Extend configure's __int128 test to check for a known gcc bug.

On Sparc64, use of __attribute__(aligned(8)) with __int128 causes faulty
code generation in gcc versions at least through 5.5.0.  We can work around
that by disabling use of __int128, so teach configure to test for the bug.

This solution doesn't fix things for the case of cross-compiling with a
buggy compiler; to support that nicely, we'd need to add a manual disable
switch.  Unless more such cases turn up, it doesn't seem worth the work.
Affected users could always edit pg_config.h manually.

In passing, fix some typos in the existing configure test for __int128.
They're harmless because we only compile that code not run it, but
they're still confusing for anyone looking at it closely.

This is needed in support of commit 751804998, so back-patch to 9.5
as that was.

Marina Polyakova, Victor Wagner, Tom Lane

Discussion: https://postgr.es/m/0d3a9fa264cebe1cb9966f37b7c06e86@postgrespro.ru

config/c-compiler.m4

@@ -106,29 +106,61 @@ AC_DEFUN([PGAC_TYPE_128BIT_INT],
 [AC_CACHE_CHECK([for __int128], [pgac_cv__128bit_int],
 [AC_LINK_IFELSE([AC_LANG_PROGRAM([
 /*
+ * We don't actually run this test, just link it to verify that any support
+ * functions needed for __int128 are present.
+ *
  * 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...
+ * convenient support for 128bit literals at this point...
  */
 __int128 a = 48828125;
-__int128 b = 97656255;
+__int128 b = 97656250;
 ],[
 __int128 c,d;
 a = (a << 12) + 1; /* 200000000001 */
 b = (b << 12) + 5; /* 400000000005 */
-/* use the most relevant arithmetic ops */
+/* try the most relevant arithmetic ops */
 c = a * b;
 d = (c + b) / b;
-/* return different values, to prevent optimizations */
+/* must use the results, else compiler may optimize arithmetic away */
 if (d != a+1)
-  return 0;
+  return 1;
-return 1;
 ])],
 [pgac_cv__128bit_int=yes],
 [pgac_cv__128bit_int=no])])
 if test x"$pgac_cv__128bit_int" = xyes ; then
-  AC_DEFINE(PG_INT128_TYPE, __int128, [Define to the name of a signed 128-bit integer type.])
+  # Use of non-default alignment with __int128 tickles bugs in some compilers.
-  AC_CHECK_ALIGNOF(PG_INT128_TYPE)
+  # If not cross-compiling, we can test for bugs and disable use of __int128
+  # with buggy compilers.  If cross-compiling, hope for the best.
+  # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83925
+  AC_CACHE_CHECK([for __int128 alignment bug], [pgac_cv__128bit_int_bug],
+  [AC_RUN_IFELSE([AC_LANG_PROGRAM([
+/* This must match the corresponding code in c.h: */
+#if defined(__GNUC__) || defined(__SUNPRO_C) || defined(__IBMC__)
+#define pg_attribute_aligned(a) __attribute__((aligned(a)))
+#endif
+typedef __int128 int128a
+#if defined(pg_attribute_aligned)
+pg_attribute_aligned(8)
+#endif
+;
+int128a holder;
+void pass_by_val(void *buffer, int128a par) { holder = par; }
+],[
+long int i64 = 97656225L << 12;
+int128a q;
+pass_by_val(main, (int128a) i64);
+q = (int128a) i64;
+if (q != holder)
+  return 1;
+])],
+  [pgac_cv__128bit_int_bug=ok],
+  [pgac_cv__128bit_int_bug=broken],
+  [pgac_cv__128bit_int_bug="assuming ok"])])
+  if test x"$pgac_cv__128bit_int_bug" != xbroken ; then
+    AC_DEFINE(PG_INT128_TYPE, __int128, [Define to the name of a signed 128-bit integer type.])
+    AC_CHECK_ALIGNOF(PG_INT128_TYPE)
+  fi
 fi])# PGAC_TYPE_128BIT_INT
 
 

configure

@@ -14355,12 +14355,15 @@ else
 /* end confdefs.h.  */
 
 /*
+ * We don't actually run this test, just link it to verify that any support
+ * functions needed for __int128 are present.
+ *
  * 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...
+ * convenient support for 128bit literals at this point...
  */
 __int128 a = 48828125;
-__int128 b = 97656255;
+__int128 b = 97656250;
 
 int
 main ()
@@ -14369,13 +14372,12 @@ main ()
 __int128 c,d;
 a = (a << 12) + 1; /* 200000000001 */
 b = (b << 12) + 5; /* 400000000005 */
-/* use the most relevant arithmetic ops */
+/* try the most relevant arithmetic ops */
 c = a * b;
 d = (c + b) / b;
-/* return different values, to prevent optimizations */
+/* must use the results, else compiler may optimize arithmetic away */
 if (d != a+1)
-  return 0;
+  return 1;
-return 1;
 
   ;
   return 0;
@@ -14392,10 +14394,65 @@ fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv__128bit_int" >&5
 $as_echo "$pgac_cv__128bit_int" >&6; }
 if test x"$pgac_cv__128bit_int" = xyes ; then
+  # Use of non-default alignment with __int128 tickles bugs in some compilers.
+  # If not cross-compiling, we can test for bugs and disable use of __int128
+  # with buggy compilers.  If cross-compiling, hope for the best.
+  # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83925
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for __int128 alignment bug" >&5
+$as_echo_n "checking for __int128 alignment bug... " >&6; }
+if ${pgac_cv__128bit_int_bug+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if test "$cross_compiling" = yes; then :
+  pgac_cv__128bit_int_bug="assuming ok"
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+/* This must match the corresponding code in c.h: */
+#if defined(__GNUC__) || defined(__SUNPRO_C) || defined(__IBMC__)
+#define pg_attribute_aligned(a) __attribute__((aligned(a)))
+#endif
+typedef __int128 int128a
+#if defined(pg_attribute_aligned)
+pg_attribute_aligned(8)
+#endif
+;
+int128a holder;
+void pass_by_val(void *buffer, int128a par) { holder = par; }
+
+int
+main ()
+{
+
+long int i64 = 97656225L << 12;
+int128a q;
+pass_by_val(main, (int128a) i64);
+q = (int128a) i64;
+if (q != holder)
+  return 1;
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_run "$LINENO"; then :
+  pgac_cv__128bit_int_bug=ok
+else
+  pgac_cv__128bit_int_bug=broken
+fi
+rm -f core *.core core.conftest.* gmon.out bb.out conftest$ac_exeext \
+  conftest.$ac_objext conftest.beam conftest.$ac_ext
+fi
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv__128bit_int_bug" >&5
+$as_echo "$pgac_cv__128bit_int_bug" >&6; }
+  if test x"$pgac_cv__128bit_int_bug" != xbroken ; then
 
 $as_echo "#define PG_INT128_TYPE __int128" >>confdefs.h
 
-  # The cast to long int works around a bug in the HP C Compiler,
+    # The cast to long int works around a bug in the HP C Compiler,
 # see AC_CHECK_SIZEOF for more information.
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking alignment of PG_INT128_TYPE" >&5
 $as_echo_n "checking alignment of PG_INT128_TYPE... " >&6; }
@@ -14430,6 +14487,7 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+  fi
 fi
 
 # Check for various atomic operations now that we have checked how to declare