.
+
+#endif // BITCOIN_COMPAT_ASSUMPTIONS_H

src/util/system.h

@@ -16,6 +16,7 @@
 
 #include <attributes.h>
 #include <compat.h>
+#include <compat/assumptions.h>
 #include <fs.h>
 #include <logging.h>
 #include <sync.h>