Issue 0463: Left-shifting into the sign bit

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: WG14, Aaron Ballman
Date: 2014-04-02
Reference document: N1817
Submitted against: C11 / C17
Status: Closed
Converted from: n2396.htm

Summary

Harmonizing left-shift with C++14

It is not uncommon to see code such as:

signed someint_t min_value = 1 << (CHAR_BIT * sizeof(someint_t));

However, left-shifting a one bit into the sign bit is undefined behavior, despite the fact that the majority of (twos-complement) architectures handle it properly.

Suggested Technical Corrigendum

6.5.7p4 should be modified to read:

The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has an unsigned type, the value of the result is E1 x 2E2, reduced modulo one more than the maximum value representable in the result type. If E1 has a signed type and nonnegative value, and E1 x 2E2 is representable in the corresponding unsigned type of the result type, then that value, converted to the result type, is the resulting value; otherwise, the behavior is undefined.

C++ addressed this in C++14 with DR1457 with identical wording modifications.

Comment from WG14 on 2015-04-17:

Apr 2014 meeting

Proposed Committee Response

This is not a defect.

The committee will track this and consider it for the next revision of the standard.