Change default # of digits for APFloat::toString
This is a re-commit of r189442; I'll follow up with clang changes.
The previous default was almost, but not quite enough digits to
represent a floating-point value in a manner which preserves the
representation when it's read back in. The larger default is much
less confusing.
I spent some time looking into printing exactly the right number of
digits if a precision isn't specified, but it's kind of complicated,
and I'm not really sure I understand what APFloat::toString is supposed
to output for FormatPrecision != 0 (or maybe the current API specification
is just silly, not sure which). I have a WIP patch if anyone is interested.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189624 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/APFloat.cpp b/lib/Support/APFloat.cpp
index 34bc6b6..676e2d4 100644
--- a/lib/Support/APFloat.cpp
+++ b/lib/Support/APFloat.cpp
@@ -3546,11 +3546,14 @@
// Set FormatPrecision if zero. We want to do this before we
// truncate trailing zeros, as those are part of the precision.
if (!FormatPrecision) {
- // It's an interesting question whether to use the nominal
- // precision or the active precision here for denormals.
+ // 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 = ceil(significandBits / lg_2(10))
- FormatPrecision = (semantics->precision * 59 + 195) / 196;
+ // FormatPrecision = 2 + floor(significandBits / lg_2(10))
+ FormatPrecision = 2 + semantics->precision * 59 / 196;
}
// Ignore trailing binary zeros.