[ARM] Allow passing/returning of __fp16 arguments

[olista01]

Summary

The ACLE (ARM C Language Extensions) 2.0 allows the __fp16 type to be
used as a functon argument or return type (ACLE 1.1 did not).

The current public release of the AAPCS (2.09) states that __fp16 values
should be converted to single-precision before being passed or returned,
but AAPCS 2.10 (to be released shortly) changes this, so that they are
passed in the least-significant 16 bits of either a GPR (for base AAPCS)
or a single-precision register (for AAPCS-VFP). This does not change how
arguments are passed if they get passed on the stack.

This patch brings clang up to compliance with the latest versions of
both of these specs.

We can now set the ARM_FP16_ARGS ACLE predefine, and we have always
been able to set the ARM_FP16_FORMAT_IEEE predefine (we do not support
the alternative format).

[jmolloy]

lib/CodeGen/TargetInfo.cpp

4717

"Top 16 bits"?

4884

"Top 16 bits" ?

[olista01]

Fixed typo

[rengolin]

lib/Driver/Tools.cpp

4296–4302

This could use the TargetParser to match the arch. If it's not invalid, it's ARM. Or a switch, maybe.

[olista01]

Use a switch statement to check the target architecture.

[olista01]

Ping?

[jmolloy]

This looks alright to me, but I'd like to wait for someone more familiar with Clang to approve it.

[rengolin]

LGTM, thanks!

[olista01]

Thanks, committed as r246755.

[olista01]

Don't make this change for OpenCL, which handles the half type natively.

[rengolin]

I don't like the idea of making it language-specific, but I don't have a better idea. I don't know if there are support flags in Clang's languages like we have for back-ends. Anyway, this should fix the current problem. Maybe adding a comment to that effect on the usage of !CL?

LGTM.

[olista01]

Committed as r246764, with an additional comment explaining why we don't do this for OpenCL.

lib/Basic/Targets.cpp

void getTargetDefines(const LangOptions &Opts,
 
     // ACLE 6.5.1 Hardware Floating Point
     if (HW_FP)
       Builder.defineMacro("__ARM_FP", "0x" + llvm::utohexstr(HW_FP));
 
     // ACLE predefines.
     Builder.defineMacro("__ARM_ACLE", "200");
 
+    // FP16 support (we currently only support IEEE format).
+    Builder.defineMacro("__ARM_FP16_FORMAT_IEEE", "1");
+    Builder.defineMacro("__ARM_FP16_ARGS", "1");
+
     // Subtarget options.
 
     // FIXME: It's more complicated than this and we don't really support
     // interworking.
     // Windows on ARM does not "support" interworking
     if (5 <= ArchVersion && ArchVersion <= 8 && !getTriple().isOSWindows())
       Builder.defineMacro("__THUMB_INTERWORK__");
 

lib/CodeGen/TargetInfo.cpp

if (isIllegalVectorType(Ty)) {
     if (Size == 128) {
       llvm::Type *ResType = llvm::VectorType::get(
           llvm::Type::getInt32Ty(getVMContext()), 4);
       return ABIArgInfo::getDirect(ResType);
     }
     return ABIArgInfo::getIndirect(0, /*ByVal=*/false);
   }
 
+  // __fp16 gets passed as if it were an int or float, but with the top 16 bits
+  // unspecified.
+  if (Ty->isHalfType() && !getContext().getLangOpts().OpenCL) {
+    llvm::Type *ResType = IsEffectivelyAAPCS_VFP ?
+      llvm::Type::getFloatTy(getVMContext()) :
+      llvm::Type::getInt32Ty(getVMContext());
+    return ABIArgInfo::getDirect(ResType);
+  }
+
   if (!isAggregateTypeForABI(Ty)) {
     // Treat an enum type as its underlying type.
     if (const EnumType *EnumTy = Ty->getAs<EnumType>()) {
       Ty = EnumTy->getDecl()->getIntegerType();
     }
 
     return (Ty->isPromotableIntegerType() ? ABIArgInfo::getExtend()
                                           : ABIArgInfo::getDirect());
ABIArgInfo ARMABIInfo::classifyReturnType(QualType RetTy,
   if (RetTy->isVoidType())
     return ABIArgInfo::getIgnore();
 
   // Large vector types should be returned via memory.
   if (RetTy->isVectorType() && getContext().getTypeSize(RetTy) > 128) {
     return ABIArgInfo::getIndirect(0);
   }
 
+  // __fp16 gets returned as if it were an int or float, but with the top 16
+  // bits unspecified.
+  if (RetTy->isHalfType() && !getContext().getLangOpts().OpenCL) {
+    llvm::Type *ResType = IsEffectivelyAAPCS_VFP ?
+      llvm::Type::getFloatTy(getVMContext()) :
+      llvm::Type::getInt32Ty(getVMContext());
+    return ABIArgInfo::getDirect(ResType);
+  }
+
   if (!isAggregateTypeForABI(RetTy)) {
     // Treat an enum type as its underlying type.
     if (const EnumType *EnumTy = RetTy->getAs<EnumType>())
       RetTy = EnumTy->getDecl()->getIntegerType();
 
     return RetTy->isPromotableIntegerType() ? ABIArgInfo::getExtend()
                                             : ABIArgInfo::getDirect();
   }

lib/Driver/Tools.cpp

if (Args.hasArg(options::OPT_mstack_probe_size)) {
     StringRef Size = Args.getLastArgValue(options::OPT_mstack_probe_size);
 
     if (!Size.empty())
       CmdArgs.push_back(Args.MakeArgString("-mstack-probe-size=" + Size));
     else
       CmdArgs.push_back("-mstack-probe-size=0");
   }
 
-  if (getToolChain().getArch() == llvm::Triple::aarch64 ||
-      getToolChain().getArch() == llvm::Triple::aarch64_be)
+  switch (getToolChain().getArch()) {
+  case llvm::Triple::aarch64:
+  case llvm::Triple::aarch64_be:
+  case llvm::Triple::arm:
+  case llvm::Triple::armeb:
+  case llvm::Triple::thumb:
+  case llvm::Triple::thumbeb:
     CmdArgs.push_back("-fallow-half-arguments-and-returns");
+    break;
+
+  default:
+    break;
+  }
 
   if (Arg *A = Args.getLastArg(options::OPT_mrestrict_it,
                                options::OPT_mno_restrict_it)) {
     if (A->getOption().matches(options::OPT_mrestrict_it)) {
       CmdArgs.push_back("-backend-option");
       CmdArgs.push_back("-arm-restrict-it");
     } else {
       CmdArgs.push_back("-backend-option");

test/CodeGen/arm-fp16-arguments.c

+// RUN: %clang_cc1 -triple armv7a--none-eabi -target-abi aapcs -mfloat-abi soft -fallow-half-arguments-and-returns -emit-llvm -o - -O1 %s | FileCheck %s --check-prefix=CHECK --check-prefix=SOFT
+// RUN: %clang_cc1 -triple armv7a--none-eabi -target-abi aapcs -mfloat-abi hard -fallow-half-arguments-and-returns -emit-llvm -o - -O1 %s | FileCheck %s --check-prefix=CHECK --check-prefix=HARD
+
+__fp16 g;
+
+void t1(__fp16 a) { g = a; }
+// SOFT: define void @t1(i32 [[PARAM:%.*]])
+// SOFT: [[TRUNC:%.*]] = trunc i32 [[PARAM]] to i16
+// HARD: define arm_aapcs_vfpcc void @t1(float [[PARAM:%.*]])
+// HARD: [[BITCAST:%.*]] = bitcast float [[PARAM]] to i32
+// HARD: [[TRUNC:%.*]] = trunc i32 [[BITCAST]] to i16
+// CHECK: store i16 [[TRUNC]], i16* bitcast (half* @g to i16*)
+
+__fp16 t2() { return g; }
+// SOFT: define i32 @t2()
+// HARD: define arm_aapcs_vfpcc float @t2()
+// CHECK: [[LOAD:%.*]] = load i16, i16* bitcast (half* @g to i16*)
+// CHECK: [[ZEXT:%.*]] = zext i16 [[LOAD]] to i32
+// SOFT: ret i32 [[ZEXT]]
+// HARD: [[BITCAST:%.*]] = bitcast i32 [[ZEXT]] to float
+// HARD: ret float [[BITCAST]]

test/Preprocessor/arm-acle-6.5.c

 // RUN: %clang -target arm-eabi -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-DEFAULT
 
-// CHECK-DEFAULT-NOT: __ARM_FP
+// CHECK-DEFAULT-NOT: __ARM_FP 0x
 
 // RUN: %clang -target arm-eabi -mfpu=vfp -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-SP-DP
 // RUN: %clang -target arm-eabi -mfpu=vfp3 -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-SP-DP
 // RUN: %clang -target arm-eabi -mfpu=vfp3-d16 -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-SP-DP
 // RUN: %clang -target arm-eabi -mfpu=neon -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-SP-DP
 // RUN: %clang -target arm-eabi -mfpu=vfp3 -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-SP-DP
 // RUN: %clang -target armv7-eabi -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-SP-DP
 

test/Preprocessor/arm-target-features.c

 // RUN: %clang -target armv8a-none-linux-gnu -x c -E -dM %s -o - | FileCheck %s
 // CHECK: __ARMEL__ 1
 // CHECK: __ARM_ARCH 8
 // CHECK: __ARM_ARCH_8A__ 1
 // CHECK: __ARM_FEATURE_CRC32 1
 // CHECK: __ARM_FEATURE_DIRECTED_ROUNDING 1
 // CHECK: __ARM_FEATURE_NUMERIC_MAXMIN 1
+// CHECK: __ARM_FP16_ARGS 1
+// CHECK: __ARM_FP16_FORMAT_IEEE 1
 
 // RUN: %clang -target armv7a-none-linux-gnu -x c -E -dM %s -o - | FileCheck --check-prefix=CHECK-V7 %s
 // CHECK-V7: __ARMEL__ 1
 // CHECK-V7: __ARM_ARCH 7
 // CHECK-V7: __ARM_ARCH_7A__ 1
 // CHECK-V7-NOT: __ARM_FEATURE_CRC32
 // CHECK-V7-NOT: __ARM_FEATURE_NUMERIC_MAXMIN                                   
 // CHECK-V7-NOT: __ARM_FEATURE_DIRECTED_ROUNDING