On Mon, 21 Dec 2020, Jonathan Wakely wrote:
> On 21/12/20 11:58 -0500, Patrick Palka via Libstdc++ wrote:
> > This patch conditionally disables the floating-point std::to_chars
> > implementation on targets whose float/double are not IEEE binary32 and
> > binary64 respectively, until a proper fallback can be added for such
> > targets. This fixes a bootstrap failure on non-IEEE-754 FP targets such
> > as vax-netbsdelf.
> >
> > The preprocessor conditions that define the new internal libstdc++
> > macros for detecting the binary32 and binary64 formats were copied from
> > the test gcc/testsuite/gcc.dg/float-exact-1.c.
> >
> > Tested on x86_64-pc-linux-gnu, with and without -m32, and also when
> > artifically undefining the below new macros. Does this look OK for
> > trunk?
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/bits/c++config (_GLIBCXX_FLOAT_IS_IEEE_BINARY_32):
> > Define this macro.
> > (_GLIBCXX_DOUBLE_IS_IEEE_BINARY_64): Likewise.
> > * include/std/charconv (to_chars): Use the macros to
> > conditionally hide the overloads for floating-point types.
> > * src/c++17/floating_to_chars.cc: Use the macros to
> > conditionally disable this file.
> > (floating_type_traits<float>): Remove redundant static assert.
> > (floating_type_traits<double>): Likewise.
> > * testsuite/20_util/to_chars/double.cc: Use the macros to
> > conditionally disable this test.
> > * testsuite/20_util/to_chars/float.cc: Likewise.
> > * testsuite/20_util/to_chars/long_double.cc: Likewise. Adjust
> > dg-do directives so that we don't execute this test on targets
> > with a large long double type and without int128.
> > ---
> > libstdc++-v3/include/bits/c++config | 14 ++++++++++++++
> > libstdc++-v3/include/std/charconv | 2 ++
> > libstdc++-v3/src/c++17/floating_to_chars.cc | 9 +++++----
> > .../testsuite/20_util/to_chars/double.cc | 2 ++
> > .../testsuite/20_util/to_chars/float.cc | 2 ++
> > .../testsuite/20_util/to_chars/long_double.cc | 17 +++++++++++++----
> > 6 files changed, 38 insertions(+), 8 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/bits/c++config
> > b/libstdc++-v3/include/bits/c++config
> > index 8cce88aa87b..f54074a2c04 100644
> > --- a/libstdc++-v3/include/bits/c++config
> > +++ b/libstdc++-v3/include/bits/c++config
> > @@ -688,6 +688,20 @@ namespace std
> > # endif
> > #endif
> >
> > +// Define if float has the IEEE binary32 format.
> > +#if __FLT_MANT_DIG__ == 24 \
> > + && __FLT_MIN_EXP__ == -125 \
> > + && __FLT_MAX_EXP == 128
> > +# define _GLIBCXX_FLOAT_IS_IEEE_BINARY32 1
> > +#endif
> > +
> > +// Define if double has the IEEE binary64 format.
> > +#if __DBL_MANT_DIG__ == 53 \
> > + && __DBL_MIN_EXP__ == -1021 \
> > + && __DBL_MAX_EXP__ == 1024
> > +# define _GLIBCXX_DOUBLE_IS_IEEE_BINARY64 1
> > +#endif
> > +
> > #ifdef __has_builtin
> > # ifdef __is_identifier
> > // Intel and older Clang require !__is_identifier for some built-ins:
> > diff --git a/libstdc++-v3/include/std/charconv
> > b/libstdc++-v3/include/std/charconv
> > index b57b0a16db2..1f005be47b1 100644
> > --- a/libstdc++-v3/include/std/charconv
> > +++ b/libstdc++-v3/include/std/charconv
> > @@ -704,6 +704,7 @@ namespace __detail
> >
> > // Floating-point std::to_chars
> >
> > +#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
> > // Overloads for float.
> > to_chars_result to_chars(char* __first, char* __last, float __value)
> > noexcept;
> > to_chars_result to_chars(char* __first, char* __last, float __value,
> > @@ -725,6 +726,7 @@ namespace __detail
> > chars_format __fmt) noexcept;
> > to_chars_result to_chars(char* __first, char* __last, long double __value,
> > chars_format __fmt, int __precision) noexcept;
> > +#endif
> >
> > _GLIBCXX_END_NAMESPACE_VERSION
> > } // namespace std
> > diff --git a/libstdc++-v3/src/c++17/floating_to_chars.cc
> > b/libstdc++-v3/src/c++17/floating_to_chars.cc
> > index b7c31c746cc..6d94d46cc0a 100644
> > --- a/libstdc++-v3/src/c++17/floating_to_chars.cc
> > +++ b/libstdc++-v3/src/c++17/floating_to_chars.cc
> > @@ -22,6 +22,10 @@
> > // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
> > // <http://www.gnu.org/licenses/>.
> >
> > +// This implementation crucially assumes float/double have the
> > +// IEEE binary32/binary64 formats.
> > +#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
> > +
> > // Activate __glibcxx_assert within this file to shake out any bugs.
> > #define _GLIBCXX_ASSERTIONS 1
> >
> > @@ -109,8 +113,6 @@ namespace
> > template<>
> > struct floating_type_traits<float>
> > {
> > - // We (and Ryu) assume float has the IEEE binary32 format.
> > - static_assert(__FLT_MANT_DIG__ == 24);
> > static constexpr int mantissa_bits = 23;
> > static constexpr int exponent_bits = 8;
> > static constexpr bool has_implicit_leading_bit = true;
> > @@ -124,8 +126,6 @@ namespace
> > template<>
> > struct floating_type_traits<double>
> > {
> > - // We (and Ryu) assume double has the IEEE binary64 format.
> > - static_assert(__DBL_MANT_DIG__ == 53);
> > static constexpr int mantissa_bits = 52;
> > static constexpr int exponent_bits = 11;
> > static constexpr bool has_implicit_leading_bit = true;
> > @@ -1565,3 +1565,4 @@ _ZSt8to_charsPcS_eSt12chars_formati(char* first, char*
> > last, double value,
> >
> > _GLIBCXX_END_NAMESPACE_VERSION
> > } // namespace std
> > +#endif // _GLIBCXX_FLOAT_IS_IEEE_BINARY32 &&
> > _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
> > diff --git a/libstdc++-v3/testsuite/20_util/to_chars/double.cc
> > b/libstdc++-v3/testsuite/20_util/to_chars/double.cc
> > index 9d1f37d3026..a52d7e7cd61 100644
> > --- a/libstdc++-v3/testsuite/20_util/to_chars/double.cc
> > +++ b/libstdc++-v3/testsuite/20_util/to_chars/double.cc
> > @@ -56952,6 +56952,7 @@ inline constexpr double_to_chars_testcase
> > double_hex_precision_to_chars_test_cas
> > void
> > test01()
> > {
> > +#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
> > auto handle_testcases = [] (const auto& testcases) {
> > for (const auto [value,fmt,precision,correct] : testcases)
> > {
> > @@ -56991,6 +56992,7 @@ test01()
> > handle_testcases(double_general_precision_to_chars_test_cases);
> >
> > handle_testcases(double_hex_precision_to_chars_test_cases);
> > +#endif
>
> If I'm reading this right, the compiler is going to churn through
> 50000 lines defining arrays of numbers, and then do nothing with them.
>
> This would mean you could { dg-require-effective-targets ieee-floats }
> so the whole testcase is skipped:
>
> --- a/libstdc++-v3/testsuite/lib/libstdc++.exp
> +++ b/libstdc++-v3/testsuite/lib/libstdc++.exp
> @@ -1323,6 +1323,14 @@ proc check_effective_target_futex { } {
> }]
> }
>
> +# Return 1 if float and double have the IEEE binary32 and binary64 formats.
> +proc check_effective_target_ieee-floats { } {
> + return [check_v3_target_prop_cached et_ieee_floats {
> + set cond "_GLIBCXX_FLOAT_IS_IEEE_BINARY32 &&
> _GLIBCXX_DOUBLE_IS_IEEE_BINARY64"
> + return [v3_check_preprocessor_condition ieee_floats $cond]
> + }]
> +}
> +
> set additional_prunes ""
>
> if { [info exists env(GCC_RUNTEST_PARALLELIZE_DIR)] \
>
> If something like that works, please do that.
>
> OK for trunk with that if it works, and as in your patch otherwise.
Thanks a lot, this cleaner approach using a new effective target seems
to work well. Here's what I've committed after another round of testing
on x86_64-pc-linux-gnu:
-- >8 --
Subject: [PATCH] libstdc++: Disable floating-point std::to_chars on
unsupported targets
This patch conditionally disables the floating-point std::to_chars
implementation on targets whose float and double aren't IEEE binary32
and binary64, until a proper fallback can be added for such targets.
This fixes a bootstrap failure on non-IEEE-754 FP targets such as
vax-netbsdelf.
The new preprocessor tests in c++config that detect the binary32 and
binary64 formats were copied from gcc/testsuite/gcc.dg/float-exact-1.c.
libstdc++-v3/ChangeLog:
* include/bits/c++config (_GLIBCXX_FLOAT_IS_IEEE_BINARY_32):
Define this macro.
(_GLIBCXX_DOUBLE_IS_IEEE_BINARY_64): Likewise.
* include/std/charconv (to_chars): Use these macros to
conditionally hide the overloads for floating-point types.
* src/c++17/floating_to_chars.cc: Use the macros to
conditionally disable this file.
(floating_type_traits<float>): Remove redundant static assert.
(floating_type_traits<double>): Likewise.
* testsuite/20_util/to_chars/double.cc: Run this test only on
ieee-floats effective targets.
* testsuite/20_util/to_chars/float.cc: Likewise.
* testsuite/20_util/to_chars/long_double.cc: Likewise.
* testsuite/lib/libstdc++.exp
(check_effective_target_ieee-floats): Define new proc for
detecting whether float and double have the IEEE binary32 and
binary64 formats.
---
libstdc++-v3/include/bits/c++config | 14 ++++++++++++++
libstdc++-v3/include/std/charconv | 2 ++
libstdc++-v3/src/c++17/floating_to_chars.cc | 10 ++++++----
libstdc++-v3/testsuite/20_util/to_chars/double.cc | 1 +
libstdc++-v3/testsuite/20_util/to_chars/float.cc | 1 +
.../testsuite/20_util/to_chars/long_double.cc | 1 +
libstdc++-v3/testsuite/lib/libstdc++.exp | 8 ++++++++
7 files changed, 33 insertions(+), 4 deletions(-)
diff --git a/libstdc++-v3/include/bits/c++config
b/libstdc++-v3/include/bits/c++config
index 8cce88aa87b..be0961a9d9e 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -688,6 +688,20 @@ namespace std
# endif
#endif
+// Define if float has the IEEE binary32 format.
+#if __FLT_MANT_DIG__ == 24 \
+ && __FLT_MIN_EXP__ == -125 \
+ && __FLT_MAX_EXP__ == 128
+# define _GLIBCXX_FLOAT_IS_IEEE_BINARY32 1
+#endif
+
+// Define if double has the IEEE binary64 format.
+#if __DBL_MANT_DIG__ == 53 \
+ && __DBL_MIN_EXP__ == -1021 \
+ && __DBL_MAX_EXP__ == 1024
+# define _GLIBCXX_DOUBLE_IS_IEEE_BINARY64 1
+#endif
+
#ifdef __has_builtin
# ifdef __is_identifier
// Intel and older Clang require !__is_identifier for some built-ins:
diff --git a/libstdc++-v3/include/std/charconv
b/libstdc++-v3/include/std/charconv
index b57b0a16db2..c6e80693aad 100644
--- a/libstdc++-v3/include/std/charconv
+++ b/libstdc++-v3/include/std/charconv
@@ -702,6 +702,7 @@ namespace __detail
chars_format __fmt = chars_format::general) noexcept;
#endif
+#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
// Floating-point std::to_chars
// Overloads for float.
@@ -725,6 +726,7 @@ namespace __detail
chars_format __fmt) noexcept;
to_chars_result to_chars(char* __first, char* __last, long double __value,
chars_format __fmt, int __precision) noexcept;
+#endif
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
diff --git a/libstdc++-v3/src/c++17/floating_to_chars.cc
b/libstdc++-v3/src/c++17/floating_to_chars.cc
index b7c31c746cc..6e154365aee 100644
--- a/libstdc++-v3/src/c++17/floating_to_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_to_chars.cc
@@ -40,6 +40,10 @@
#include <string_view>
#include <type_traits>
+// This implementation crucially assumes float/double have the
+// IEEE binary32/binary64 formats.
+#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
+
// Determine the binary format of 'long double'.
// We support the binary64, float80 (i.e. x86 80-bit extended precision),
@@ -109,8 +113,6 @@ namespace
template<>
struct floating_type_traits<float>
{
- // We (and Ryu) assume float has the IEEE binary32 format.
- static_assert(__FLT_MANT_DIG__ == 24);
static constexpr int mantissa_bits = 23;
static constexpr int exponent_bits = 8;
static constexpr bool has_implicit_leading_bit = true;
@@ -124,8 +126,6 @@ namespace
template<>
struct floating_type_traits<double>
{
- // We (and Ryu) assume double has the IEEE binary64 format.
- static_assert(__DBL_MANT_DIG__ == 53);
static constexpr int mantissa_bits = 52;
static constexpr int exponent_bits = 11;
static constexpr bool has_implicit_leading_bit = true;
@@ -1565,3 +1565,5 @@ _ZSt8to_charsPcS_eSt12chars_formati(char* first, char*
last, double value,
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
+
+#endif // _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
diff --git a/libstdc++-v3/testsuite/20_util/to_chars/double.cc
b/libstdc++-v3/testsuite/20_util/to_chars/double.cc
index 9d1f37d3026..bb6f74424ed 100644
--- a/libstdc++-v3/testsuite/20_util/to_chars/double.cc
+++ b/libstdc++-v3/testsuite/20_util/to_chars/double.cc
@@ -32,6 +32,7 @@
// DEALINGS IN THE SOFTWARE.
// { dg-do run { target c++17 } }
+// { dg-require-effective-target ieee-floats }
#include <charconv>
diff --git a/libstdc++-v3/testsuite/20_util/to_chars/float.cc
b/libstdc++-v3/testsuite/20_util/to_chars/float.cc
index b8901063ea0..0c8dd4f66df 100644
--- a/libstdc++-v3/testsuite/20_util/to_chars/float.cc
+++ b/libstdc++-v3/testsuite/20_util/to_chars/float.cc
@@ -32,6 +32,7 @@
// DEALINGS IN THE SOFTWARE.
// { dg-do run { target c++17 } }
+// { dg-require-effective-target ieee-floats }
#include <charconv>
diff --git a/libstdc++-v3/testsuite/20_util/to_chars/long_double.cc
b/libstdc++-v3/testsuite/20_util/to_chars/long_double.cc
index 12ac8ae7822..f89daa2c665 100644
--- a/libstdc++-v3/testsuite/20_util/to_chars/long_double.cc
+++ b/libstdc++-v3/testsuite/20_util/to_chars/long_double.cc
@@ -19,6 +19,7 @@
// hexadecimal floating-point literals.
// { dg-do run { target c++17 } }
// { dg-xfail-run-if "Ryu needs __int128" { large_long_double && { ! int128 }
} }
+// { dg-require-effective-target ieee-floats }
#include <charconv>
diff --git a/libstdc++-v3/testsuite/lib/libstdc++.exp
b/libstdc++-v3/testsuite/lib/libstdc++.exp
index b7d7b906de4..6e5634ef1d2 100644
--- a/libstdc++-v3/testsuite/lib/libstdc++.exp
+++ b/libstdc++-v3/testsuite/lib/libstdc++.exp
@@ -1323,6 +1323,14 @@ proc check_effective_target_futex { } {
}]
}
+# Return 1 if float and double have the IEEE binary32 and binary64 formats.
+proc check_effective_target_ieee-floats { } {
+ return [check_v3_target_prop_cached et_ieee_floats {
+ set cond "_GLIBCXX_FLOAT_IS_IEEE_BINARY32 &&
_GLIBCXX_DOUBLE_IS_IEEE_BINARY64"
+ return [v3_check_preprocessor_condition ieee_floats $cond]
+ }]
+}
+
set additional_prunes ""
if { [info exists env(GCC_RUNTEST_PARALLELIZE_DIR)] \
--
2.30.0.rc0
