Introduce overflow-safe integer comparison functions.

This commit adds integer comparison functions that are designed to
be as efficient as possible while avoiding overflow.  A follow-up
commit will make use of these functions in many of the in-tree
qsort() comparators.  The new functions are not better in all cases
(e.g., when the comparator function is inlined), so it is important
to consider the context before using them.

Author: Mats Kindahl
Reviewed-by: Tom Lane, Heikki Linnakangas, Andres Freund, Thomas Munro, Andrey Borodin, Fabrízio de Royes Mello
Discussion: https://postgr.es/m/CA%2B14426g2Wa9QuUpmakwPxXFWG_1FaY0AsApkvcTBy-YfS6uaw%40mail.gmail.com

src/include/common/int.h

@@ -1,7 +1,7 @@
 /*-------------------------------------------------------------------------
  *
  * int.h
- *	  Routines to perform integer math, while checking for overflows.
+ *	  Overflow-aware integer math and integer comparison routines.
  *
  * The routines in this file are intended to be well defined C, without
  * relying on compiler flags like -fwrapv.
@@ -22,7 +22,7 @@
 
 
 /*---------
- * The following guidelines apply to all the routines:
+ * The following guidelines apply to all the overflow routines:
  * - 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.
@@ -438,4 +438,75 @@ pg_mul_u64_overflow(uint64 a, uint64 b, uint64 *result)
 #endif
 }
 
+/*------------------------------------------------------------------------
+ *
+ * Comparison routines for integer types.
+ *
+ * These routines are primarily intended for use in qsort() comparator
+ * functions and therefore return a positive integer, 0, or a negative
+ * integer depending on whether "a" is greater than, equal to, or less
+ * than "b", respectively.  These functions are written to be as efficient
+ * as possible without introducing overflow risks, thereby helping ensure
+ * the comparators that use them are transitive.
+ *
+ * Types with fewer than 32 bits are cast to signed integers and
+ * subtracted.  Other types are compared using > and <, and the results of
+ * those comparisons (which are either (int) 0 or (int) 1 per the C
+ * standard) are subtracted.
+ *
+ * NB: If the comparator function is inlined, some compilers may produce
+ * worse code with these helper functions than with code with the
+ * following form:
+ *
+ *     if (a < b)
+ *         return -1;
+ *     if (a > b)
+ *         return 1;
+ *     return 0;
+ *
+ *------------------------------------------------------------------------
+ */
+
+static inline int
+pg_cmp_s16(int16 a, int16 b)
+{
+	return (int32) a - (int32) b;
+}
+
+static inline int
+pg_cmp_u16(uint16 a, uint16 b)
+{
+	return (int32) a - (int32) b;
+}
+
+static inline int
+pg_cmp_s32(int32 a, int32 b)
+{
+	return (a > b) - (a < b);
+}
+
+static inline int
+pg_cmp_u32(uint32 a, uint32 b)
+{
+	return (a > b) - (a < b);
+}
+
+static inline int
+pg_cmp_s64(int64 a, int64 b)
+{
+	return (a > b) - (a < b);
+}
+
+static inline int
+pg_cmp_u64(uint64 a, uint64 b)
+{
+	return (a > b) - (a < b);
+}
+
+static inline int
+pg_cmp_size(size_t a, size_t b)
+{
+	return (a > b) - (a < b);
+}
+
 #endif							/* COMMON_INT_H */

src/include/lib/sort_template.h

@@ -34,6 +34,16 @@
  *	  - ST_COMPARE(a, b, arg) - variant that takes an extra argument
  *	  - ST_COMPARE_RUNTIME_POINTER - sort function takes a function pointer
  *
+ *	  NB: If the comparator function is inlined, some compilers may produce
+ *	  worse code with the optimized comparison routines in common/int.h than
+ *	  with code with the following form:
+ *
+ *	      if (a < b)
+ *	          return -1;
+ *	      if (a > b)
+ *	          return 1;
+ *	      return 0;
+ *
  *	  To say that the comparator and therefore also sort function should
  *	  receive an extra pass-through argument, specify the type of the
  *	  argument.
@@ -243,6 +253,9 @@ ST_SCOPE void ST_SORT(ST_ELEMENT_TYPE * first, size_t n
  * Find the median of three values.  Currently, performance seems to be best
  * if the comparator is inlined here, but the med3 function is not inlined
  * in the qsort function.
+ *
+ * Refer to the comment at the top of this file for known caveats to consider
+ * when writing inlined comparator functions.
  */
 static pg_noinline ST_ELEMENT_TYPE *
 ST_MED3(ST_ELEMENT_TYPE * a,