[gcc r12-5848] libstdc++: Fix undefined shift when _Atomic_word is 64-bit

[Jonathan Wakely]

https://gcc.gnu.org/g:c15aa46cca0649b68613d3292cf71c7cc57ef78f

commit r12-5848-gc15aa46cca0649b68613d3292cf71c7cc57ef78f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Dec 8 19:36:24 2021 +0000

    libstdc++: Fix undefined shift when _Atomic_word is 64-bit
    
    The check for _Atomic_word being 32-bit is just a normal runtime
    condition for C++11 and C++14, because it doesn't use if-constexpr. That
    means the 1LL << (CHAR_BIT * sizeof(_Atomic_word)) expression expands to
    1LL << 64 on Solaris, which is ill-formed.
    
    This adds another indirection so that the shift width is zero if the
    code is unreachable.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/shared_ptr_base.h (_Sp_counted_base::_M_release()):
            Make shift width conditional on __double_word condition.

Diff:
---
 libstdc++-v3/include/bits/shared_ptr_base.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
index 90ad30947b0..f315d8f354f 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -325,8 +325,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr bool __aligned = __alignof(long long) <= alignof(void*);
       if _GLIBCXX17_CONSTEXPR (__lock_free && __double_word && __aligned)
 	{
-	  constexpr long long __unique_ref
-	    = 1LL + (1LL << (__CHAR_BIT__ * sizeof(_Atomic_word)));
+	  constexpr int __wordbits = __CHAR_BIT__ * sizeof(_Atomic_word);
+	  constexpr int __shiftbits = __double_word ? __wordbits : 0;
+	  constexpr long long __unique_ref = 1LL + (1LL << __shiftbits);
 	  auto __both_counts = reinterpret_cast<long long*>(&_M_use_count);
 
 	  _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);