make/autoconf/flags-cflags.m4

@  rev 51624 : 8210416: [linux] Poor StrictMath performance due to non-optimized compilation
|  Summary: Compile fdlibm with -O2 -ffp-contract=off on gcc/clang arches.
~  Reviewed-by: aph, erikj, dholmes, darcy

*** 371,380 ****
--- 371,394 ----
    FLAGS_CPU_LEGACY=$OPENJDK_BUILD_CPU_LEGACY
    FLAGS_CPU_LEGACY_LIB=$OPENJDK_BUILD_CPU_LEGACY_LIB
  
    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"
    CFLAGS_TESTEXE="$CFLAGS_JDKEXE"
    CXXFLAGS_TESTEXE="$CXXFLAGS_JDKEXE"