[WebAssembly] Add half-precision feature

This currently only defines a constant, but in the future will be used
to gate builtins for experimenting and prototyping half-precision proposal
(https://github.com/WebAssembly/half-precision).

Author: brendandahl

clang/include/clang/Driver/Options.td

@@ -4876,6 +4876,8 @@ def msimd128 : Flag<["-"], "msimd128">, Group<m_wasm_Features_Group>;
 def mno_simd128 : Flag<["-"], "mno-simd128">, Group<m_wasm_Features_Group>;
 def mrelaxed_simd : Flag<["-"], "mrelaxed-simd">, Group<m_wasm_Features_Group>;
 def mno_relaxed_simd : Flag<["-"], "mno-relaxed-simd">, Group<m_wasm_Features_Group>;
+def mhalf_precision : Flag<["-"], "mhalf-precision">, Group<m_wasm_Features_Group>;
+def mno_half_precision : Flag<["-"], "mno-half-precision">, Group<m_wasm_Features_Group>;
 def mnontrapping_fptoint : Flag<["-"], "mnontrapping-fptoint">, Group<m_wasm_Features_Group>;
 def mno_nontrapping_fptoint : Flag<["-"], "mno-nontrapping-fptoint">, Group<m_wasm_Features_Group>;
 def msign_ext : Flag<["-"], "msign-ext">, Group<m_wasm_Features_Group>;

clang/lib/Basic/Targets/WebAssembly.cpp

@@ -47,6 +47,7 @@ bool WebAssemblyTargetInfo::hasFeature(StringRef Feature) const {
   return llvm::StringSwitch<bool>(Feature)
       .Case("simd128", SIMDLevel >= SIMD128)
       .Case("relaxed-simd", SIMDLevel >= RelaxedSIMD)
+      .Case("half-precision", HasHalfPrecision)
       .Case("nontrapping-fptoint", HasNontrappingFPToInt)
       .Case("sign-ext", HasSignExt)
       .Case("exception-handling", HasExceptionHandling)
@@ -156,6 +157,7 @@ bool WebAssemblyTargetInfo::initFeatureMap(
     Features["reference-types"] = true;
     Features["sign-ext"] = true;
     Features["tail-call"] = true;
+    Features["half-precision"] = true;
     setSIMDLevel(Features, SIMD128, true);
   } else if (CPU == "generic") {
     Features["mutable-globals"] = true;
@@ -216,6 +218,15 @@ bool WebAssemblyTargetInfo::handleTargetFeatures(
       HasBulkMemory = false;
       continue;
     }
+    if (Feature == "+half-precision") {
+      SIMDLevel = std::max(SIMDLevel, SIMD128);
+      HasHalfPrecision = true;
+      continue;
+    }
+    if (Feature == "-half-precision") {
+      HasHalfPrecision = false;
+      continue;
+    }
     if (Feature == "+atomics") {
       HasAtomics = true;
       continue;

clang/lib/Basic/Targets/WebAssembly.h

@@ -64,6 +64,7 @@ class LLVM_LIBRARY_VISIBILITY WebAssemblyTargetInfo : public TargetInfo {
   bool HasReferenceTypes = false;
   bool HasExtendedConst = false;
   bool HasMultiMemory = false;
+  bool HasHalfPrecision = false;
 
   std::string ABI;
 

llvm/lib/Target/WebAssembly/WebAssembly.td

@@ -28,6 +28,9 @@ def FeatureSIMD128 : SubtargetFeature<"simd128", "SIMDLevel", "SIMD128",
 def FeatureRelaxedSIMD : SubtargetFeature<"relaxed-simd", "SIMDLevel", "RelaxedSIMD",
                                       "Enable relaxed-simd instructions">;
 
+def FeatureHalfPrecision : SubtargetFeature<"half-precision", "HasHalfPrecision", "true",
+                                            "Enable half precision instructions">;
+
 def FeatureAtomics : SubtargetFeature<"atomics", "HasAtomics", "true",
                                       "Enable Atomics">;
 

llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.td

@@ -30,6 +30,10 @@ def HasRelaxedSIMD :
     Predicate<"Subtarget->hasRelaxedSIMD()">,
     AssemblerPredicate<(all_of FeatureRelaxedSIMD), "relaxed-simd">;
 
+def HasHalfPrecision :
+    Predicate<"Subtarget->hasHalfPrecision()">,
+    AssemblerPredicate<(all_of FeatureHalfPrecision), "half-precision">;
+
 def HasAtomics :
     Predicate<"Subtarget->hasAtomics()">,
     AssemblerPredicate<(all_of FeatureAtomics), "atomics">;

llvm/lib/Target/WebAssembly/WebAssemblySubtarget.h

@@ -50,6 +50,7 @@ class WebAssemblySubtarget final : public WebAssemblyGenSubtargetInfo {
   bool HasReferenceTypes = false;
   bool HasExtendedConst = false;
   bool HasMultiMemory = false;
+  bool HasHalfPrecision = false;
 
   /// What processor and OS we're targeting.
   Triple TargetTriple;
@@ -93,6 +94,7 @@ class WebAssemblySubtarget final : public WebAssemblyGenSubtargetInfo {
   bool hasAddr64() const { return TargetTriple.isArch64Bit(); }
   bool hasSIMD128() const { return SIMDLevel >= SIMD128; }
   bool hasRelaxedSIMD() const { return SIMDLevel >= RelaxedSIMD; }
+  bool hasHalfPrecision() const { return HasHalfPrecision; }
   bool hasAtomics() const { return HasAtomics; }
   bool hasNontrappingFPToInt() const { return HasNontrappingFPToInt; }
   bool hasSignExt() const { return HasSignExt; }