Le 04/03/2021 à 20:24, Segher Boessenkool a écrit :
On Thu, Mar 04, 2021 at 09:54:44AM -0800, Nick Desaulniers wrote:
On Thu, Mar 4, 2021 at 9:42 AM Marco Elver <el...@google.com> wrote:
include/linux/compiler.h:246:
prevent_tail_call_optimization
commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")
https://github.com/linuxppc/linux/commit/a9a3ed1eff36
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.
In the commit mentionned at the top, it is said:
The next attempt to prevent compilers from tail-call optimizing
the last function call cpu_startup_entry(), ... , was to add an empty asm("").
This current solution was short and sweet, and reportedly, is supported
by both compilers but we didn't get very far this time: future (LTO?)
optimization passes could potentially eliminate this, which leads us
to the third attempt: having an actual memory barrier there which the
compiler cannot ignore or move around etc.
Christophe
