[PATCH] libstdc++: Fix __max_diff_type::operator>>= for negative values

[Patrick Palka]

On Mon, 24 Apr 2023, Patrick Palka wrote:

> This patch fixes sign bit propagation when right-shifting a negative
> __max_diff_type value by more than one, a bug which our test coverage
> failed to uncover until r14-159-g03cebd304955a6 fixed the front end's
> 'signed typedef-name' handling (which is a non-standard extension to
> the language grammar).
> 
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk/13.2/12/11?
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/max_size_type.h (max_diff_type::operator>>=):
> 	Fix propagation of sign bit.
> 	* testsuite/std/ranges/iota/max_size_type.cc: Avoid using
> 	'signed typedef-name'.  Add compile-time tests for
> 	right-shifting a negative __max_diff_type value by more than
> 	one.
> ---
>  libstdc++-v3/include/bits/max_size_type.h             |  3 ++-
>  .../testsuite/std/ranges/iota/max_size_type.cc        | 11 +++++++++--
>  2 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/max_size_type.h b/libstdc++-v3/include/bits/max_size_type.h
> index 92b8168d02f..4796135d073 100644
> --- a/libstdc++-v3/include/bits/max_size_type.h
> +++ b/libstdc++-v3/include/bits/max_size_type.h
> @@ -560,7 +560,8 @@ namespace ranges
>  	// Arithmetic right shift.
>  	const auto __msb = _M_rep._M_msb;
>  	_M_rep >>= __r._M_rep;
> -	_M_rep._M_msb |= __msb;
> +	if (__msb)
> +	  _M_rep |= ~(__max_size_type(-1) >> __r._M_rep);
>  	return *this;
>        }
>  
> diff --git a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
> index 06114c22cae..985acd5a803 100644
> --- a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
> @@ -26,6 +26,11 @@
>  using max_size_t = std::ranges::__detail::__max_size_type;
>  using max_diff_t = std::ranges::__detail::__max_diff_type;
>  using rep_t = max_size_t::__rep;
> +#if __SIZEOF_INT128__
> +using signed_rep_t = signed __int128;

Whoops, I suppose this 'signed' qualifier is redundant so consider it
removed.

> +#else
> +using signed_rep_t = long long;
> +#endif
>  
>  static_assert(sizeof(max_size_t) == sizeof(max_diff_t));
>  
> @@ -54,6 +59,8 @@ test01()
>    static_assert(max_diff_t(3) % -2 == 1);
>    static_assert(max_diff_t(-3) << 1 == -6);
>    static_assert(max_diff_t(-3) >> 1 == -2);
> +  static_assert(max_diff_t(-3) >> 2 == -1);
> +  static_assert(max_diff_t(-3) >> 10 == -1);
>    static_assert(max_diff_t(3) >> 1 == 1);
>    static_assert(max_diff_t(3) >> 2 == 0);
>  
> @@ -188,7 +195,7 @@ template<bool signed_p, bool shorten_p>
>  void
>  test02()
>  {
> -  using hw_type = std::conditional_t<signed_p, signed rep_t, rep_t>;
> +  using hw_type = std::conditional_t<signed_p, signed_rep_t, rep_t>;
>    using max_type = std::conditional_t<signed_p, max_diff_t, max_size_t>;
>    using shorten_type = std::conditional_t<shorten_p, hw_type, max_type>;
>    const int hw_type_bit_size = sizeof(hw_type) * __CHAR_BIT__;
> @@ -246,7 +253,7 @@ template<bool signed_p, bool toggle_base_p>
>  void
>  test03()
>  {
> -  using hw_type = std::conditional_t<signed_p, signed rep_t, rep_t>;
> +  using hw_type = std::conditional_t<signed_p, signed_rep_t, rep_t>;
>    using max_type = std::conditional_t<signed_p, max_diff_t, max_size_t>;
>    using base_type = std::conditional_t<toggle_base_p, hw_type, max_type>;
>    constexpr int hw_type_bit_size = sizeof(hw_type) * __CHAR_BIT__;
> -- 
> 2.40.0.374.g7580f92ffa
> 
>