Provide overflow safe integer math inline functions.

It's not easy to get signed integer overflow checks correct and
fast. Therefore abstract the necessary infrastructure into a common
header providing addition, subtraction and multiplication for 16, 32,
64 bit signed integers.

The new macros aren't yet used, but a followup commit will convert
several open coded overflow checks.

Author: Andres Freund, with some code stolen from Greg Stark
Reviewed-By: Robert Haas
Discussion: https://postgr.es/m/20171024103954.ztmatprlglz3rwke@alap3.anarazel.de

3 files changed, 245 insertions, 0 deletions

diff --git a/src/include/common/int.h b/src/include/common/int.h
new file mode 100644
index 0000000000..e44d42f7da
--- /dev/null
+++ b/src/include/common/int.h
@@ -0,0 +1,239 @@
+/*-------------------------------------------------------------------------
+ *
+ * int.h
+ *	  Routines to perform integer math, while checking for overflows.
+ *
+ * The routines in this file are intended to be well defined C, without
+ * relying on compiler flags like -fwrapv.
+ *
+ * To reduce the overhead of these routines try to use compiler intrinsics
+ * where available. That's not that important for the 16, 32 bit cases, but
+ * the 64 bit cases can be considerably faster with intrinsics. In case no
+ * intrinsics are available 128 bit math is used where available.
+ *
+ * Copyright (c) 2017, PostgreSQL Global Development Group
+ *
+ * src/include/common/int.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef COMMON_INT_H
+#define COMMON_INT_H
+
+/*
+ * If a + b overflows, return true, otherwise store the result of a + b into
+ * *result. The content of *result is implementation defined in case of
+ * overflow.
+ */
+static inline bool
+pg_add_s16_overflow(int16 a, int16 b, int16 *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_add_overflow(a, b, result);
+#else
+	int32		res = (int32) a + (int32) b;
+
+	if (res > PG_INT16_MAX || res < PG_INT16_MIN)
+		return true;
+	*result = (int16) res;
+	return false;
+#endif
+}
+
+/*
+ * If a - b overflows, return true, otherwise store the result of a + b into
+ * *result. The content of *result is implementation defined in case of
+ * overflow.
+ */
+static inline bool
+pg_sub_s16_overflow(int16 a, int16 b, int16 *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_sub_overflow(a, b, result);
+#else
+	int32		res = (int32) a - (int32) b;
+
+	if (res > PG_INT16_MAX || res < PG_INT16_MIN)
+		return true;
+	*result = (int16) res;
+	return false;
+#endif
+}
+
+/*
+ * If a * b overflows, return true, otherwise store the result of a + b into
+ * *result. The content of *result is implementation defined in case of
+ * overflow.
+ */
+static inline bool
+pg_mul_s16_overflow(int16 a, int16 b, int16 *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_mul_overflow(a, b, result);
+#else
+	int32		res = (int32) a * (int32) b;
+
+	if (res > PG_INT16_MAX || res < PG_INT16_MIN)
+		return true;
+	*result = (int16) res;
+	return false;
+#endif
+}
+
+/*
+ * If a + b overflows, return true, otherwise store the result of a + b into
+ * *result. The content of *result is implementation defined in case of
+ * overflow.
+ */
+static inline bool
+pg_add_s32_overflow(int32 a, int32 b, int32 *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_add_overflow(a, b, result);
+#else
+	int64		res = (int64) a + (int64) b;
+
+	if (res > PG_INT32_MAX || res < PG_INT32_MIN)
+		return true;
+	*result = (int32) res;
+	return false;
+#endif
+}
+
+/*
+ * If a - b overflows, return true, otherwise store the result of a + b into
+ * *result. The content of *result is implementation defined in case of
+ * overflow.
+ */
+static inline bool
+pg_sub_s32_overflow(int32 a, int32 b, int32 *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_sub_overflow(a, b, result);
+#else
+	int64		res = (int64) a - (int64) b;
+
+	if (res > PG_INT32_MAX || res < PG_INT32_MIN)
+		return true;
+	*result = (int32) res;
+	return false;
+#endif
+}
+
+/*
+ * If a * b overflows, return true, otherwise store the result of a + b into
+ * *result. The content of *result is implementation defined in case of
+ * overflow.
+ */
+static inline bool
+pg_mul_s32_overflow(int32 a, int32 b, int32 *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_mul_overflow(a, b, result);
+#else
+	int64		res = (int64) a * (int64) b;
+
+	if (res > PG_INT32_MAX || res < PG_INT32_MIN)
+		return true;
+	*result = (int32) res;
+	return false;
+#endif
+}
+
+/*
+ * If a + b overflows, return true, otherwise store the result of a + b into
+ * *result. The content of *result is implementation defined in case of
+ * overflow.
+ */
+static inline bool
+pg_add_s64_overflow(int64 a, int64 b, int64 *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_add_overflow(a, b, result);
+#elif defined(HAVE_INT128)
+	int128		res = (int128) a + (int128) b;
+
+	if (res > PG_INT64_MAX || res < PG_INT64_MIN)
+		return true;
+	*result = (int64) res;
+	return false;
+#else
+	if ((a > 0 && b > 0 && a > PG_INT64_MAX - b) ||
+		(a < 0 && b < 0 && a < PG_INT64_MIN - b))
+		return true;
+	*result = a + b;
+	return false;
+#endif
+}
+
+/*
+ * If a - b overflows, return true, otherwise store the result of a + b into
+ * *result. The content of *result is implementation defined in case of
+ * overflow.
+ */
+static inline bool
+pg_sub_s64_overflow(int64 a, int64 b, int64 *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_sub_overflow(a, b, result);
+#elif defined(HAVE_INT128)
+	int128		res = (int128) a - (int128) b;
+
+	if (res > PG_INT64_MAX || res < PG_INT64_MIN)
+		return true;
+	*result = (int64) res;
+	return false;
+#else
+	if ((a < 0 && b > 0 && a < PG_INT64_MIN + b) ||
+		(a > 0 && b < 0 && a > PG_INT64_MAX + b))
+		return true;
+	*result = a - b;
+	return false;
+#endif
+}
+
+/*
+ * If a * b overflows, return true, otherwise store the result of a + b into
+ * *result. The content of *result is implementation defined in case of
+ * overflow.
+ */
+static inline bool
+pg_mul_s64_overflow(int64 a, int64 b, int64 *result)
+{
+#if defined(HAVE__BUILTIN_OP_OVERFLOW)
+	return __builtin_mul_overflow(a, b, result);
+#elif defined(HAVE_INT128)
+	int128		res = (int128) a * (int128) b;
+
+	if (res > PG_INT64_MAX || res < PG_INT64_MIN)
+		return true;
+	*result = (int64) res;
+	return false;
+#else
+	/*
+	 * Overflow can only happen if at least one value is outside the range
+	 * sqrt(min)..sqrt(max) so check that first as the division can be quite a
+	 * bit more expensive than the multiplication.
+	 *
+	 * Multiplying by 0 or 1 can't overflow of course and checking for 0
+	 * separately avoids any risk of dividing by 0.  Be careful about dividing
+	 * INT_MIN by -1 also, note reversing the a and b to ensure we're always
+	 * dividing it by a positive value.
+	 *
+	 */
+	if ((a > PG_INT32_MAX || a < PG_INT32_MIN ||
+		 b > PG_INT32_MAX || b < PG_INT32_MIN) &&
+		a != 0 && a != 1 && b != 0 && b != 1 &&
+		((a > 0 && b > 0 && a > PG_INT64_MAX / b) ||
+		 (a > 0 && b < 0 && b < PG_INT64_MIN / a) ||
+		 (a < 0 && b > 0 && a < PG_INT64_MIN / b) ||
+		 (a < 0 && b < 0 && a < PG_INT64_MAX / b)))
+	{
+		return true;
+	}
+	*result = a * b;
+	return false;
+#endif
+}
+
+#endif							/* COMMON_INT_H */
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 84d59f12b2..0aa6be4666 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -690,6 +690,9 @@
 /* Define to 1 if your compiler understands __builtin_constant_p. */
 #undef HAVE__BUILTIN_CONSTANT_P
 
+/* Define to 1 if your compiler understands __builtin_$op_overflow. */
+#undef HAVE__BUILTIN_OP_OVERFLOW
+
 /* Define to 1 if your compiler understands __builtin_types_compatible_p. */
 #undef HAVE__BUILTIN_TYPES_COMPATIBLE_P
 
diff --git a/src/include/pg_config.h.win32 b/src/include/pg_config.h.win32
index e192d98c5a..22d19ed794 100644
--- a/src/include/pg_config.h.win32
+++ b/src/include/pg_config.h.win32
@@ -515,6 +515,9 @@
 /* Define to 1 if your compiler understands __builtin_constant_p. */
 /* #undef HAVE__BUILTIN_CONSTANT_P */
 
+/* Define to 1 if your compiler understands __builtin_$op_overflow. */
+/* #undef HAVE__BUILTIN_OP_OVERFLOW */
+
 /* Define to 1 if your compiler understands __builtin_types_compatible_p. */
 /* #undef HAVE__BUILTIN_TYPES_COMPATIBLE_P */