Try to detect runtime unavailability of __builtin_mul_overflow(int64).

anarazel · anarazel · commit c04d35f442a8 · 2017-12-16T12:49:41.000-08:00
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
diff --git a/config/c-compiler.m4 b/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])])
diff --git a/configure b/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;

Original file line number	Diff line number	Diff line change
`@@ -14488,8 +14488,10 @@ else`
`14488`	`14488`	`int`
`14489`	`14489`	`main ()`
`14490`	`14490`	`{`
	`14491`	`+PG_INT64_TYPE a = 1;`
	`14492`	`+PG_INT64_TYPE b = 1;`
`14491`	`14493`	`PG_INT64_TYPE result;`
`14492`		`-__builtin_mul_overflow((PG_INT64_TYPE) 1, (PG_INT64_TYPE) 2, &result);`
	`14494`	`+__builtin_mul_overflow((volatile PG_INT64_TYPE ) a, (volatile PG_INT64_TYPE ) b, &result);`
`14493`	`14495`
`14494`	`14496`	`;`
`14495`	`14497`	`return 0;`