Disable _Float16 for non ARM/SPIR Targets

As Discussed here:
http://lists.llvm.org/pipermail/llvm-dev/2019-January/129543.html

There are problems exposing the _Float16 type on architectures that
haven't defined the ABI/ISel for the type yet, so we're temporarily
disabling the type and making it opt-in.

Differential Revision: https://reviews.llvm.org/D57188

Change-Id: I5db7366dedf1deb9485adb8948b1deb7e612a736
llvm-svn: 352221

clang/docs/LanguageExtensions.rst

@@ -474,44 +474,58 @@ Half-Precision Floating Point
 =============================
 
 Clang supports two half-precision (16-bit) floating point types: ``__fp16`` and
-``_Float16``. ``__fp16`` is defined in the ARM C Language Extensions (`ACLE
-<http://infocenter.arm.com/help/topic/com.arm.doc.ihi0053d/IHI0053D_acle_2_1.pdf>`_)
-and ``_Float16`` in ISO/IEC TS 18661-3:2015.
+``_Float16``.  These types are supported in all language modes.
+
+``__fp16`` is supported on every target, as it is purely a storage format; see below.
+``_Float16`` is currently only supported on the following targets, with further
+targets pending ABI standardization:
+- 32-bit ARM
+- 64-bit ARM (AArch64)
+- SPIR
+``_Float16`` will be supported on more targets as they define ABIs for it.
 
 ``__fp16`` is a storage and interchange format only.  This means that values of
-``__fp16`` promote to (at least) float when used in arithmetic operations.
-There are two ``__fp16`` formats. Clang supports the IEEE 754-2008 format and
-not the ARM alternative format.
+``__fp16`` are immediately promoted to (at least) ``float`` when used in arithmetic
+operations, so that e.g. the result of adding two ``__fp16`` values has type ``float``.
+The behavior of ``__fp16`` is specified by the ARM C Language Extensions (`ACLE <http://infocenter.arm.com/help/topic/com.arm.doc.ihi0053d/IHI0053D_acle_2_1.pdf>`_).
+Clang uses the ``binary16`` format from IEEE 754-2008 for ``__fp16``, not the ARM
+alternative format.
 
-ISO/IEC TS 18661-3:2015 defines C support for additional floating point types.
-``_FloatN`` is defined as a binary floating type, where the N suffix denotes
-the number of bits and is 16, 32, 64, or greater and equal to 128 and a
-multiple of 32. Clang supports ``_Float16``. The difference from ``__fp16`` is
-that arithmetic on ``_Float16`` is performed in half-precision, thus it is not
-a storage-only format. ``_Float16`` is available as a source language type in
-both C and C++ mode.
+``_Float16`` is an extended floating-point type.  This means that, just like arithmetic on
+``float`` or ``double``, arithmetic on ``_Float16`` operands is formally performed in the
+``_Float16`` type, so that e.g. the result of adding two ``_Float16`` values has type
+``_Float16``.  The behavior of ``_Float16`` is specified by ISO/IEC TS 18661-3:2015
+("Floating-point extensions for C").  As with ``__fp16``, Clang uses the ``binary16``
+format from IEEE 754-2008 for ``_Float16``.
 
-It is recommended that portable code use the ``_Float16`` type because
-``__fp16`` is an ARM C-Language Extension (ACLE), whereas ``_Float16`` is
-defined by the C standards committee, so using ``_Float16`` will not prevent
-code from being ported to architectures other than Arm.  Also, ``_Float16``
-arithmetic and operations will directly map on half-precision instructions when
-they are available (e.g. Armv8.2-A), avoiding conversions to/from
-single-precision, and thus will result in more performant code. If
-half-precision instructions are unavailable, values will be promoted to
-single-precision, similar to the semantics of ``__fp16`` except that the
-results will be stored in single-precision.
+``_Float16`` arithmetic will be performed using native half-precision support
+when available on the target (e.g. on ARMv8.2a); otherwise it will be performed
+at a higher precision (currently always ``float``) and then truncated down to
+``_Float16``.  Note that C and C++ allow intermediate floating-point operands
+of an expression to be computed with greater precision than is expressible in
+their type, so Clang may avoid intermediate truncations in certain cases; this may
+lead to results that are inconsistent with native arithmetic.
 
-In an arithmetic operation where one operand is of ``__fp16`` type and the
-other is of ``_Float16`` type, the ``_Float16`` type is first converted to
-``__fp16`` type and then the operation is completed as if both operands were of
-``__fp16`` type.
+It is recommended that portable code use ``_Float16`` instead of ``__fp16``,
+as it has been defined by the C standards committee and has behavior that is
+more familiar to most programmers.
 
-To define a ``_Float16`` literal, suffix ``f16`` can be appended to the compile-time
-constant declaration. There is no default argument promotion for ``_Float16``; this
-applies to the standard floating types only. As a consequence, for example, an
-explicit cast is required for printing a ``_Float16`` value (there is no string
-format specifier for ``_Float16``).
+Because ``__fp16`` operands are always immediately promoted to ``float``, the
+common real type of ``__fp16`` and ``_Float16`` for the purposes of the usual
+arithmetic conversions is ``float``.
+
+A literal can be given ``_Float16`` type using the suffix ``f16``; for example:
+```
+  3.14f16
+  ```
+
+Because default argument promotion only applies to the standard floating-point
+types, ``_Float16`` values are not promoted to ``double`` when passed as variadic
+or untyped arguments.  As a consequence, some caution must be taken when using
+certain library facilities with ``_Float16``; for example, there is no ``printf`` format
+specifier for ``_Float16``, and (unlike ``float``) it will not be implicitly promoted to
+``double`` when passed to ``printf``, so the programmer must explicitly cast it to
+``double`` before using it with an ``%f`` or similar specifier.
 
 Messages on ``deprecated`` and ``unavailable`` Attributes
 =========================================================

clang/include/clang/Basic/TargetInfo.h

@@ -63,6 +63,7 @@ protected:
   bool HasLegalHalfType; // True if the backend supports operations on the half
                          // LLVM IR type.
   bool HasFloat128;
+  bool HasFloat16;
   unsigned char PointerWidth, PointerAlign;
   unsigned char BoolWidth, BoolAlign;
   unsigned char IntWidth, IntAlign;
@@ -516,6 +517,9 @@ public:
   /// Determine whether the __float128 type is supported on this target.
   virtual bool hasFloat128Type() const { return HasFloat128; }
 
+  /// Determine whether the _Float16 type is supported on this target.
+  virtual bool hasFloat16Type() const { return HasFloat16; }
+
   /// Return the alignment that is suitable for storing any
   /// object with a fundamental alignment requirement.
   unsigned getSuitableAlign() const { return SuitableAlign; }

clang/lib/Basic/TargetInfo.cpp

@@ -34,6 +34,7 @@ TargetInfo::TargetInfo(const llvm::Triple &T) : TargetOpts(), Triple(T) {
   NoAsmVariants = false;
   HasLegalHalfType = false;
   HasFloat128 = false;
+  HasFloat16 = false;
   PointerWidth = PointerAlign = 32;
   BoolWidth = BoolAlign = 8;
   IntWidth = IntAlign = 32;

clang/lib/Basic/Targets/AArch64.cpp

@@ -49,6 +49,7 @@ AArch64TargetInfo::AArch64TargetInfo(const llvm::Triple &Triple,
 
   // All AArch64 implementations support ARMv8 FP, which makes half a legal type.
   HasLegalHalfType = true;
+  HasFloat16 = true;
 
   LongWidth = LongAlign = PointerWidth = PointerAlign = 64;
   MaxVectorAlign = 128;

clang/lib/Basic/Targets/ARM.cpp

@@ -396,6 +396,7 @@ bool ARMTargetInfo::handleTargetFeatures(std::vector<std::string> &Features,
   SoftFloat = SoftFloatABI = false;
   HWDiv = 0;
   DotProd = 0;
+  HasFloat16 = true;
 
   // This does not diagnose illegal cases like having both
   // "+vfpv2" and "+vfpv3" or having "+neon" and "+fp-only-sp".

clang/lib/Basic/Targets/SPIR.h

@@ -47,6 +47,7 @@ public:
     AddrSpaceMap = &SPIRAddrSpaceMap;
     UseAddrSpaceMapMangling = true;
     HasLegalHalfType = true;
+    HasFloat16 = true;
     // Define available target features
     // These must be defined in sorted order!
     NoAsmVariants = true;

clang/lib/Sema/SemaType.cpp

@@ -1441,7 +1441,12 @@ static QualType ConvertDeclSpecToType(TypeProcessingState &state) {
     else
       Result = Context.Int128Ty;
     break;
-  case DeclSpec::TST_float16: Result = Context.Float16Ty; break;
+  case DeclSpec::TST_float16:
+    if (!S.Context.getTargetInfo().hasFloat16Type())
+      S.Diag(DS.getTypeSpecTypeLoc(), diag::err_type_unsupported)
+        << "_Float16";
+    Result = Context.Float16Ty;
+    break;
   case DeclSpec::TST_half:    Result = Context.HalfTy; break;
   case DeclSpec::TST_float:   Result = Context.FloatTy; break;
   case DeclSpec::TST_double:

clang/test/AST/float16.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -std=c++11 -ast-dump %s | FileCheck %s --strict-whitespace
-// RUN: %clang_cc1 -std=c++11 -ast-dump -fnative-half-type %s | FileCheck %s --check-prefix=CHECK-NATIVE --strict-whitespace
+// RUN: %clang_cc1 -std=c++11 -ast-dump -triple aarch64-linux-gnu %s | FileCheck %s --strict-whitespace
+// RUN: %clang_cc1 -std=c++11 -ast-dump -triple aarch64-linux-gnu -fnative-half-type %s | FileCheck %s --check-prefix=CHECK-NATIVE --strict-whitespace
 
 /*  Various contexts where type _Float16 can appear. */
 

clang/test/CodeGenCXX/float16-declarations.cpp

@@ -1,5 +1,4 @@
 // RUN: %clang -std=c++11 --target=aarch64-arm--eabi -S -emit-llvm %s -o - | FileCheck %s  --check-prefix=CHECK --check-prefix=CHECK-AARCH64
-// RUN: %clang -std=c++11 --target=x86_64 -S -emit-llvm %s -o - | FileCheck %s  --check-prefix=CHECK --check-prefix=CHECK-X86
 
 /*  Various contexts where type _Float16 can appear. */
 
@@ -15,7 +14,6 @@ namespace {
 
   _Float16 arr1n[10];
 // CHECK-AARCH64-DAG: @_ZN12_GLOBAL__N_15arr1nE = internal global [10 x half] zeroinitializer, align 2
-// CHECK-X86-DAG:     @_ZN12_GLOBAL__N_15arr1nE = internal global [10 x half] zeroinitializer, align 16
 
   _Float16 arr2n[] = { 1.2, 3.0, 3.e4 };
 // CHECK-DAG: @_ZN12_GLOBAL__N_15arr2nE = internal global [3 x half] [half 0xH3CCD, half 0xH4200, half 0xH7753], align 2
@@ -30,14 +28,12 @@ namespace {
 
 _Float16 f1f;
 // CHECK-AARCH64-DAG: @f1f = dso_local global half 0xH0000, align 2
-// CHECK-X86-DAG: @f1f = dso_local global half 0xH0000, align 2
 
 _Float16 f2f = 32.4;
 // CHECK-DAG: @f2f = dso_local global half 0xH500D, align 2
 
 _Float16 arr1f[10];
 // CHECK-AARCH64-DAG: @arr1f = dso_local global [10 x half] zeroinitializer, align 2
-// CHECK-X86-DAG: @arr1f = dso_local global [10 x half] zeroinitializer, align 16
 
 _Float16 arr2f[] = { -1.2, -3.0, -3.e4 };
 // CHECK-DAG: @arr2f = dso_local global [3 x half] [half 0xHBCCD, half 0xHC200, half 0xHF753], align 2
@@ -137,8 +133,6 @@ int main(void) {
   long double cvtld = f2n;
 //CHECK-AARCh64-DAG: [[H2LD:%[a-z0-9]+]] = fpext half {{%[0-9]+}} to fp128
 //CHECK-AARCh64-DAG: store fp128 [[H2LD]], fp128* %{{.*}}, align 16
-//CHECK-X86-DAG:     [[H2LD:%[a-z0-9]+]] = fpext half {{%[0-9]+}} to x86_fp80
-//CHECK-X86-DAG:     store x86_fp80 [[H2LD]], x86_fp80* %{{.*}}, align 16
 
   _Float16 f2h = 42.0f;
 //CHECK-DAG: store half 0xH5140, half* %{{.*}}, align 2

clang/test/CodeGenCXX/mangle-ms.cpp

@@ -1,5 +1,6 @@
 // RUN: %clang_cc1 -fblocks -emit-llvm %s -o - -triple=i386-pc-win32 -std=c++98 | FileCheck %s
 // RUN: %clang_cc1 -fblocks -emit-llvm %s -o - -triple=x86_64-pc-win32 -std=c++98| FileCheck -check-prefix X64 %s
+// RUN: %clang_cc1 -fblocks -emit-llvm %s -o - -triple=aarch64-pc-win32 -std=c++98 -DARM | FileCheck -check-prefixes=X64,ARM %s
 
 int a;
 // CHECK-DAG: @"?a@@3HA"
@@ -466,10 +467,12 @@ namespace Complex {
 // CHECK-DAG: define dso_local void @"?f@Complex@@YAXU?$_Complex@H@__clang@@@Z"(
 void f(_Complex int) {}
 }
+#ifdef ARM
 namespace Float16 {
-// CHECK-DAG: define dso_local void @"?f@Float16@@YAXU_Float16@__clang@@@Z"(
+// ARM-DAG: define dso_local void @"?f@Float16@@YAXU_Float16@__clang@@@Z"(
 void f(_Float16) {}
 }
+#endif // ARM
 
 namespace PR26029 {
 template <class>

clang/test/Lexer/half-literal.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -pedantic %s
+// RUN: %clang_cc1 -fsyntax-only -verify -pedantic -triple aarch64-linux-gnu %s
 float a = 1.0h; // expected-error{{no matching literal operator for call to 'operator""h' with argument of type 'long double' or 'const char *', and no matching literal operator template}}
 float b = 1.0H; // expected-error{{invalid suffix 'H' on floating constant}}
 

clang/test/Sema/Float16.c

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -triple x86_64-linux-pc %s
+// RUN: %clang_cc1 -fsyntax-only -verify -triple spir-unknown-unknown %s -DHAVE
+// RUN: %clang_cc1 -fsyntax-only -verify -triple armv7a-linux-gnu %s -DHAVE
+// RUN: %clang_cc1 -fsyntax-only -verify -triple aarch64-linux-gnu %s -DHAVE
+
+#ifdef HAVE
+// expected-no-diagnostics
+#else
+// expected-error@+2{{_Float16 is not supported on this target}}
+#endif // HAVE
+_Float16 f;