[APFloat] Add APFloat support for E8M0 type (llvm#107127)

This patch adds an APFloat type for unsigned E8M0 format. This format is
used for representing the "scale-format" in the MX specification:
(section 5.4)

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 <durgadossr@nvidia.com>

llvm/include/llvm/ADT/APFloat.h

@@ -195,6 +195,13 @@ 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.
+    // This format has unsigned representation only. (U -> Unsigned only).
+    // NaN is represented with all bits set to 1. Bias is 127.
+    // This format 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_Float8E8M0FNU,
     // 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 +236,7 @@ struct APFloatBase {
   static const fltSemantics &Float8E4M3B11FNUZ() LLVM_READNONE;
   static const fltSemantics &Float8E3M4() LLVM_READNONE;
   static const fltSemantics &FloatTF32() LLVM_READNONE;
+  static const fltSemantics &Float8E8M0FNU() LLVM_READNONE;
   static const fltSemantics &Float6E3M2FN() LLVM_READNONE;
   static const fltSemantics &Float6E2M3FN() LLVM_READNONE;
   static const fltSemantics &Float4E2M1FN() LLVM_READNONE;
@@ -581,7 +589,8 @@ class IEEEFloat final : public APFloatBase {
   integerPart addSignificand(const IEEEFloat &);
   integerPart subtractSignificand(const IEEEFloat &, integerPart);
   lostFraction addOrSubtractSignificand(const IEEEFloat &, bool subtract);
-  lostFraction multiplySignificand(const IEEEFloat &, IEEEFloat);
+  lostFraction multiplySignificand(const IEEEFloat &, IEEEFloat,
+                                   bool ignoreAddend = false);
   lostFraction multiplySignificand(const IEEEFloat&);
   lostFraction divideSignificand(const IEEEFloat &);
   void incrementSignificand();
@@ -591,6 +600,7 @@ 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 +662,7 @@ class IEEEFloat final : public APFloatBase {
   APInt convertFloat8E4M3B11FNUZAPFloatToAPInt() const;
   APInt convertFloat8E3M4APFloatToAPInt() const;
   APInt convertFloatTF32APFloatToAPInt() const;
+  APInt convertFloat8E8M0FNUAPFloatToAPInt() const;
   APInt convertFloat6E3M2FNAPFloatToAPInt() const;
   APInt convertFloat6E2M3FNAPFloatToAPInt() const;
   APInt convertFloat4E2M1FNAPFloatToAPInt() const;
@@ -672,6 +683,7 @@ class IEEEFloat final : public APFloatBase {
   void initFromFloat8E4M3B11FNUZAPInt(const APInt &api);
   void initFromFloat8E3M4APInt(const APInt &api);
   void initFromFloatTF32APInt(const APInt &api);
+  void initFromFloat8E8M0FNUAPInt(const APInt &api);
   void initFromFloat6E3M2FNAPInt(const APInt &api);
   void initFromFloat6E2M3FNAPInt(const APInt &api);
   void initFromFloat4E2M1FNAPInt(const APInt &api);
@@ -1079,6 +1091,9 @@ 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 +1106,13 @@ class APFloat : public APFloatBase {
     }
   }
 
+  /// Returns true if the given semantics has actual significand.
+  ///
+  /// \param Sem - type float semantics
+  static bool hasSignificand(const fltSemantics &Sem) {
+    return &Sem != &Float8E8M0FNU();
+  }
+
   /// Used to insert APFloat objects, or objects that contain APFloat objects,
   /// into FoldingSets.
   void Profile(FoldingSetNodeID &NID) const;