Question about sizeof after struct change

Jonathan Wakely jwakely.gcc@gmail.com
Mon Jan 6 11:03:00 GMT 2020


On Fri, 3 Jan 2020 at 16:09, Erick Ochoa wrote:
> Do you mean something like taking the address of a struct and adding an offset
> to access a field?

Yes, the following code is valid:

struct S { int unused; int used; };
static_assert( sizeof(S) == 8 );
S s[4];
int* s2_used = (int*)((char*)s + 20);

Accessing the field doesn't refer to S::used or S::unused directly,
and doesn't use sizeof(S) or sizeof(int) explicitly.

So if your pass removes S::unused, then you'd need to analyse
((char*)s + 20) to determine that it accesses s[2].used and then
adjust it to ((char*)s + 8) instead.



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