Support -fexcess-precision=16 which will enable FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16 when backend supports _Float16.

gcc/ada/ChangeLog:

	* gcc-interface/misc.c (gnat_post_options): Issue an error for
	-fexcess-precision=16.

gcc/c-family/ChangeLog:

	* c-common.c (excess_precision_mode_join): Update below comments.
	(c_ts18661_flt_eval_method): Set excess_precision_type to
	EXCESS_PRECISION_TYPE_FLOAT16 when -fexcess-precision=16.
	* c-cppbuiltin.c (cpp_atomic_builtins): Update below comments.
	(c_cpp_flt_eval_method_iec_559): Set excess_precision_type to
	EXCESS_PRECISION_TYPE_FLOAT16 when -fexcess-precision=16.

gcc/ChangeLog:

	* common.opt: Support -fexcess-precision=16.
	* config/aarch64/aarch64.c (aarch64_excess_precision): Return
	FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16 when
	EXCESS_PRECISION_TYPE_FLOAT16.
	* config/arm/arm.c (arm_excess_precision): Ditto.
	* config/i386/i386.c (ix86_get_excess_precision): Ditto.
	* config/m68k/m68k.c (m68k_excess_precision): Issue an error
	when EXCESS_PRECISION_TYPE_FLOAT16.
	* config/s390/s390.c (s390_excess_precision): Ditto.
	* coretypes.h (enum excess_precision_type): Add
	EXCESS_PRECISION_TYPE_FLOAT16.
	* doc/tm.texi (TARGET_C_EXCESS_PRECISION): Update documents.
	* doc/tm.texi.in (TARGET_C_EXCESS_PRECISION): Ditto.
	* doc/extend.texi (Half-Precision): Document
	-fexcess-precision=16.
	* flag-types.h (enum excess_precision): Add
	EXCESS_PRECISION_FLOAT16.
	* target.def (excess_precision): Update document.
	* tree.c (excess_precision_type): Set excess_precision_type to
	EXCESS_PRECISION_FLOAT16 when -fexcess-precision=16.

gcc/fortran/ChangeLog:

	* options.c (gfc_post_options): Issue an error for
	-fexcess-precision=16.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/float16-6.c: New test.
	* gcc.target/i386/float16-7.c: New test.

3 files changed, 15 insertions, 5 deletions

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 777dc0f..f08238b 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -1160,7 +1160,8 @@ On x86 targets with SSE2 enabled, without @option{-mavx512fp16},
 software emulation and the @code{float} instructions. The default behavior
 for @code{FLT_EVAL_METHOD} is to keep the intermediate result of the operation
 as 32-bit precision. This may lead to inconsistent behavior between software
-emulation and AVX512-FP16 instructions.
+emulation and AVX512-FP16 instructions. Using @option{-fexcess-precision=16}
+will force round back after each operation.
 
 @node Decimal Float
 @section Decimal Floating Types
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index f68f426..be81485 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -982,20 +982,26 @@ Do not define this macro if it would never modify @var{m}.
 Return a value, with the same meaning as the C99 macro
 @code{FLT_EVAL_METHOD} that describes which excess precision should be
 applied.  @var{type} is either @code{EXCESS_PRECISION_TYPE_IMPLICIT},
-@code{EXCESS_PRECISION_TYPE_FAST}, or
-@code{EXCESS_PRECISION_TYPE_STANDARD}.  For
+@code{EXCESS_PRECISION_TYPE_FAST},
+@code{EXCESS_PRECISION_TYPE_STANDARD}, or
+@code{EXCESS_PRECISION_TYPE_FLOAT16}.  For
 @code{EXCESS_PRECISION_TYPE_IMPLICIT}, the target should return which
 precision and range operations will be implictly evaluated in regardless
 of the excess precision explicitly added.  For
-@code{EXCESS_PRECISION_TYPE_STANDARD} and
+@code{EXCESS_PRECISION_TYPE_STANDARD}, 
+@code{EXCESS_PRECISION_TYPE_FLOAT16}, and
 @code{EXCESS_PRECISION_TYPE_FAST}, the target should return the
 explicit excess precision that should be added depending on the
 value set for @option{-fexcess-precision=@r{[}standard@r{|}fast@r{]}}.
 Note that unpredictable explicit excess precision does not make sense,
 so a target should never return @code{FLT_EVAL_METHOD_UNPREDICTABLE}
-when @var{type} is @code{EXCESS_PRECISION_TYPE_STANDARD} or
+when @var{type} is @code{EXCESS_PRECISION_TYPE_STANDARD},
+@code{EXCESS_PRECISION_TYPE_FLOAT16} or
 @code{EXCESS_PRECISION_TYPE_FAST}.
 @end deftypefn
+Return a value, with the same meaning as the C99 macro
+@code{FLT_EVAL_METHOD} that describes which excess precision should be
+applied.
 
 @deftypefn {Target Hook} machine_mode TARGET_PROMOTE_FUNCTION_MODE (const_tree @var{type}, machine_mode @var{mode}, int *@var{punsignedp}, const_tree @var{funtype}, int @var{for_return})
 Like @code{PROMOTE_MODE}, but it is applied to outgoing function arguments or
diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index fdf16b9..d088eee 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -929,6 +929,9 @@ Do not define this macro if it would never modify @var{m}.
 @end defmac
 
 @hook TARGET_C_EXCESS_PRECISION
+Return a value, with the same meaning as the C99 macro
+@code{FLT_EVAL_METHOD} that describes which excess precision should be
+applied.
 
 @hook TARGET_PROMOTE_FUNCTION_MODE