Try to detect runtime unavailability of __builtin_mul_overflow(int64).

On some systems the results of 64 bit __builtin_mul_overflow()
operations can be computed at compile time, but not at runtime. The
known cases are arm buildfar animals using clang where the runtime
operation is implemented in a unavailable function.

Try to avoid compile-time computation by using volatile arguments to
__builtin_mul_overflow(). In that case we hopefully will get a link
error when unavailable, similar to what buildfarm animals dangomushi
and gull are reporting.

Author: Andres Freund
Discussion: https://postgr.es/m/20171213213754.pydkyjs6bt2hvsdb@alap3.anarazel.de

config/c-compiler.m4

@@ -310,13 +310,17 @@ fi])# PGAC_C_BUILTIN_CONSTANT_P
 # and define HAVE__BUILTIN_OP_OVERFLOW if so.
 #
 # Check for the most complicated case, 64 bit multiplication, as a
-# proxy for all of the operations. Have to link to be sure to
-# recognize a missing __builtin_mul_overflow.
+# proxy for all of the operations. Use volatile variables to avoid the
+# compiler computing result at compile time, even though the runtime
+# might not supply operation. Have to link to be sure to recognize a
+# missing __builtin_mul_overflow.
 AC_DEFUN([PGAC_C_BUILTIN_OP_OVERFLOW],
 [AC_CACHE_CHECK(for __builtin_mul_overflow, pgac_cv__builtin_op_overflow,
 [AC_LINK_IFELSE([AC_LANG_PROGRAM([],
-[PG_INT64_TYPE result;
-__builtin_mul_overflow((PG_INT64_TYPE) 1, (PG_INT64_TYPE) 2, &result);]
+[PG_INT64_TYPE a = 1;
+PG_INT64_TYPE b = 1;
+PG_INT64_TYPE result;
+__builtin_mul_overflow(*(volatile PG_INT64_TYPE *) a, *(volatile PG_INT64_TYPE *) b, &result);]
 )],
 [pgac_cv__builtin_op_overflow=yes],
 [pgac_cv__builtin_op_overflow=no])])

configure

@@ -14488,8 +14488,10 @@ else
 int
 main ()
 {
+PG_INT64_TYPE a = 1;
+PG_INT64_TYPE b = 1;
 PG_INT64_TYPE result;
-__builtin_mul_overflow((PG_INT64_TYPE) 1, (PG_INT64_TYPE) 2, &result);
+__builtin_mul_overflow(*(volatile PG_INT64_TYPE *) a, *(volatile PG_INT64_TYPE *) b, &result);
 
   ;
   return 0;