Avoid undefined shifts in ceil2 operations

For values where the result cannot be represented the shift width would
be equal to the width of the type, which is undefined. Perform two
well-defined shifts instead of one possible undefined shift.

	* include/bits/hashtable_policy.h (__clp2): Fix calculation for LLP64
	targets where sizeof(size_t) > sizeof(long). Avoid undefined shifts
	of the number of bits in the type.
	* include/std/bit (__ceil2): Avoid undefined shifts.
	* testsuite/26_numerics/bit/bit.pow.two/ceil2.cc: Test values with
	the most signifiant bit set.

From-SVN: r263986

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index a198549f0f3120fe78e90c5dd5fcad213dd539cd..98288521abd41eaa7b06af546bb058af039375ef 100644 (file)
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,5 +1,12 @@
 2018-08-30  Jonathan Wakely  <jwakely@redhat.com>
 
+       * include/bits/hashtable_policy.h (__clp2): Fix calculation for LLP64
+       targets where sizeof(size_t) > sizeof(long). Avoid undefined shifts
+       of the number of bits in the type.
+       * include/std/bit (__ceil2): Avoid undefined shifts.
+       * testsuite/26_numerics/bit/bit.pow.two/ceil2.cc: Test values with
+       the most signifiant bit set.
+
        * config/abi/pre/gnu.ver: Add missing exports for mingw.
 
        * include/ext/pointer.h (_Pointer_adapter): Define operators for

diff --git a/libstdc++-v3/include/bits/hashtable_policy.h b/libstdc++-v3/include/bits/hashtable_policy.h
index d7497711071bc02278dcb4fcbfbe79f7968bfb38..66fbfbe5f21746566abd6f608976a1f9bfa8056e 100644 (file)
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -511,8 +511,11 @@ namespace __detail
     // Equivalent to return __n ? std::ceil2(__n) : 0;
     if (__n < 2)
       return __n;
-    return 1ul << (numeric_limits<unsigned long>::digits
-                   - __builtin_clzl(__n - 1ul));
+    const unsigned __lz = sizeof(size_t) > sizeof(long)
+      ? __builtin_clzll(__n - 1ull)
+      : __builtin_clzl(__n - 1ul);
+    // Doing two shifts avoids undefined behaviour when __lz == 0.
+    return (size_t(1) << (numeric_limits<size_t>::digits - __lz - 1)) << 1;
   }
 
   /// Rehash policy providing power of 2 bucket numbers. Avoids modulo

diff --git a/libstdc++-v3/include/std/bit b/libstdc++-v3/include/std/bit
index 0aebac2875861a7983a7e1f5dd59ffaf434e6350..bc2ade75b3586dd819f2328806f93af5a1cee2bf 100644 (file)
--- a/libstdc++-v3/include/std/bit
+++ b/libstdc++-v3/include/std/bit
@@ -195,9 +195,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     __ceil2(_Tp __x) noexcept
     {
       constexpr auto _Nd = numeric_limits<_Tp>::digits;
-      if (__x == 0)
+      if (__x == 0 || __x == 1)
         return 1;
-      return (_Tp)1u << (_Nd - std::__countl_zero((_Tp)(__x - 1u)));
+      const unsigned __n = _Nd - std::__countl_zero((_Tp)(__x - 1u));
+      const _Tp __y_2 = (_Tp)1u << (__n - 1u);
+      return __y_2 << 1u;
     }
 
   template<typename _Tp>

diff --git a/libstdc++-v3/testsuite/26_numerics/bit/bit.pow.two/ceil2.cc b/libstdc++-v3/testsuite/26_numerics/bit/bit.pow.two/ceil2.cc
index 65e1569c2778eddfaee946f2a7d79fc98a48435a..e41f82c8bb8314df816aa33ff9f0baa4085821a8 100644 (file)
--- a/libstdc++-v3/testsuite/26_numerics/bit/bit.pow.two/ceil2.cc
+++ b/libstdc++-v3/testsuite/26_numerics/bit/bit.pow.two/ceil2.cc
@@ -55,6 +55,14 @@ test(UInt x)
     static_assert( std::ceil2(UInt(3) << 64) == (UInt(4) << 64) );
   }
 
+  constexpr UInt msb = UInt(1) << (std::numeric_limits<UInt>::digits - 1);
+  static_assert( std::ceil2( msb ) == msb );
+  // Larger values cannot be represented so the return value is unspecified,
+  // but must still be valid in constant expressions, i.e. not undefined.
+  static_assert( std::ceil2( UInt(msb + 1) ) != 77 );
+  static_assert( std::ceil2( UInt(msb + 2) ) != 77 );
+  static_assert( std::ceil2( UInt(msb + 77) ) != 77 );
+
   return true;
 }