Daniel Berlin wrote:
If you built an AST that included sizeof before doing template
instantiation (which may not even be possible), you could at least
determine whether sizeof was used on a given type other than in a
malloc/new call (which we replace anyway).
Even if you did, I am allowed to know the ABI.
I can know that struct S has size 12 on this ABI, and do things like:
struct S a[3];
struct S* p = &a[0];
/* This points at a[1]. */
struct S* q = ((char *)p) + 12;
You cannot shrink S in that situation. In C, you also have to handle
the "common prefix" problem, where the members of S are the first few
members of a larger structure T. I'm then allowed to cast between an S*
and a T*, and if you shrink S, things will break.
So, these kinds of optimizations, where you rip fields out of a
structure are only safe if you can prove that addresses don't escape --
or if the user explicitly tells you that they are.
--
Mark Mitchell
CodeSourcery
[EMAIL PROTECTED]
(650) 331-3385 x713
