[PATCH] libstdc++: Add pretty printer for std::span

[Jonathan Wakely]

On Mon, 4 Apr 2022 at 11:54, Philipp Fent via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> This improves the debug output for C++20 spans.
> Before:
> {static extent = 18446744073709551615, _M_ptr = 0x7fffffffb9a8,
> _M_extent = {_M_extent_value = 2}}
> Now with StdSpanPrinter:
> std::span of length 2 = {1, 2}

Nice, thanks. I'll get this committed in time for GCC 12 (and backport
it to release branches too).


> ---
>  libstdc++-v3/python/libstdcxx/v6/printers.py  | 38 +++++++++++++++++++
>  .../libstdc++-prettyprinters/cxx20.cc         | 11 ++++++
>  2 files changed, 49 insertions(+)
>
> diff --git a/libstdc++-v3/python/libstdcxx/v6/printers.py b/libstdc++-v3/python/libstdcxx/v6/printers.py
> index f7a7f9961..6d8b765f2 100644
> --- a/libstdc++-v3/python/libstdcxx/v6/printers.py
> +++ b/libstdc++-v3/python/libstdcxx/v6/printers.py
> @@ -1654,6 +1654,43 @@ class StdRegexStatePrinter:
>              s = "{}, {}={}".format(s, v, self.val['_M_' + v])
>          return "{%s}" % (s)
>
> +class StdSpanPrinter:
> +    "Print a std::span"
> +
> +    class _iterator(Iterator):
> +        def __init__(self, begin, size):
> +            self.count = 0
> +            self.begin = begin
> +            self.size = size
> +
> +        def __iter__ (self):
> +            return self
> +
> +        def __next__ (self):
> +            if self.count == self.size:
> +                raise StopIteration
> +
> +            count = self.count
> +            self.count = self.count + 1
> +            return '[%d]' % count, (self.begin + count).dereference()
> +
> +    def __init__(self, typename, val):
> +        self.typename = typename
> +        self.val = val
> +        if val.type.template_argument(1) == gdb.parse_and_eval('static_cast<std::size_t>(-1)'):
> +            self.size = val['_M_extent']['_M_extent_value']
> +        else:
> +            self.size = val.type.template_argument(1)
> +
> +    def to_string(self):
> +        return '%s of length %d' % (self.typename, self.size)
> +
> +    def children(self):
> +        return self._iterator(self.val['_M_ptr'], self.size)
> +
> +    def display_hint(self):
> +        return 'array'
> +
>  # A "regular expression" printer which conforms to the
>  # "SubPrettyPrinter" protocol from gdb.printing.
>  class RxPrinter(object):
> @@ -2170,6 +2207,7 @@ def build_libstdcxx_dictionary ():
>      libstdcxx_printer.add_version('std::', 'partial_ordering', StdCmpCatPrinter)
>      libstdcxx_printer.add_version('std::', 'weak_ordering', StdCmpCatPrinter)
>      libstdcxx_printer.add_version('std::', 'strong_ordering', StdCmpCatPrinter)
> +    libstdcxx_printer.add_version('std::', 'span', StdSpanPrinter)
>
>      # Extensions.
>      libstdcxx_printer.add_version('__gnu_cxx::', 'slist', StdSlistPrinter)
> diff --git a/libstdc++-v3/testsuite/libstdc++-prettyprinters/cxx20.cc b/libstdc++-v3/testsuite/libstdc++-prettyprinters/cxx20.cc
> index b0de25c27..76023df93 100644
> --- a/libstdc++-v3/testsuite/libstdc++-prettyprinters/cxx20.cc
> +++ b/libstdc++-v3/testsuite/libstdc++-prettyprinters/cxx20.cc
> @@ -18,8 +18,10 @@
>  // with this library; see the file COPYING3.  If not see
>  // <http://www.gnu.org/licenses/>.
>
> +#include <array>
>  #include <compare>
>  #include <iostream>
> +#include <span>
>
>  struct X
>  {
> @@ -54,6 +56,15 @@ main()
>    auto c10 = 0.0 <=> __builtin_nan("");
>  // { dg-final { note-test c10 "std::partial_ordering::unordered" } }
>
> +  auto il = {1, 2};
> +  auto s1 = std::span(il);
> +  static_assert(s1.extent == std::size_t(-1));
> +// { dg-final { note-test s1 {std::span of length 2 = {1, 2}} } }
> +  auto a = std::array{3, 4};
> +  auto s2 = std::span(a);
> +  static_assert(s2.extent == std::size_t(2));
> +// { dg-final { note-test s2 {std::span of length 2 = {3, 4}} } }
> +
>    std::cout << "\n";
>    return 0;                    // Mark SPOT
>  }
> --
> 2.35.1
>