From 1053a0b9b4e0a4f459667c110f40f5314c30f2db Mon Sep 17 00:00:00 2001 From: Eli Friedman Date: Thu, 29 Aug 2013 23:44:34 +0000 Subject: [PATCH] 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 --- lib/Support/APFloat.cpp | 13 ++++++++----- unittests/ADT/APFloatTest.cpp | 9 +++++---- 2 files changed, 13 insertions(+), 9 deletions(-) diff --git a/lib/Support/APFloat.cpp b/lib/Support/APFloat.cpp index 34bc6b6dd6b..676e2d4ba00 100644 --- a/lib/Support/APFloat.cpp +++ b/lib/Support/APFloat.cpp @@ -3546,11 +3546,14 @@ void APFloat::toString(SmallVectorImpl &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. diff --git a/unittests/ADT/APFloatTest.cpp b/unittests/ADT/APFloatTest.cpp index 3b69de2a52f..e57c8d4b931 100644 --- a/unittests/ADT/APFloatTest.cpp +++ b/unittests/ADT/APFloatTest.cpp @@ -866,10 +866,11 @@ TEST(APFloatTest, toString) { ASSERT_EQ("0.0101", convertToString(1.01E-2, 5, 2)); ASSERT_EQ("0.0101", convertToString(1.01E-2, 4, 2)); ASSERT_EQ("1.01E-2", convertToString(1.01E-2, 5, 1)); - ASSERT_EQ("0.7853981633974483", convertToString(0.78539816339744830961, 0, 3)); - ASSERT_EQ("4.940656458412465E-324", convertToString(4.9406564584124654e-324, 0, 3)); - ASSERT_EQ("873.1834", convertToString(873.1834, 0, 1)); - ASSERT_EQ("8.731834E+2", convertToString(873.1834, 0, 0)); + ASSERT_EQ("0.78539816339744828", convertToString(0.78539816339744830961, 0, 3)); + ASSERT_EQ("4.9406564584124654E-324", convertToString(4.9406564584124654e-324, 0, 3)); + ASSERT_EQ("873.18340000000001", convertToString(873.1834, 0, 1)); + ASSERT_EQ("8.7318340000000001E+2", convertToString(873.1834, 0, 0)); + ASSERT_EQ("1.7976931348623157E+308", convertToString(1.7976931348623157E+308, 0, 0)); } TEST(APFloatTest, toInteger) { -- 2.34.1