APFloat: Add getExactLog2

[foad]

llvm/include/llvm/ADT/APFloat.h

495

"getExactLog2" or similar would make more sense to me.

[arsenm]

Rename

[jcranmer-intel]

llvm/lib/Support/APFloat.cpp

4306

I would like to see a test that exercises the case of two different parts each having popcount = 1. Testing next(false) on the existing subnormal checks should do the trick, I think.

[arsenm]

llvm/lib/Support/APFloat.cpp

4306

if (PartCount == 2) {
  assert(!(llvm::popcount(Parts[0]) == 1 && llvm::popcount(Parts[0]) == 1));
}

already fires on the existing tests

[foad]

llvm/include/llvm/ADT/APFloat.h

493

Do you intentionally ignore the sign bit? Should at least document that, since most people would not call -8 a power of two.

[arsenm]

Fail on negative

[arsenm]

0b57c3a7b8dc7407174a5ef5644e493493c8a8ce

llvm/include/llvm/ADT/APFloat.h

public:
   /// 1.01E-2              4             1       1.01E-2
   void toString(SmallVectorImpl<char> &Str, unsigned FormatPrecision = 0,
                 unsigned FormatMaxPadding = 3, bool TruncateZero = true) const;
 
   /// If this value has an exact multiplicative inverse, store it in inv and
   /// return true.
   bool getExactInverse(APFloat *inv) const;
 
+  // If this is an exact power of two, return the exponent. If it's not an exact
+  // power of 2, return INT_MIN
+  LLVM_READONLY
+  int getExactLog2() const;
+
   /// Returns the exponent of the internal representation of the APFloat.
   ///
   /// Because the radix of APFloat is 2, this is equivalent to floor(log2(x)).
   /// For special APFloat values, this returns special error codes:
   ///
   ///   NaN -> \c IEK_NaN
   ///   0   -> \c IEK_Zero
   ///   Inf -> \c IEK_Inf
public:
   bool isLargest() const;
   bool isInteger() const;
 
   void toString(SmallVectorImpl<char> &Str, unsigned FormatPrecision,
                 unsigned FormatMaxPadding, bool TruncateZero = true) const;
 
   bool getExactInverse(APFloat *inv) const;
 
+  LLVM_READONLY
+  int getExactLog2() const;
+
   friend DoubleAPFloat scalbn(const DoubleAPFloat &X, int Exp, roundingMode);
   friend DoubleAPFloat frexp(const DoubleAPFloat &X, int &Exp, roundingMode);
   friend hash_code hash_value(const DoubleAPFloat &Arg);
 };
 
 hash_code hash_value(const DoubleAPFloat &Arg);
 
 } // End detail namespace
public:
 
   void print(raw_ostream &) const;
   void dump() const;
 
   bool getExactInverse(APFloat *inv) const {
     APFLOAT_DISPATCH_ON_SEMANTICS(getExactInverse(inv));
   }
 
+  LLVM_READONLY
+  int getExactLog2() const {
+    APFLOAT_DISPATCH_ON_SEMANTICS(getExactLog2());
+  }
+
   friend hash_code hash_value(const APFloat &Arg);
   friend int ilogb(const APFloat &Arg) { return ilogb(Arg.getIEEE()); }
   friend APFloat scalbn(APFloat X, int Exp, roundingMode RM);
   friend APFloat frexp(const APFloat &X, int &Exp, roundingMode RM);
   friend IEEEFloat;
   friend DoubleAPFloat;
 };
 

llvm/lib/Support/APFloat.cpp

assert(reciprocal.isFiniteNonZero() &&
          reciprocal.significandLSB() == reciprocal.semantics->precision - 1);
 
   if (inv)
     *inv = APFloat(reciprocal, *semantics);
 
   return true;
 }
 
+int IEEEFloat::getExactLog2() const {
+  if (!isFinite() || isZero() || isNegative())
+    return INT_MIN;
+
+  const integerPart *Parts = significandParts();
+  const int PartCount = partCountForBits(semantics->precision);
+
+  int PopCount = 0;
+  for (int i = 0; i < PartCount; ++i) {
+    PopCount += llvm::popcount(Parts[i]);
+    if (PopCount > 1)
+      return INT_MIN;
+  }
+
+  if (exponent != semantics->minExponent)
+    return exponent;
+
+  int CountrParts = 0;
+  for (int i = 0; i < PartCount;
+       ++i, CountrParts += APInt::APINT_BITS_PER_WORD) {
+    if (Parts[i] != 0) {
+      return exponent - semantics->precision + CountrParts +
+             llvm::countr_zero(Parts[i]) + 1;
+    }
+  }
+
+  llvm_unreachable("didn't find the set bit");
+}
+
 bool IEEEFloat::isSignaling() const {
   if (!isNaN())
     return false;
   if (semantics->nonFiniteBehavior == fltNonfiniteBehavior::NanOnly)
     return false;
 
   // IEEE-754R 2008 6.2.1: A signaling NaN bit string should be encoded with the
   // first bit of the trailing significand being 0.
bool DoubleAPFloat::getExactInverse(APFloat *inv) const {
   if (!inv)
     return Tmp.getExactInverse(nullptr);
   APFloat Inv(semPPCDoubleDoubleLegacy);
   auto Ret = Tmp.getExactInverse(&Inv);
   *inv = APFloat(semPPCDoubleDouble, Inv.bitcastToAPInt());
   return Ret;
 }
 
+int DoubleAPFloat::getExactLog2() const {
+  // TODO: Implement me
+  return INT_MIN;
+}
+
 DoubleAPFloat scalbn(const DoubleAPFloat &Arg, int Exp,
                      APFloat::roundingMode RM) {
   assert(Arg.Semantics == &semPPCDoubleDouble && "Unexpected Semantics");
   return DoubleAPFloat(semPPCDoubleDouble, scalbn(Arg.Floats[0], Exp, RM),
                        scalbn(Arg.Floats[1], Exp, RM));
 }
 
 DoubleAPFloat frexp(const DoubleAPFloat &Arg, int &Exp,

llvm/unittests/ADT/APFloatTest.cpp

TEST(APFloatTest, FloatTF32ToFloat) {
   APFloat SmallestDenorm = APFloat::getSmallest(APFloat::FloatTF32(), false);
   EXPECT_TRUE(SmallestDenorm.isDenormal());
   EXPECT_EQ(0x0.004p-126, SmallestDenorm.convertToFloat());
 
   APFloat QNaN = APFloat::getQNaN(APFloat::FloatTF32());
   EXPECT_TRUE(std::isnan(QNaN.convertToFloat()));
 }
 
+TEST(APFloatTest, getExactLog2) {
+  for (unsigned I = 0; I != APFloat::S_MaxSemantics + 1; ++I) {
+    auto SemEnum = static_cast<APFloat::Semantics>(I);
+    const fltSemantics &Semantics = APFloat::EnumToSemantics(SemEnum);
+
+    APFloat One(Semantics, "1.0");
+
+    if (I == APFloat::S_PPCDoubleDouble) {
+      // Not implemented
+      EXPECT_EQ(INT_MIN, One.getExactLog2());
+      continue;
+    }
+
+    int MinExp = APFloat::semanticsMinExponent(Semantics);
+    int MaxExp = APFloat::semanticsMaxExponent(Semantics);
+    int Precision = APFloat::semanticsPrecision(Semantics);
+
+    EXPECT_EQ(0, One.getExactLog2());
+    EXPECT_EQ(INT_MIN, APFloat(Semantics, "3.0").getExactLog2());
+    EXPECT_EQ(INT_MIN, APFloat(Semantics, "-3.0").getExactLog2());
+    EXPECT_EQ(3, APFloat(Semantics, "8.0").getExactLog2());
+    EXPECT_EQ(INT_MIN, APFloat(Semantics, "-8.0").getExactLog2());
+    EXPECT_EQ(INT_MIN, APFloat(Semantics, "-0.25").getExactLog2());
+    EXPECT_EQ(-2, APFloat(Semantics, "0.25").getExactLog2());
+
+    EXPECT_EQ(INT_MIN, APFloat::getZero(Semantics, false).getExactLog2());
+    EXPECT_EQ(INT_MIN, APFloat::getZero(Semantics, true).getExactLog2());
+    EXPECT_EQ(INT_MIN, APFloat::getInf(Semantics).getExactLog2());
+    EXPECT_EQ(INT_MIN, APFloat::getInf(Semantics, true).getExactLog2());
+    EXPECT_EQ(INT_MIN, APFloat::getNaN(Semantics, false).getExactLog2());
+    EXPECT_EQ(INT_MIN, APFloat::getNaN(Semantics, true).getExactLog2());
+
+    EXPECT_EQ(INT_MIN,
+              scalbn(One, MinExp - Precision - 1, APFloat::rmNearestTiesToEven)
+                  .getExactLog2());
+    EXPECT_EQ(INT_MIN,
+              scalbn(One, MinExp - Precision, APFloat::rmNearestTiesToEven)
+                  .getExactLog2());
+
+    EXPECT_EQ(
+        INT_MIN,
+        scalbn(One, MaxExp + 1, APFloat::rmNearestTiesToEven).getExactLog2());
+
+    for (int i = MinExp - Precision + 1; i <= MaxExp; ++i) {
+      EXPECT_EQ(i, scalbn(One, i, APFloat::rmNearestTiesToEven).getExactLog2());
+    }
+  }
+}
+
 } // namespace