--- old/make/autoconf/flags-cflags.m4	2018-09-11 18:11:25.925792189 +0200
+++ new/make/autoconf/flags-cflags.m4	2018-09-11 18:11:25.600792824 +0200
@@ -373,6 +373,20 @@
 
   FLAGS_SETUP_CFLAGS_CPU_DEP([BUILD], [OPENJDK_BUILD_])
 
+  COMPILER_FP_CONTRACT_OFF_FLAG="-ffp-contract=off"
+  # Check that the compiler supports -ffp-contract=off flag
+  # Set FDLIBM_CFLAGS to -ffp-contract=off if it does. Empty
+  # otherwise.
+  # These flags are required for GCC-based builds of
+  # fdlibm with optimization without losing precision.
+  # Notably, -ffp-contract=off needs to be added for GCC >= 4.6.
+  if test "x$TOOLCHAIN_TYPE" = xgcc || test "x$TOOLCHAIN_TYPE" = xclang; then
+    FLAGS_COMPILER_CHECK_ARGUMENTS(ARGUMENT: [${COMPILER_FP_CONTRACT_OFF_FLAG}],
+	IF_TRUE: [FDLIBM_CFLAGS=${COMPILER_FP_CONTRACT_OFF_FLAG}],
+	IF_FALSE: [FDLIBM_CFLAGS=""])
+  fi
+  AC_SUBST(FDLIBM_CFLAGS)
+
   # Tests are only ever compiled for TARGET
   CFLAGS_TESTLIB="$CFLAGS_JDKLIB"
   CXXFLAGS_TESTLIB="$CXXFLAGS_JDKLIB"
--- old/make/autoconf/spec.gmk.in	2018-09-11 18:11:26.692790691 +0200
+++ new/make/autoconf/spec.gmk.in	2018-09-11 18:11:26.376791308 +0200
@@ -450,6 +450,7 @@
 LIBJSIG_HASHSTYLE_LDFLAGS := @LIBJSIG_HASHSTYLE_LDFLAGS@
 LIBJSIG_NOEXECSTACK_LDFLAGS := @LIBJSIG_NOEXECSTACK_LDFLAGS@
 
+FDLIBM_CFLAGS := @FDLIBM_CFLAGS@
 JVM_CFLAGS := @JVM_CFLAGS@
 JVM_LDFLAGS := @JVM_LDFLAGS@
 JVM_ASFLAGS := @JVM_ASFLAGS@
--- old/make/lib/CoreLibraries.gmk	2018-09-11 18:11:27.499789114 +0200
+++ new/make/lib/CoreLibraries.gmk	2018-09-11 18:11:27.137789821 +0200
@@ -39,20 +39,15 @@
   BUILD_LIBFDLIBM_OPTIMIZATION := HIGH
 endif
 
-ifeq ($(OPENJDK_TARGET_OS), linux)
-  ifeq ($(OPENJDK_TARGET_CPU), ppc64)
-    BUILD_LIBFDLIBM_OPTIMIZATION := HIGH
-  else ifeq ($(OPENJDK_TARGET_CPU), ppc64le)
-    BUILD_LIBFDLIBM_OPTIMIZATION := HIGH
-  else ifeq ($(OPENJDK_TARGET_CPU), s390x)
-    BUILD_LIBFDLIBM_OPTIMIZATION := HIGH
-  else ifeq ($(OPENJDK_TARGET_CPU), aarch64)
-    BUILD_LIBFDLIBM_OPTIMIZATION := HIGH
-  endif
+# If FDLIBM_CFLAGS is non-empty we know that we can optimize
+# fdlibm by adding those extra C flags. Currently GCC,
+# and clang only.
+ifneq ($(FDLIBM_CFLAGS), )
+  BUILD_LIBFDLIBM_OPTIMIZATION := LOW
 endif
 
 LIBFDLIBM_SRC := $(TOPDIR)/src/java.base/share/native/libfdlibm
-LIBFDLIBM_CFLAGS := -I$(LIBFDLIBM_SRC)
+LIBFDLIBM_CFLAGS := -I$(LIBFDLIBM_SRC) $(FDLIBM_CFLAGS)
 
 ifneq ($(OPENJDK_TARGET_OS), macosx)
   $(eval $(call SetupNativeCompilation, BUILD_LIBFDLIBM, \
@@ -64,10 +59,6 @@
       CFLAGS := $(CFLAGS_JDKLIB) $(LIBFDLIBM_CFLAGS), \
       CFLAGS_windows_debug := -DLOGGING, \
       CFLAGS_aix := -qfloat=nomaf, \
-      CFLAGS_linux_ppc64 := -ffp-contract=off, \
-      CFLAGS_linux_ppc64le := -ffp-contract=off, \
-      CFLAGS_linux_s390x := -ffp-contract=off, \
-      CFLAGS_linux_aarch64 := -ffp-contract=off, \
       DISABLED_WARNINGS_gcc := sign-compare misleading-indentation, \
       DISABLED_WARNINGS_microsoft := 4146 4244 4018, \
       ARFLAGS := $(ARFLAGS), \