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As mentioned in the PR, different standards have different definition on what is an UB left shift. They all agree on out of bounds (including negative) shift count. The rules used by ubsan are: C99-C2x ((unsigned) x >> (uprecm1 - y)) != 0 then UB C++11-C++17 x < 0 || ((unsigned) x >> (uprecm1 - y)) > 1 then UB C++20 and later everything is well defined Now, for C++20, I've in the P1236R1 implementation added an early exit for -Wshift-overflow* warning so that it never warns, but apparently -Wshift-negative-value remained as is. As it is well defined in C++20, the following patch doesn't enable -Wshift-negative-value from -Wextra anymore for C++20 and later, if users want for compatibility with C++17 and earlier get the warning, they still can by using -Wshift-negative-value explicitly. Another thing is -fwrapv, that is an extension to the standards, so it is up to us how exactly we define that case. Our ubsan code treats TYPE_OVERFLOW_WRAPS (type0) and cxx_dialect >= cxx20 the same as only diagnosing out of bounds shift count and nothing else and IMHO it is most sensical to treat -fwrapv signed left shifts the same as C++20 treats them, https://eel.is/c++draft/expr.shift#2 "The value of E1 << E2 is the unique value congruent to E1×2^E2 modulo 2^N, where N is the width of the type of the result. [Note 1: E1 is left-shifted E2 bit positions; vacated bits are zero-filled. — end note]" with no UB dependent on the E1 values. The UB is only "The behavior is undefined if the right operand is negative, or greater than or equal to the width of the promoted left operand." Under the hood (except for FEs and ubsan from FEs) GCC middle-end doesn't consider UB in left shifts dependent on the first operand's value, only the out of bounds shifts. While this change isn't a regression, I'd think it is useful for GCC 12, it doesn't add new warnings, but just removes warnings that aren't appropriate. 2022-03-09 Jakub Jelinek <jakub@redhat.com> PR c/104711 gcc/ * doc/invoke.texi (-Wextra): Document that -Wshift-negative-value is enabled by it only for C++11 to C++17 rather than for C++03 or later. (-Wshift-negative-value): Similarly (except here we stated that it is enabled for C++11 or later). gcc/c-family/ * c-opts.cc (c_common_post_options): Don't enable -Wshift-negative-value from -Wextra for C++20 or later. * c-ubsan.cc (ubsan_instrument_shift): Adjust comments. * c-warn.cc (maybe_warn_shift_overflow): Use TYPE_OVERFLOW_WRAPS instead of TYPE_UNSIGNED. gcc/c/ * c-fold.cc (c_fully_fold_internal): Don't emit -Wshift-negative-value warning if TYPE_OVERFLOW_WRAPS. * c-typeck.cc (build_binary_op): Likewise. gcc/cp/ * constexpr.cc (cxx_eval_check_shift_p): Use TYPE_OVERFLOW_WRAPS instead of TYPE_UNSIGNED. * typeck.cc (cp_build_binary_op): Don't emit -Wshift-negative-value warning if TYPE_OVERFLOW_WRAPS. gcc/testsuite/ * c-c++-common/Wshift-negative-value-1.c: Remove dg-additional-options, instead in target selectors of each diagnostic check for exact C++ versions where it should be diagnosed. * c-c++-common/Wshift-negative-value-2.c: Likewise. * c-c++-common/Wshift-negative-value-3.c: Likewise. * c-c++-common/Wshift-negative-value-4.c: Likewise. * c-c++-common/Wshift-negative-value-7.c: New test. * c-c++-common/Wshift-negative-value-8.c: New test. * c-c++-common/Wshift-negative-value-9.c: New test. * c-c++-common/Wshift-negative-value-10.c: New test. * c-c++-common/Wshift-overflow-1.c: Remove dg-additional-options, instead in target selectors of each diagnostic check for exact C++ versions where it should be diagnosed. * c-c++-common/Wshift-overflow-2.c: Likewise. * c-c++-common/Wshift-overflow-5.c: Likewise. * c-c++-common/Wshift-overflow-6.c: Likewise. * c-c++-common/Wshift-overflow-7.c: Likewise. * c-c++-common/Wshift-overflow-8.c: New test. * c-c++-common/Wshift-overflow-9.c: New test. * c-c++-common/Wshift-overflow-10.c: New test. * c-c++-common/Wshift-overflow-11.c: New test. * c-c++-common/Wshift-overflow-12.c: New test.
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