diff --git a/configure b/configure
index 4c789bd9289..f69331f0748 100755
--- a/configure
+++ b/configure
@@ -7700,29 +7700,6 @@ fi
 rm -f core conftest.err conftest.$ac_objext \
     conftest$ac_exeext conftest.$ac_ext
 
-# Defend against gcc -ffast-math
-if test "$GCC" = yes; then
-cat confdefs.h - <<_ACEOF >conftest.$ac_ext
-/* end confdefs.h.  */
-
-int
-main ()
-{
-#ifdef __FAST_MATH__
-choke me
-#endif
-  ;
-  return 0;
-}
-_ACEOF
-if ac_fn_c_try_compile "$LINENO"; then :
-
-else
-  as_fn_error $? "do not put -ffast-math in CFLAGS" "$LINENO" 5
-fi
-rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
-fi
-
 # Defend against clang being used on x86-32 without SSE2 enabled.  As current
 # versions of clang do not understand -fexcess-precision=standard, the use of
 # x87 floating point operations leads to problems like isinf possibly returning
diff --git a/configure.ac b/configure.ac
index 9edffe481a6..fead9a6ce99 100644
--- a/configure.ac
+++ b/configure.ac
@@ -786,13 +786,6 @@ AC_LINK_IFELSE([AC_LANG_PROGRAM([], [return 0;])],
   [AC_MSG_RESULT(no)
    AC_MSG_ERROR([cannot proceed])])
 
-# Defend against gcc -ffast-math
-if test "$GCC" = yes; then
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [@%:@ifdef __FAST_MATH__
-choke me
-@%:@endif])], [], [AC_MSG_ERROR([do not put -ffast-math in CFLAGS])])
-fi
-
 # Defend against clang being used on x86-32 without SSE2 enabled.  As current
 # versions of clang do not understand -fexcess-precision=standard, the use of
 # x87 floating point operations leads to problems like isinf possibly returning
diff --git a/src/backend/utils/adt/date.c b/src/backend/utils/adt/date.c
index 621b9175c12..87c063fa0cd 100644
--- a/src/backend/utils/adt/date.c
+++ b/src/backend/utils/adt/date.c
@@ -38,14 +38,6 @@
 #include "utils/skipsupport.h"
 #include "utils/sortsupport.h"
 
-/*
- * gcc's -ffast-math switch breaks routines that expect exact results from
- * expressions like timeval / SECS_PER_HOUR, where timeval is double.
- */
-#ifdef __FAST_MATH__
-#error -ffast-math is known to break this code
-#endif
-
 
 /* common code for timetypmodin and timetztypmodin */
 static int32
diff --git a/src/backend/utils/adt/float.c b/src/backend/utils/adt/float.c
index 641e7de21a0..f77f73820d7 100644
--- a/src/backend/utils/adt/float.c
+++ b/src/backend/utils/adt/float.c
@@ -29,6 +29,23 @@
 #include "utils/sortsupport.h"
 
 
+/*
+ * Reject building with gcc's -ffast-math switch.  It breaks our handling of
+ * float Infinity and NaN values (via -ffinite-math-only), causes results to
+ * be less accurate than expected (via -funsafe-math-optimizations and
+ * -fexcess-precision=fast), and causes some math error reports to be missed
+ * (via -fno-math-errno).  Unfortunately we can't easily detect cases where
+ * those options were given individually, but this at least catches the most
+ * obvious case.
+ *
+ * We test this only here, not in any header file, to allow extensions to use
+ * -ffast-math if they need to.  But the inline functions in float.h will
+ * misbehave in such an extension, so its authors had better be careful.
+ */
+#ifdef __FAST_MATH__
+#error -ffast-math is known to break this code
+#endif
+
 /*
  * Configurable GUC parameter
  *
diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index 6f8cf29c910..50a73fd24ec 100644
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -40,13 +40,6 @@
 #include "utils/skipsupport.h"
 #include "utils/sortsupport.h"
 
-/*
- * gcc's -ffast-math switch breaks routines that expect exact results from
- * expressions like timeval / SECS_PER_HOUR, where timeval is double.
- */
-#ifdef __FAST_MATH__
-#error -ffast-math is known to break this code
-#endif
 
 /* Set at postmaster start */
 TimestampTz PgStartTime;
diff --git a/src/interfaces/ecpg/pgtypeslib/interval.c b/src/interfaces/ecpg/pgtypeslib/interval.c
index e452a088f9e..463455398f1 100644
--- a/src/interfaces/ecpg/pgtypeslib/interval.c
+++ b/src/interfaces/ecpg/pgtypeslib/interval.c
@@ -6,10 +6,6 @@
 #include <math.h>
 #include <limits.h>
 
-#ifdef __FAST_MATH__
-#error -ffast-math is known to break this code
-#endif
-
 #include "common/string.h"
 #include "dt.h"
 #include "pgtypes_error.h"
diff --git a/src/interfaces/ecpg/pgtypeslib/timestamp.c b/src/interfaces/ecpg/pgtypeslib/timestamp.c
index 7cf433266f4..9bf1b914553 100644
--- a/src/interfaces/ecpg/pgtypeslib/timestamp.c
+++ b/src/interfaces/ecpg/pgtypeslib/timestamp.c
@@ -7,10 +7,6 @@
 #include <limits.h>
 #include <math.h>
 
-#ifdef __FAST_MATH__
-#error -ffast-math is known to break this code
-#endif
-
 #include "common/int.h"
 #include "dt.h"
 #include "pgtypes_date.h"