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

[Patrick Palka]

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?

libstdc++-v3/ChangeLog:

	PR libstdc++/98384
	* testsuite/20_util/to_chars/long_double.cc: Include <optional>.
	(test01): Simplify verifying the nearby values by using a
	2-iteration loop and a dedicated output buffer to check that the
	nearby values are different.  Factor out the printf-based
	verification into a local function, and check that the leading
	hex digits agree before comparing with the output of printf.
	Also verify the output by round-tripping it through from_chars.
---
 .../testsuite/20_util/to_chars/long_double.cc | 73 ++++++++++++-------
 1 file changed, 47 insertions(+), 26 deletions(-)

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 4f72cb65400..da847ae5401 100644
--- a/libstdc++-v3/testsuite/20_util/to_chars/long_double.cc
+++ b/libstdc++-v3/testsuite/20_util/to_chars/long_double.cc
@@ -26,6 +26,7 @@
 #include <cmath>
 #include <cstring>
 #include <iterator>
+#include <optional>
 #include <limits>
 
 #include <testsuite_hooks.h>
@@ -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
+    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
+  };
+
+  // Verifies correctness of the null-terminated hexadecimal form at BEGIN
+  // for VALUE and PRECISION by comparing it with the output of printf's %La
+  // conversion specifier.
+  auto verify_via_printf = [] (char *begin, long double value,
+			       optional<int> precision = nullopt) {
+    char printf_buffer[1024] = {};
+    if (precision.has_value())
+      sprintf(printf_buffer, "%.*La", precision.value(), value);
+    else
+      sprintf(printf_buffer, "%La", value);
+
+    // Only compare with the output of printf if the leading hex digits agree.
+    // If the leading hex digit of our form doesn't agree with that of printf,
+    // then the two forms may still be equivalent (e.g. 1.1p+0 vs 8.8p-3).  But
+    // if the leading hex digits do agree, then we do expect the two forms to be
+    // the same.
+    if (printf_buffer[strlen("0x")] == begin[0])
+      VERIFY( !strcmp(begin, printf_buffer+strlen("0x")) );
+  };
+
   const long double hex_testcases[]
     = { detail::nextdownl(numeric_limits<long double>::max()),
 	detail::nextupl(numeric_limits<long double>::min()),
@@ -92,38 +125,27 @@ test01()
 	if (testcase == 0.0L || isinf(testcase))
 	  continue;
 
-	char to_chars_buffer[1024], printf_buffer[1024];
-	memset(to_chars_buffer, '\0', sizeof(to_chars_buffer));
-	memset(printf_buffer, '\0', sizeof(printf_buffer));
-
+	char to_chars_buffer[1024] = {};
 	auto result = to_chars(begin(to_chars_buffer), end(to_chars_buffer),
 			       testcase, chars_format::hex);
 	VERIFY( result.ec == errc{} );
 	*result.ptr = '\0';
-	sprintf(printf_buffer, "%La", testcase);
-	VERIFY( !strcmp(to_chars_buffer, printf_buffer+strlen("0x")) );
+	verify_via_from_chars(begin(to_chars_buffer), result.ptr, testcase);
+	verify_via_printf(to_chars_buffer, testcase);
 
+	// Verify the nearby values, and also check they have a different
+	// shortest form.
+	for (long double nearby
+	     : { detail::nextdownl(testcase), detail::nextupl(testcase) })
 	  {
-	    // Verify that the nearby values have a different shortest form.
-	    testcase = detail::nextdownl(testcase);
-	    result = to_chars(begin(to_chars_buffer), end(to_chars_buffer),
-			      testcase, chars_format::hex);
-	    VERIFY( result.ec == errc{} );
-	    *result.ptr = '\0';
-	    VERIFY( strcmp(to_chars_buffer, printf_buffer+strlen("0x")) != 0);
-	    sprintf(printf_buffer, "%La", testcase);
-	    VERIFY( !strcmp(to_chars_buffer, printf_buffer+strlen("0x")) );
-
-	    testcase = detail::nextupl(detail::nextupl(testcase));
-	    result = to_chars(begin(to_chars_buffer), end(to_chars_buffer),
-			      testcase, chars_format::hex);
+	    char nearby_buffer[1024] = {};
+	    result = to_chars(begin(nearby_buffer), end(nearby_buffer),
+			      nearby, chars_format::hex);
 	    VERIFY( result.ec == errc{} );
 	    *result.ptr = '\0';
-	    VERIFY( strcmp(to_chars_buffer, printf_buffer+strlen("0x")) != 0);
-	    sprintf(printf_buffer, "%La", testcase);
-	    VERIFY( !strcmp(to_chars_buffer, printf_buffer+strlen("0x")) );
-
-	    testcase = detail::nextdownl(testcase);
+	    VERIFY( strcmp(nearby_buffer, to_chars_buffer) != 0);
+	    verify_via_from_chars(begin(nearby_buffer), result.ptr, nearby);
+	    verify_via_printf(nearby_buffer, nearby);
 	  }
 
 	for (int precision = -1; precision < 50; precision++)
@@ -132,8 +154,7 @@ test01()
 			      testcase, chars_format::hex, precision);
 	    VERIFY( result.ec == errc{} );
 	    *result.ptr = '\0';
-	    sprintf(printf_buffer, "%.*La", precision, testcase);
-	    VERIFY( !strcmp(to_chars_buffer, printf_buffer+strlen("0x")) );
+	    verify_via_printf(to_chars_buffer, testcase, precision);
 	  }
       }
 }
-- 
2.30.1.602.g966e671106