APFloat: Fix scalbn handling of denormals

This was incorrect for denormals, and also failed
on longer exponent ranges.

llvm-svn: 263369

1 files changed, 14 insertions, 12 deletions

diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
index 744506ccfa8..95953b6d071 100644
--- a/llvm/lib/Support/APFloat.cpp
+++ b/llvm/lib/Support/APFloat.cpp
@@ -3945,19 +3945,21 @@ APFloat::makeZero(bool Negative) {
   APInt::tcSet(significandParts(), 0, partCount());  
 }
 
-APFloat llvm::scalbn(APFloat X, int Exp) {
-  if (X.isInfinity() || X.isZero() || X.isNaN())
-    return X;
-
+APFloat llvm::scalbn(APFloat X, int Exp, APFloat::roundingMode RoundingMode) {
   auto MaxExp = X.getSemantics().maxExponent;
   auto MinExp = X.getSemantics().minExponent;
-  if (Exp > (MaxExp - X.exponent))
-    // Overflow saturates to infinity.
-    return APFloat::getInf(X.getSemantics(), X.isNegative());
-  if (Exp < (MinExp - X.exponent))
-    // Underflow saturates to zero.
-    return APFloat::getZero(X.getSemantics(), X.isNegative());
-
-  X.exponent += Exp;
+
+  // If Exp is wildly out-of-scale, simply adding it to X.exponent will
+  // overflow; clamp it to a safe range before adding, but ensure that the range
+  // is large enough that the clamp does not change the result. The range we
+  // need to support is the difference between the largest possible exponent and
+  // the normalized exponent of half the smallest denormal.
+
+  int SignificandBits = X.getSemantics().precision - 1;
+  int MaxIncrement = MaxExp - (MinExp - SignificandBits) + 1;
+
+  // Clamp to one past the range ends to let normalize handle overlflow.
+  X.exponent += std::min(std::max(Exp, -MaxIncrement - 1), MaxIncrement);
+  X.normalize(RoundingMode, lfExactlyZero);
   return X;
 }