Messages in this thread | | | Date | Tue, 6 Sep 2022 10:30:47 -0500 | From | Segher Boessenkool <> | Subject | Re: [PATCH net-next v4] net: skb: prevent the split of kfree_skb_reason() by gcc |
| |
On Tue, Sep 06, 2022 at 02:37:47PM +0200, Florian Weimer wrote: > > On Mon, Aug 22, 2022 at 4:01 PM Florian Weimer <fweimer@redhat.com> wrote: > > I did some research on the 'sibcalls' you mentioned above. Feel like > > It's a little similar to 'inline', and makes the callee use the same stack > > frame with the caller, which obviously will influence the result of > > '__builtin_return_address'.
Sibling calls are essentially calls that can be replaced by jumps (aka "tail call"), without needing a separate entry point to the callee.
Different targets can have a slightly different implementation and definition of what exactly is a sibling call, but that's the gist.
> > Hmm......but I'm not able to find any attribute to disable this optimization. > > Do you have any ideas? > > Unless something changed quite recently, GCC does not allow disabling > the optimization with a simple attribute (which would have to apply to > function pointers as well, not functions).
It isn't specified what a sibling call exactly *is*, certainly not on C level (only in the generated machine code), and the details differs per target.
> asm ("") barriers that move > out a call out of the tail position are supposed to prevent the > optimization.
Not just "supposed": they work 100%. The asm has to stay after the function call by the fundamental rules of C (the function call having a sequence point, and the asm a side effect).
void g(void); void f(void) { g(); // This can not be optimised to a jump... asm(""); // ... because it has to stay before this. }
Segher
| |