| Summary: | [C++11] overflow in signed left shift isn't diagnosed | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Jeffrey Yasskin <jyasskin> |
| Component: | c++ | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED FIXED | ||
| Severity: | normal | CC: | daniel.kruegler, jason, msebor, richard-gccbugzilla |
| Priority: | P3 | Keywords: | accepts-invalid |
| Version: | 4.7.0 | ||
| Target Milestone: | --- | ||
| Host: | Target: | ||
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | 2012-02-04 00:00:00 | |
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Description
Jeffrey Yasskin
2012-02-04 01:16:27 UTC
In more detail: Under the C++11 FDIS, this is not a (core) constant expression due to the "— a result that is not mathematically defined or not in the range of representable values for its type;" bullet in [expr.const]p2. Under DR1313, that rule is generalized to all undefined behavior. And a constexpr function which can't produce a constant expression is ill-formed by [dcl.constexpr]p5. The problematic macro is extremely old. Using #define __glibcxx_min(T) \ (__glibcxx_signed (T) ? -__glibcxx_max(T) - 1 : (T)0) seems indeed ok to me. Did you actually test it with clang? And of course the interesting issue here is C++ front-end, which doesn't produce any diagnostics (thus nobody would ever notice the library issue with GCC only). Let's add Jason in CC for confirmation. I hadn't tested #define __glibcxx_min(T) \ (__glibcxx_signed (T) ? -__glibcxx_max(T) - 1 : (T)0) but now I have, and it works with clang. Author: paolo Date: Sun Feb 5 12:58:51 2012 New Revision: 183905 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=183905 Log: 2012-02-05 Jeffrey Yasskin <jyasskin@gcc.gnu.org> Paolo Carlini <paolo.carlini@oracle.com> PR libstdc++/52119 * include/std/limits (__glibcxx_min): Fix to avoid undefined behavior. Modified: trunk/libstdc++-v3/ChangeLog trunk/libstdc++-v3/include/std/limits This is now a C++ front-end issue. At the C++ meeting last week we changed that shift from undefined to implementation-defined, so there is no front end bug. Signed left shift which overflows the corresponding unsigned type is still undefined, so g++ should reject this: constexpr int n = 2 << 31; Good point. (In reply to Jeffrey Yasskin from comment #0) > numeric_limits<T>::min() is defined as (__glibcxx_signed (T) ? (T)1 << > __glibcxx_digits (T) : (T)0). Unfortunately, shifting into the sign bit is > undefined behavior (C++11[expr.shift]p2), and undefined behavior makes an > expression non-constant. MY apologies if I am missing something obvious, but what exactly makes shifting a 1 into the sign bit undefined behavior? Seems rather defined to me, assuming it's actually being using as a integral numerical data-type (in this case, 32-bit int, long), which would result in a signed numerical value.In other words, the sign bit, like any other bit, can only be 0 or 1, so I honestly do not understand how undefined behavior enters the equation, or what impact it may have. (In reply to ajf from comment #10) > MY apologies if I am missing something obvious, but what exactly makes > shifting a 1 into the sign bit undefined behavior? You're right, it isn't: 5.8/2: ... if E1 has a signed type and non-negative value, and E1 × 2^E2 is representable in the corresponding unsigned type of the result type, then that value, converted to the result type, is the resulting value .... http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3675.html#1457 changed this between C++11 and C++14. (In reply to Jeffrey Yasskin from comment #12) > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3675.html#1457 > changed this between C++11 and C++14. True, but the defect report applies to C++11. Whoops, I missed your comment 7 where you already knew the history. Sorry. Recent versions of G++ diagnose shifting into the sign bit (and reject it in constexpr contexts) so this looks resolved:
warning: result of ‘(2 << 31)’ requires 34 bits to represent, but ‘int’ only has 32 bits [-Wshift-overflow=]
int n = 2 << 31;
~~^~~~~
Fixed by r225998 PR c++/55095 * c-common.c (c_fully_fold_internal): Warn about left shift overflows. Use EXPR_LOC_OR_LOC. (maybe_warn_shift_overflow): New function. * c-common.h (maybe_warn_shift_overflow): Declare. * c-opts.c (c_common_post_options): Set warn_shift_overflow. * c.opt (Wshift-overflow): New option. * c-typeck.c (digest_init): Pass OPT_Wpedantic to pedwarn_init. (build_binary_op): Warn about left shift overflows. * typeck.c (cp_build_binary_op): Warn about left shift overflows. * doc/invoke.texi: Document -Wshift-overflow and -Wshift-overflow=. Author: redi Date: Fri May 3 19:13:31 2019 New Revision: 270858 URL: https://gcc.gnu.org/viewcvs?rev=270858&root=gcc&view=rev Log: Avoid -Woverflow warning in __numeric_limits_integer This is the same fix as was done for std::numeric_limits in r183905. PR libstdc++/52119 * include/ext/numeric_traits.h (__glibcxx_min): Avoid integer overflow warning with -Wpedantic -Wsystem-headers. Modified: trunk/libstdc++-v3/ChangeLog trunk/libstdc++-v3/include/ext/numeric_traits.h |