[APFloat] Add APFloat support for E8M0 type

This patch adds an APFloat type for unsigned E8M0 format.
This format is used for representing the "scale-format"
in the MX specification:
https://www.opencompute.org/documents/ocp-microscaling-formats-mx-v1-0-spec-final-pdf

This format does not support {Inf, denorms, zeroes}.
Like FP32, this format's exponents are 8-bits (all bits here)
and the bias value is 127. However, it differs from IEEE-FP32
in that the minExponent is -127 (instead of -126).
There are updates done in the APFloat utility functions
to handle these constraints for this format.

* The bias calculation is different and convertIEEE* APIs
  are updated to handle this.
* Since there are no significand bits, the
  isSignificandAll{Zeroes/Ones} methods are updated accordingly.
* Although the format does not have any precision, the precision
  bit in the fltSemantics is set to 1 for consistency with
  APFloat's internal representation.
* Many utility functions are updated to handle the fact that this
  format does not support Zero.
* Provide a separate initFromAPInt() implementation to
  handle the quirks of the format.
* Add specific tests to verify the range of values for this format.

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

Author: durga4github

llvm/include/llvm/ADT/APFloat.h

@@ -195,6 +195,12 @@ struct APFloatBase {
     // improved range compared to half (16-bit) formats, at (potentially)
     // greater throughput than single precision (32-bit) formats.
     S_FloatTF32,
+    // 8-bit floating point number with (all the) 8 bits for the exponent
+    // like in FP32. There are no zeroes, no infinities, and no denormal values.
+    // NaN is represented with all bits set to 1. Bias is 127.
+    // This represents the scale data type in the MX specification from
+    // https://www.opencompute.org/documents/ocp-microscaling-formats-mx-v1-0-spec-final-pdf
+    S_Float8E8M0FN,
     // 6-bit floating point number with bit layout S1E3M2. 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
@@ -229,6 +235,7 @@ struct APFloatBase {
   static const fltSemantics &Float8E4M3B11FNUZ() LLVM_READNONE;
   static const fltSemantics &Float8E3M4() LLVM_READNONE;
   static const fltSemantics &FloatTF32() LLVM_READNONE;
+  static const fltSemantics &Float8E8M0FN() LLVM_READNONE;
   static const fltSemantics &Float6E3M2FN() LLVM_READNONE;
   static const fltSemantics &Float6E2M3FN() LLVM_READNONE;
   static const fltSemantics &Float4E2M1FN() LLVM_READNONE;
@@ -591,6 +598,7 @@ class IEEEFloat final : public APFloatBase {
   unsigned int significandLSB() const;
   unsigned int significandMSB() const;
   void zeroSignificand();
+  unsigned int getNumHighBits() const;
   /// Return true if the significand excluding the integral bit is all ones.
   bool isSignificandAllOnes() const;
   bool isSignificandAllOnesExceptLSB() const;
@@ -652,6 +660,7 @@ class IEEEFloat final : public APFloatBase {
   APInt convertFloat8E4M3B11FNUZAPFloatToAPInt() const;
   APInt convertFloat8E3M4APFloatToAPInt() const;
   APInt convertFloatTF32APFloatToAPInt() const;
+  APInt convertFloat8E8M0FNAPFloatToAPInt() const;
   APInt convertFloat6E3M2FNAPFloatToAPInt() const;
   APInt convertFloat6E2M3FNAPFloatToAPInt() const;
   APInt convertFloat4E2M1FNAPFloatToAPInt() const;
@@ -672,6 +681,7 @@ class IEEEFloat final : public APFloatBase {
   void initFromFloat8E4M3B11FNUZAPInt(const APInt &api);
   void initFromFloat8E3M4APInt(const APInt &api);
   void initFromFloatTF32APInt(const APInt &api);
+  void initFromFloat8E8M0FNAPInt(const APInt &api);
   void initFromFloat6E3M2FNAPInt(const APInt &api);
   void initFromFloat6E2M3FNAPInt(const APInt &api);
   void initFromFloat4E2M1FNAPInt(const APInt &api);
@@ -1079,6 +1089,9 @@ class APFloat : public APFloatBase {
   /// \param Semantics - type float semantics
   static APFloat getAllOnesValue(const fltSemantics &Semantics);
 
+  /// Returns true if the given semantics supports either NaN or Infinity.
+  ///
+  /// \param Sem - type float semantics
   static bool hasNanOrInf(const fltSemantics &Sem) {
     switch (SemanticsToEnum(Sem)) {
     default:
@@ -1091,6 +1104,28 @@ class APFloat : public APFloatBase {
     }
   }
 
+  /// Returns true if the given semantics can represent Zero.
+  ///
+  /// \param Sem - type float semantics
+  static bool hasZero(const fltSemantics &Sem) {
+    return &Sem != &Float8E8M0FN();
+  }
+
+  /// Returns true if the given semantics has actual significand.
+  ///
+  /// \param Sem - type float semantics
+  static bool hasSignificand(const fltSemantics &Sem) {
+    return &Sem != &Float8E8M0FN();
+  }
+
+  /// Returns true if the given semantics has only exponent
+  /// and no significand.
+  ///
+  /// \param Sem - type float semantics
+  static bool hasExponentOnly(const fltSemantics &Sem) {
+    return !hasSignificand(Sem);
+  }
+
   /// Used to insert APFloat objects, or objects that contain APFloat objects,
   /// into FoldingSets.
   void Profile(FoldingSetNodeID &NID) const;