Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends

Segher Boessenkool
2021-03-04T19:30:09Z

On Thu, Mar 04, 2021 at 09:54:44AM -0800, Nick Desaulniers wrote:
> On Thu, Mar 4, 2021 at 9:42 AM Marco Elver <elver@google.com> wrote:
> include/linux/compiler.h:246:
> prevent_tail_call_optimization
>
> commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")

That is much heavier than needed (an mb()). You can just put an empty
inline asm after a call before a return, and that call cannot be
optimised to a sibling call: (the end of a function is an implicit
return:)

Instead of:

void g(void);
void f(int x)
if (x)
g();
}

Do:

void g(void);
void f(int x)
if (x)
g();
asm("");
}

This costs no extra instructions, and certainly not something as heavy
as an mb()! It works without the "if" as well, of course, but with it
it is a more interesting example of a tail call.

Segher