[APFloat] Add APFloat support for FP4 data type

This patch adds APFloat type support for the E2M1
FP4 datatype. The definitions for this format are
detailed in section 5.3.3 of the OCP specification,
which can be accessed here:
https://www.opencompute.org/documents/ocp-microscaling-formats-mx-v1-0-spec-final-pdf

Signed-off-by: Durgadoss R <[email protected]>

Author: durga4github

clang/lib/AST/MicrosoftMangle.cpp

@@ -901,6 +901,7 @@ void MicrosoftCXXNameMangler::mangleFloat(llvm::APFloat Number) {
   case APFloat::S_FloatTF32:
   case APFloat::S_Float6E3M2FN:
   case APFloat::S_Float6E2M3FN:
+  case APFloat::S_Float4E2M1FN:
     llvm_unreachable("Tried to mangle unexpected APFloat semantics");
   }
 

llvm/include/llvm/ADT/APFloat.h

@@ -197,6 +197,10 @@ struct APFloatBase {
     // types, there are no infinity or NaN values. The format is detailed in
     // https://www.opencompute.org/documents/ocp-microscaling-formats-mx-v1-0-spec-final-pdf
     S_Float6E2M3FN,
+    // 4-bit floating point number with bit layout S1E2M1. Unlike IEEE-754
+    // types, there are no infinity or NaN values. The format is detailed in
+    // https://www.opencompute.org/documents/ocp-microscaling-formats-mx-v1-0-spec-final-pdf
+    S_Float4E2M1FN,
 
     S_x87DoubleExtended,
     S_MaxSemantics = S_x87DoubleExtended,
@@ -219,6 +223,7 @@ struct APFloatBase {
   static const fltSemantics &FloatTF32() LLVM_READNONE;
   static const fltSemantics &Float6E3M2FN() LLVM_READNONE;
   static const fltSemantics &Float6E2M3FN() LLVM_READNONE;
+  static const fltSemantics &Float4E2M1FN() LLVM_READNONE;
   static const fltSemantics &x87DoubleExtended() LLVM_READNONE;
 
   /// A Pseudo fltsemantic used to construct APFloats that cannot conflict with
@@ -639,6 +644,7 @@ class IEEEFloat final : public APFloatBase {
   APInt convertFloatTF32APFloatToAPInt() const;
   APInt convertFloat6E3M2FNAPFloatToAPInt() const;
   APInt convertFloat6E2M3FNAPFloatToAPInt() const;
+  APInt convertFloat4E2M1FNAPFloatToAPInt() const;
   void initFromAPInt(const fltSemantics *Sem, const APInt &api);
   template <const fltSemantics &S> void initFromIEEEAPInt(const APInt &api);
   void initFromHalfAPInt(const APInt &api);
@@ -656,6 +662,7 @@ class IEEEFloat final : public APFloatBase {
   void initFromFloatTF32APInt(const APInt &api);
   void initFromFloat6E3M2FNAPInt(const APInt &api);
   void initFromFloat6E2M3FNAPInt(const APInt &api);
+  void initFromFloat4E2M1FNAPInt(const APInt &api);
 
   void assign(const IEEEFloat &);
   void copySignificand(const IEEEFloat &);
@@ -1067,6 +1074,7 @@ class APFloat : public APFloatBase {
     // Below Semantics do not support {NaN or Inf}
     case APFloat::S_Float6E3M2FN:
     case APFloat::S_Float6E2M3FN:
+    case APFloat::S_Float4E2M1FN:
       return false;
     }
   }

llvm/lib/Support/APFloat.cpp

@@ -69,8 +69,8 @@ enum class fltNonfiniteBehavior {
   // encodings do not distinguish between signalling and quiet NaN.
   NanOnly,
 
-  // This behavior is present in Float6E3M2FN and Float6E2M3FN types,
-  // which do not support Inf or NaN values.
+  // This behavior is present in Float6E3M2FN, Float6E2M3FN and
+  // Float4E2M1FN types, which do not support Inf or NaN values.
   FiniteOnly,
 };
 
@@ -147,6 +147,8 @@ static constexpr fltSemantics semFloat6E3M2FN = {
     4, -2, 3, 6, fltNonfiniteBehavior::FiniteOnly};
 static constexpr fltSemantics semFloat6E2M3FN = {
     2, 0, 4, 6, fltNonfiniteBehavior::FiniteOnly};
+static constexpr fltSemantics semFloat4E2M1FN = {
+    2, 0, 2, 4, fltNonfiniteBehavior::FiniteOnly};
 static constexpr fltSemantics semX87DoubleExtended = {16383, -16382, 64, 80};
 static constexpr fltSemantics semBogus = {0, 0, 0, 0};
 
@@ -218,6 +220,8 @@ const llvm::fltSemantics &APFloatBase::EnumToSemantics(Semantics S) {
     return Float6E3M2FN();
   case S_Float6E2M3FN:
     return Float6E2M3FN();
+  case S_Float4E2M1FN:
+    return Float4E2M1FN();
   case S_x87DoubleExtended:
     return x87DoubleExtended();
   }
@@ -254,6 +258,8 @@ APFloatBase::SemanticsToEnum(const llvm::fltSemantics &Sem) {
     return S_Float6E3M2FN;
   else if (&Sem == &llvm::APFloat::Float6E2M3FN())
     return S_Float6E2M3FN;
+  else if (&Sem == &llvm::APFloat::Float4E2M1FN())
+    return S_Float4E2M1FN;
   else if (&Sem == &llvm::APFloat::x87DoubleExtended())
     return S_x87DoubleExtended;
   else
@@ -278,6 +284,7 @@ const fltSemantics &APFloatBase::Float8E4M3B11FNUZ() {
 const fltSemantics &APFloatBase::FloatTF32() { return semFloatTF32; }
 const fltSemantics &APFloatBase::Float6E3M2FN() { return semFloat6E3M2FN; }
 const fltSemantics &APFloatBase::Float6E2M3FN() { return semFloat6E2M3FN; }
+const fltSemantics &APFloatBase::Float4E2M1FN() { return semFloat4E2M1FN; }
 const fltSemantics &APFloatBase::x87DoubleExtended() {
   return semX87DoubleExtended;
 }
@@ -3640,6 +3647,11 @@ APInt IEEEFloat::convertFloat6E2M3FNAPFloatToAPInt() const {
   return convertIEEEFloatToAPInt<semFloat6E2M3FN>();
 }
 
+APInt IEEEFloat::convertFloat4E2M1FNAPFloatToAPInt() const {
+  assert(partCount() == 1);
+  return convertIEEEFloatToAPInt<semFloat4E2M1FN>();
+}
+
 // This function creates an APInt that is just a bit map of the floating
 // point constant as it would appear in memory.  It is not a conversion,
 // and treating the result as a normal integer is unlikely to be useful.
@@ -3687,6 +3699,9 @@ APInt IEEEFloat::bitcastToAPInt() const {
   if (semantics == (const llvm::fltSemantics *)&semFloat6E2M3FN)
     return convertFloat6E2M3FNAPFloatToAPInt();
 
+  if (semantics == (const llvm::fltSemantics *)&semFloat4E2M1FN)
+    return convertFloat4E2M1FNAPFloatToAPInt();
+
   assert(semantics == (const llvm::fltSemantics*)&semX87DoubleExtended &&
          "unknown format!");
   return convertF80LongDoubleAPFloatToAPInt();
@@ -3911,6 +3926,10 @@ void IEEEFloat::initFromFloat6E2M3FNAPInt(const APInt &api) {
   initFromIEEEAPInt<semFloat6E2M3FN>(api);
 }
 
+void IEEEFloat::initFromFloat4E2M1FNAPInt(const APInt &api) {
+  initFromIEEEAPInt<semFloat4E2M1FN>(api);
+}
+
 /// Treat api as containing the bits of a floating point number.
 void IEEEFloat::initFromAPInt(const fltSemantics *Sem, const APInt &api) {
   assert(api.getBitWidth() == Sem->sizeInBits);
@@ -3944,6 +3963,8 @@ void IEEEFloat::initFromAPInt(const fltSemantics *Sem, const APInt &api) {
     return initFromFloat6E3M2FNAPInt(api);
   if (Sem == &semFloat6E2M3FN)
     return initFromFloat6E2M3FNAPInt(api);
+  if (Sem == &semFloat4E2M1FN)
+    return initFromFloat4E2M1FNAPInt(api);
 
   llvm_unreachable(nullptr);
 }