APFloat: Add getExactLog2

https://reviews.llvm.org/D157108
llvm · Aug 7, 2023 · 0b57c3a · 0b57c3a
1 parent 4b1702e
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diff --git a/llvm/include/llvm/ADT/APFloat.h b/llvm/include/llvm/ADT/APFloat.h
@@ -489,6 +489,11 @@ class IEEEFloat final : public APFloatBase {
   /// 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)).
@@ -747,6 +752,9 @@ class DoubleAPFloat final : public APFloatBase {
 
   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);
@@ -1316,6 +1324,11 @@ class APFloat : public APFloatBase {
     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);

diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
@@ -4292,6 +4292,35 @@ bool IEEEFloat::getExactInverse(APFloat *inv) const {
   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;
@@ -5087,6 +5116,11 @@ bool DoubleAPFloat::getExactInverse(APFloat *inv) const {
   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");

diff --git a/llvm/unittests/ADT/APFloatTest.cpp b/llvm/unittests/ADT/APFloatTest.cpp
@@ -6587,4 +6587,53 @@ TEST(APFloatTest, FloatTF32ToFloat) {
   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