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908edc1309
N3517 (array subscripting without decay) has been added to C2y (via a remote vote in May, not at a meeting). Implement this in GCC. The conceptual change, that the array subscripting operator [] no longer involves an array operand decaying to a pointer, is something GCC has done for a very long time. The main effect in terms of what is made possible in the language, subscripting a register array (undefined behavior in C23 and before), was available as a GNU extension, but only with constant indices. There is also a new constraint that array indices must not be negative when they are integer constant expressions and the array operand has array type (negative indices are fine with pointers) - an access out of bounds of an array (even when contained within a larger object) has undefined behavior at runtime when not a constraint violation. Thus, the previous GCC extension is adapted to allow the cases of register arrays not previously allowed, clearing DECL_REGISTER on them as needed (similar to what is done with register declarations of structures with volatile members) and restricting the pedwarn to pedwarn_c23. That pedwarn_c23 is also extended to cover the C23 case of register compound literals (although not strictly needed since it was undefined behavior rather than a constraint violation in C23). The new error is added (only for flag_isoc2y) for negative array indices with an operand of array type. N3517 has some specific wording about the type of the result of non-lvalue array element access. It's unclear what's actually desired there in the case where the array element is itself of array type; see C23 issue 1001 regarding types of qualified members of rvalue structures and unions more generally. Rather than implementing the specific wording about this in N3517, that is deferred until there's an accepted resolution to issue 1001 and can be dealt with as part of implementing such a resolution. Nothing specific is done about the obsolescence in that paper of writing index[array] or index[pointer] as opposed to array[index] or pointer[index], although that seems like a reasonable enough thing to warn about. Bootstrapped with no regressions for x86_64-pc-linux-gnu. gcc/c/ * c-typeck.cc (c_mark_addressable): New parameter override_register. (build_array_ref): Update calls to c_mark_addressable. Give error in C2Y mode for negative array indices when array expression is an array not a pointer. Use pedwarn_c23 for subscripting register array; diagnose that also for register compound literal. * c-tree.h (c_mark_addressable): Update prototype. gcc/testsuite/ * gcc.dg/c23-array-negative-1.c, gcc.dg/c23-register-array-1.c, gcc.dg/c23-register-array-2.c, gcc.dg/c23-register-array-3.c, gcc.dg/c23-register-array-4.c, gcc.dg/c2y-array-negative-1.c, gcc.dg/c2y-register-array-2.c, gcc.dg/c2y-register-array-3.c: New tests. |
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