On 2020-02-03 01:49 -0800, J Decker wrote: > On Mon, Feb 3, 2020 at 1:41 AM Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> wrote: > > > On 2020-02-03 01:34 -0800, J Decker wrote: > > > On Mon, Feb 3, 2020 at 1:29 AM aotto <aotto1968@xxxxxxxxxxx> wrote: > > > > > > > Hi, > > > > > > > > this is a test: https://godbolt.org/z/8PpX8W > > > > > > > > > > why is method_base(&aO->obj, 1); so much worse than method_base(aO, 1); > > > it's the same resulting code... > > > > > > > > > method_base(aO, 1); > > > mov rax, QWORD PTR [rbp-8] > > > mov esi, 1 > > > mov rdi, rax > > > call method_base > > > > > > method_base(&aO->obj, 1); > > > mov rax, QWORD PTR [rbp-8] > > > mov esi, 1 > > > mov rdi, rax > > > call method_base > > > > > > and there's no forced cast.... > > > > If a0 is NULL, the standard specifies "(base *) a0" will also be NULL > > (6.3.2.3 > > p4). But, &a0->obj is undefined behavior. > > > > While true; that's not necessarily any sort of issue... other than perhaps > finding words to describe it. > It doesn't require dereferencing the pointer to get the value that is > 'obj'. It's just (address that is a pointer of some type) + ( offsetof( > type, member ) ) which does yield a non-NULL result After reading the standard more carefully I found you are right... It's not undefined, if a0 is NULL but the value of a0->obj is not used. Only actually using the value is undefined. Sorry for being so paranoid :(. -- Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
