Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 2 Mar 2011 11:48:03 -0800 | Subject | Re: [PATCH v3 0/4] exec: unify native/compat code |
| |
On Wed, Mar 2, 2011 at 11:40 AM, David Miller <davem@davemloft.net> wrote: > > We purposely don't do that "page table entry typedef'd to aggregate" stuff > on sparc32 because otherwise such values get passed on the stack. > > Architectures can currently avoid this bad code generation for the > page table case, but with this new code they won't be able to avoid > pass-by-value.
Well, the thing is, on architectures that _can_ pass by value, it avoids one indirection.
And if you do pass it on stack, then the code generated will be the same as if we passed a pointer. So sparc may not be able to take advantage of the optimization, but I don't think the code generation would be worse.
For the page table case, we don't have that kind of trade-off: the trade-off there is literally just between "pass in registers, or pass on stack". Here the trade-off is "pass as an aggregate value or pass as a pointer to an aggregate value".
That said, since I suspect that the main user will always just get inlined (ie the helper function that actually fetches the pointers), I suspect even sparc will see the advantage of the pass-by-value model.
But you might want to actually test the difference and look at the code generation.
Linus
| |