Revert r189442 "Change default # of digits for APFloat::toString"

This is breaking numerous Clang tests on the buildbot.

llvm-svn: 189447

1 files changed, 5 insertions, 8 deletions

diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
index 676e2d4ba00..34bc6b6dd6b 100644
--- a/llvm/lib/Support/APFloat.cpp
+++ b/llvm/lib/Support/APFloat.cpp
@@ -3546,14 +3546,11 @@ void APFloat::toString(SmallVectorImpl<char> &Str,
   // Set FormatPrecision if zero.  We want to do this before we
   // truncate trailing zeros, as those are part of the precision.
   if (!FormatPrecision) {
-    // We use enough digits so the number can be round-tripped back to an
-    // APFloat. The formula comes from "How to Print Floating-Point Numbers
-    // Accurately" by Steele and White.
-    // FIXME: Using a formula based purely on the precision is conservative;
-    // we can print fewer digits depending on the actual value being printed.
-
-    // FormatPrecision = 2 + floor(significandBits / lg_2(10))
-    FormatPrecision = 2 + semantics->precision * 59 / 196;
+    // It's an interesting question whether to use the nominal
+    // precision or the active precision here for denormals.
+
+    // FormatPrecision = ceil(significandBits / lg_2(10))
+    FormatPrecision = (semantics->precision * 59 + 195) / 196;
   }
 
   // Ignore trailing binary zeros.