On 08/16/2017 08:44 AM, Jonathan Wakely wrote: > On 16 August 2017 at 15:40, Jonathan Wakely wrote: >> On 16 August 2017 at 15:27, Oleg Endo wrote: >>> On Wed, 2017-08-16 at 13:30 +0200, Paolo Carlini wrote: >>>> >>>> I didn't understand why we don't already handle the easy case: >>>> >>>> constexpr int* ptr = nullptr; >>>> delete ptr; >>>> >>> >>> What about overriding the global delete operator with some user defined >>> implementation? Is there something in the C++ standard that says the >>> invocation can be completely omitted, i.e. on which side of the call >>> the nullptr check is being done? >>> >>> One possible use case could be overriding the global delete operator to >>> count the number of invocations, incl. for nullptr. Not sure how >>> useful that is though. >> >> Users can replace the deallocation function "operator delete" but this >> is the delete operator ... a subtly different thing. >> >> Anyway, the standard says: >> >> "If the value of the operand of the delete-expression is a null >> pointer value, it is unspecified whether a deallocation function will >> be called as described above." >> >> So it's permitted to omit the call to operator delete. > > Before C++11 the call was required: > > "The delete-expression will call a deallocation function (3.7.3.2)." > > > This was changed by > http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#348 Which I think argues that we can safely remove a call to operator delete when we know the pointer is null. However, we can not assume that an object passed to operator delete is non-null.
ISTM this would be better implemented in the optimizers rather than in the front-end. tree-ssa-dce.c would seem fairly natural. The only wrinkle is we can't do it in C++03 or earlier mode. jeff
