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From: Samuel Benzaquen <sbenza@google.com>
Subject: Fix float conversion for PPC.
Origin: backport, https://github.com/abseil/abseil-cpp/commit/c36d825d9a5443f81d2656685ae021d6326da90c
In PPC `long double` is a double-double representation which behaves weirdly
wrt numeric_limits. Don't take `long double` into account when we are not
handling `long double` natively anyway.
Fix the convert test to always run the conversion even if we are not going to
compare against libc's printf result. This allows exercising the code itself to
make sure we don't trigger assertions or UB found by sanitizers.
The author works at Google. Upstream applied this patch as Piper revision
355857729 and exported it to GitHub; the Applied-Upstream URL above points to
the exported commit.
--- a/absl/strings/internal/str_format/convert_test.cc
+++ b/absl/strings/internal/str_format/convert_test.cc
@@ -540,7 +540,8 @@
}
template <typename Floating>
-void TestWithMultipleFormatsHelper(const std::vector<Floating> &floats) {
+void TestWithMultipleFormatsHelper(const std::vector<Floating> &floats,
+ const std::set<Floating> &skip_verify) {
const NativePrintfTraits &native_traits = VerifyNativeImplementation();
// Reserve the space to ensure we don't allocate memory in the output itself.
std::string str_format_result;
@@ -588,7 +589,16 @@
AppendPack(&str_format_result, format, absl::MakeSpan(args));
}
- if (string_printf_result != str_format_result) {
+#ifdef _MSC_VER
+ // MSVC has a different rounding policy than us so we can't test our
+ // implementation against the native one there.
+ continue;
+#elif defined(__APPLE__)
+ // Apple formats NaN differently (+nan) vs. (nan)
+ if (std::isnan(d)) continue;
+#endif
+ if (string_printf_result != str_format_result &&
+ skip_verify.find(d) == skip_verify.end()) {
// We use ASSERT_EQ here because failures are usually correlated and a
// bug would print way too many failed expectations causing the test
// to time out.
@@ -602,12 +612,6 @@
}
TEST_F(FormatConvertTest, Float) {
-#ifdef _MSC_VER
- // MSVC has a different rounding policy than us so we can't test our
- // implementation against the native one there.
- return;
-#endif // _MSC_VER
-
std::vector<float> floats = {0.0f,
-0.0f,
.9999999f,
@@ -621,7 +625,8 @@
std::numeric_limits<float>::epsilon(),
std::numeric_limits<float>::epsilon() + 1.0f,
std::numeric_limits<float>::infinity(),
- -std::numeric_limits<float>::infinity()};
+ -std::numeric_limits<float>::infinity(),
+ std::nanf("")};
// Some regression tests.
floats.push_back(0.999999989f);
@@ -650,21 +655,14 @@
std::sort(floats.begin(), floats.end());
floats.erase(std::unique(floats.begin(), floats.end()), floats.end());
-#ifndef __APPLE__
- // Apple formats NaN differently (+nan) vs. (nan)
- floats.push_back(std::nan(""));
-#endif
-
- TestWithMultipleFormatsHelper(floats);
+ TestWithMultipleFormatsHelper(floats, {});
}
TEST_F(FormatConvertTest, Double) {
-#ifdef _MSC_VER
- // MSVC has a different rounding policy than us so we can't test our
- // implementation against the native one there.
- return;
-#endif // _MSC_VER
-
+ // For values that we know won't match the standard library implementation we
+ // skip verification, but still run the algorithm to catch asserts/sanitizer
+ // bugs.
+ std::set<double> skip_verify;
std::vector<double> doubles = {0.0,
-0.0,
.99999999999999,
@@ -678,7 +676,8 @@
std::numeric_limits<double>::epsilon(),
std::numeric_limits<double>::epsilon() + 1,
std::numeric_limits<double>::infinity(),
- -std::numeric_limits<double>::infinity()};
+ -std::numeric_limits<double>::infinity(),
+ std::nan("")};
// Some regression tests.
doubles.push_back(0.99999999999999989);
@@ -708,33 +707,29 @@
"5084551339423045832369032229481658085593321233482747978262041447231"
"68738177180919299881250404026184124858368.000000";
- if (!gcc_bug_22142) {
- for (int exp = -300; exp <= 300; ++exp) {
- const double all_ones_mantissa = 0x1fffffffffffff;
- doubles.push_back(std::ldexp(all_ones_mantissa, exp));
+ for (int exp = -300; exp <= 300; ++exp) {
+ const double all_ones_mantissa = 0x1fffffffffffff;
+ doubles.push_back(std::ldexp(all_ones_mantissa, exp));
+ if (gcc_bug_22142) {
+ skip_verify.insert(doubles.back());
}
}
if (gcc_bug_22142) {
- for (auto &d : doubles) {
- using L = std::numeric_limits<double>;
- double d2 = std::abs(d);
- if (d2 == L::max() || d2 == L::min() || d2 == L::denorm_min()) {
- d = 0;
- }
- }
+ using L = std::numeric_limits<double>;
+ skip_verify.insert(L::max());
+ skip_verify.insert(L::min()); // NOLINT
+ skip_verify.insert(L::denorm_min());
+ skip_verify.insert(-L::max());
+ skip_verify.insert(-L::min()); // NOLINT
+ skip_verify.insert(-L::denorm_min());
}
// Remove duplicates to speed up the logic below.
std::sort(doubles.begin(), doubles.end());
doubles.erase(std::unique(doubles.begin(), doubles.end()), doubles.end());
-#ifndef __APPLE__
- // Apple formats NaN differently (+nan) vs. (nan)
- doubles.push_back(std::nan(""));
-#endif
-
- TestWithMultipleFormatsHelper(doubles);
+ TestWithMultipleFormatsHelper(doubles, skip_verify);
}
TEST_F(FormatConvertTest, DoubleRound) {
@@ -1055,11 +1050,6 @@
}
TEST_F(FormatConvertTest, LongDouble) {
-#ifdef _MSC_VER
- // MSVC has a different rounding policy than us so we can't test our
- // implementation against the native one there.
- return;
-#endif // _MSC_VER
const NativePrintfTraits &native_traits = VerifyNativeImplementation();
const char *const kFormats[] = {"%", "%.3", "%8.5", "%9", "%.5000",
"%.60", "%+", "% ", "%-10"};
@@ -1120,10 +1110,18 @@
for (auto d : doubles) {
FormatArgImpl arg(d);
UntypedFormatSpecImpl format(fmt_str);
+ std::string result = FormatPack(format, {&arg, 1});
+
+#ifdef _MSC_VER
+ // MSVC has a different rounding policy than us so we can't test our
+ // implementation against the native one there.
+ continue;
+#endif // _MSC_VER
+
// We use ASSERT_EQ here because failures are usually correlated and a
// bug would print way too many failed expectations causing the test to
// time out.
- ASSERT_EQ(StrPrint(fmt_str.c_str(), d), FormatPack(format, {&arg, 1}))
+ ASSERT_EQ(StrPrint(fmt_str.c_str(), d), result)
<< fmt_str << " " << StrPrint("%.18Lg", d) << " "
<< StrPrint("%La", d) << " " << StrPrint("%.1080Lf", d);
}
--- a/absl/strings/internal/str_format/float_conversion.cc
+++ b/absl/strings/internal/str_format/float_conversion.cc
@@ -98,12 +98,22 @@
return next_carry % divisor;
}
+constexpr bool IsDoubleDouble() {
+ // This is the `double-double` representation of `long double`.
+ // We do not handle it natively. Fallback to snprintf.
+ return std::numeric_limits<long double>::digits ==
+ 2 * std::numeric_limits<double>::digits;
+}
+
+using MaxFloatType =
+ typename std::conditional<IsDoubleDouble(), double, long double>::type;
+
// Generates the decimal representation for an integer of the form `v * 2^exp`,
// where `v` and `exp` are both positive integers.
// It generates the digits from the left (ie the most significant digit first)
// to allow for direct printing into the sink.
//
-// Requires `0 <= exp` and `exp <= numeric_limits<long double>::max_exponent`.
+// Requires `0 <= exp` and `exp <= numeric_limits<MaxFloatType>::max_exponent`.
class BinaryToDecimal {
static constexpr int ChunksNeeded(int exp) {
// We will left shift a uint128 by `exp` bits, so we need `128+exp` total
@@ -118,10 +128,10 @@
static void RunConversion(uint128 v, int exp,
absl::FunctionRef<void(BinaryToDecimal)> f) {
assert(exp > 0);
- assert(exp <= std::numeric_limits<long double>::max_exponent);
+ assert(exp <= std::numeric_limits<MaxFloatType>::max_exponent);
static_assert(
StackArray::kMaxCapacity >=
- ChunksNeeded(std::numeric_limits<long double>::max_exponent),
+ ChunksNeeded(std::numeric_limits<MaxFloatType>::max_exponent),
"");
StackArray::RunWithCapacity(
@@ -218,14 +228,14 @@
// Converts a value of the form `x * 2^-exp` into a sequence of decimal digits.
// Requires `-exp < 0` and
-// `-exp >= limits<long double>::min_exponent - limits<long double>::digits`.
+// `-exp >= limits<MaxFloatType>::min_exponent - limits<MaxFloatType>::digits`.
class FractionalDigitGenerator {
public:
// Run the conversion for `v * 2^exp` and call `f(generator)`.
// This function will allocate enough stack space to perform the conversion.
static void RunConversion(
uint128 v, int exp, absl::FunctionRef<void(FractionalDigitGenerator)> f) {
- using Limits = std::numeric_limits<long double>;
+ using Limits = std::numeric_limits<MaxFloatType>;
assert(-exp < 0);
assert(-exp >= Limits::min_exponent - 128);
static_assert(StackArray::kMaxCapacity >=
@@ -858,10 +868,10 @@
// This buffer holds the "0x1.ab1de3" portion of "0x1.ab1de3pe+2". Compute the
// size with long double which is the largest of the floats.
constexpr size_t kBufSizeForHexFloatRepr =
- 2 // 0x
- + std::numeric_limits<long double>::digits / 4 // number of hex digits
- + 1 // round up
- + 1; // "." (dot)
+ 2 // 0x
+ + std::numeric_limits<MaxFloatType>::digits / 4 // number of hex digits
+ + 1 // round up
+ + 1; // "." (dot)
char digits_buffer[kBufSizeForHexFloatRepr];
char *digits_iter = digits_buffer;
const char *const digits =
@@ -1380,10 +1390,7 @@
bool ConvertFloatImpl(long double v, const FormatConversionSpecImpl &conv,
FormatSinkImpl *sink) {
- if (std::numeric_limits<long double>::digits ==
- 2 * std::numeric_limits<double>::digits) {
- // This is the `double-double` representation of `long double`.
- // We do not handle it natively. Fallback to snprintf.
+ if (IsDoubleDouble()) {
return FallbackToSnprintf(v, conv, sink);
}
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