Add, optional, support for 128bit integers.

We will, for the foreseeable future, not expose 128 bit datatypes to
SQL. But being able to use 128bit math will allow us, in a later patch,
to use 128bit accumulators for some aggregates; leading to noticeable
speedups over using numeric.

So far we only detect a gcc/clang extension that supports 128bit math,
but no 128bit literals, and no *printf support. We might want to expand
this in the future to further compilers; if there are any that that
provide similar support.

Discussion: 544BB5F1.50709@proxel.se
Author: Andreas Karlsson, with significant editorializing by me
Reviewed-By: Peter Geoghegan, Oskari Saarenmaa

1 files changed, 37 insertions, 0 deletions

diff --git a/config/c-compiler.m4 b/config/c-compiler.m4
index 509f96139ff..38aab11bc65 100644
--- a/config/c-compiler.m4
+++ b/config/c-compiler.m4
@@ -125,6 +125,43 @@ undefine([Ac_cachevar])dnl
 ])# PGAC_TYPE_64BIT_INT
 
 
+# PGAC_TYPE_128BIT_INT
+# ---------------------
+# Check if __int128 is a working 128 bit integer type, and if so
+# define PG_INT128_TYPE to that typename.  This currently only detects
+# a GCC/clang extension, but support for different environments may be
+# added in the future.
+#
+# For the moment we only test for support for 128bit math; support for
+# 128bit literals and snprintf is not required.
+AC_DEFUN([PGAC_TYPE_128BIT_INT],
+[AC_CACHE_CHECK([for __int128], [pgac_cv__128bit_int],
+[AC_LINK_IFELSE([AC_LANG_PROGRAM([
+/*
+ * These are globals to discourage the compiler from folding all the
+ * arithmetic tests down to compile-time constants.  We do not have
+ * convenient support for 64bit literals at this point...
+ */
+__int128 a = 48828125;
+__int128 b = 97656255;
+],[
+__int128 c,d;
+a = (a << 12) + 1; /* 200000000001 */
+b = (b << 12) + 5; /* 400000000005 */
+/* use the most relevant arithmetic ops */
+c = a * b;
+d = (c + b) / b;
+/* return different values, to prevent optimizations */
+if (d != a+1)
+  return 0;
+return 1;
+])],
+[pgac_cv__128bit_int=yes],
+[pgac_cv__128bit_int=no])])
+if test x"$pgac_cv__128bit_int" = xyes ; then
+  AC_DEFINE(PG_INT128_TYPE, __int128, [Define to the name of a signed 128-bit integer type.])
+fi])# PGAC_TYPE_128BIT_INT
+
 
 # PGAC_C_FUNCNAME_SUPPORT
 # -----------------------