[PATCH 1/2] libstdc++: Robustify long double std::to_chars testcase [PR98384]

[Jonathan Wakely]

On 23/02/21 11:30 -0500, Patrick Palka via Libstdc++ wrote:
>On Mon, 22 Feb 2021, Patrick Palka wrote:
>
>> This makes the hexadecimal section of the long double std::to_chars
>> testcase more robust by avoiding false-negative FAILs due to printf
>> using a different leading hex digit than us, and by additionally
>> verifying the correctness of the hexadecimal form via round-tripping
>> through std::from_chars.
>>
>> Tested on x86, x86_64, powerpc64be, powerpc64le and aarch64.  Does this
>> look OK for trunk?
>
>The commit message could explain the issue better, so here's v2 with a
>more detailed commit message.
>
>-- >8 --
>
>Subject: [PATCH] libstdc++: Robustify long double std::to_chars testcase
> [PR98384]
>
>The long double std::to_chars testcase currently verifies the
>correctness of its output by comparing it to that of printf, so if
>there's a mismatch between to_chars and printf, the test FAILs.  This
>works well for the scientific, fixed and general formatting modes,
>because the corresponding printf conversion specifiers (%e, %f and %g)
>are rigidly specified.
>
>But this doesn't work so well for the hex formatting mode because the
>corresponding printf conversion specifier %a is more flexibly specified.
>For instance, the hexadecimal forms 0x1p+0, 0x2p-1, 0x4p-2 and 0x8p-3
>are all equivalent and valid outputs of the %a specifier for the number
>1.  The apparent freedom here is the choice of leading hex digit -- the
>standard just requires that the leading hex digit is nonzero for
>normalized numbers.
>
>Currently, our hexadecimal formatting implementation uses 0/1/2 as the
>leading hex digit for floating point types that have an implicit leading
>mantissa bit which in practice means all supported floating point types
>except x86 long double.  The latter type has a 64 bit mantissa with an
>explicit leading mantissa bit, and for this type our implementation uses
>the most significant four bits of the mantissa as leading hex digit.
>This seems to be consistent with most printf implementations, but not
>all, as PR98384 illustrates.
>
>In order to avoid false-positive FAILs due to arbitrary disagreement
>between to_chars and printf about the choice of leading hex digit, this
>patch makes the testcase's verification via printf conditional on the
>leading hex digits first agreeing.  An additional verification step is
>also added: round-tripping the output of to_chars through from_chars
>should yield the original value.
>
>Tested on x86, x86_64, powerpc64be, powerpc64le and aarch64.  Does this
>look OK for trunk?

>@@ -50,6 +51,38 @@ namespace detail
> void
> test01()
> {
>+  // Verifies correctness of the hexadecimal form [BEGIN,END) for VALUE by
>+  // round-tripping it through from_chars (if available).
>+  auto verify_via_from_chars = [] (char *begin, char *end, long double value) {
>+#if __cpp_lib_to_chars >= 201611L || _GLIBCXX_HAVE_USELOCALE

This is currently going to fail, because we don't actually define
__cpp_lib_to_chars yet (we should fix that!)

Is checking the feature test macro here useful? We know that
floating-point from_chars was committed before to_chars, so if this
test is running, we should have from_chars (modulo uselocale being
available, so that check is right). Is this to make the test usable
for other C++ std::lib implementations?

>+    long double roundtrip;
>+    auto result = from_chars(begin, end, roundtrip, chars_format::hex);
>+    VERIFY( result.ec == errc{} );
>+    VERIFY( result.ptr == end );
>+    VERIFY( roundtrip == value );
>+#endif