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author | Eli Friedman <eli.friedman@gmail.com> | 2013-08-29 23:44:34 +0000 |
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committer | Eli Friedman <eli.friedman@gmail.com> | 2013-08-29 23:44:34 +0000 |
commit | 1053a0b9b4e0a4f459667c110f40f5314c30f2db (patch) | |
tree | 2a03ae0ad689506361a95b1a783f76003dcb74ce /lib/Support | |
parent | 3a0e9b50f3a9031217a8fe209398ec37c54774eb (diff) | |
download | llvm-1053a0b9b4e0a4f459667c110f40f5314c30f2db.tar.gz llvm-1053a0b9b4e0a4f459667c110f40f5314c30f2db.tar.bz2 llvm-1053a0b9b4e0a4f459667c110f40f5314c30f2db.tar.xz |
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
Diffstat (limited to 'lib/Support')
-rw-r--r-- | lib/Support/APFloat.cpp | 13 |
1 files changed, 8 insertions, 5 deletions
diff --git a/lib/Support/APFloat.cpp b/lib/Support/APFloat.cpp index 34bc6b6dd6..676e2d4ba0 100644 --- a/lib/Support/APFloat.cpp +++ b/lib/Support/APFloat.cpp @@ -3546,11 +3546,14 @@ 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) { - // 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; + // 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; } // Ignore trailing binary zeros. |