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From: "lukas.graetz@tu-darmstadt.de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/38534] gcc 4.2.1 and above: No need to save
 called-saved registers in 'noreturn' function
Date: Thu, 01 Feb 2024 12:23:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D38534

--- Comment #25 from Lukas Gr=C3=A4tz <lukas.graetz@tu-darmstadt.de> ---
(In reply to Jakub Jelinek from comment #19)
> (In reply to H.J. Lu from comment #18)
> > (In reply to Jakub Jelinek from comment #17)
> > > E.g. shouldn't it at least be disabled for -O0 and -Og and shouldn't =
we
> >=20
> > We can disable this for -O0 and -Og.
>=20
> I think we should go for that.
>=20

This is independent from debugging, but I thought the patch was only meant =
for
-O3. Have you thought about the following situation:


Compile with gcc -O1 (and --fno-exception is implicit):
---- library.c ----------------
void __attribute__((noreturn))
foo(void (*bar)(void)) {
    ...
    bar();
    while (1);
}
---------------------


Compile with g++ (and -fexception is implicit):
---- app.c++ ----------------
#include <iostream>

extern void foo(void (*bar)(void));
extern void bar_throws_exception(void) throw ();

int main() {
    ...
    try
    {
        foo(bar_throws_exception);
    }
    catch (const std::exception& e)
    {
        ...
    }
}
----------------------


It is not hart to fill the ... to make it use some callee-saved registers (=
e.g.
with LOOP_BODY as in this issue report). And then the program would crash.

One might argue that either the library.c is to blame for the missing
-fexceptions? Or that the app.c++ is to blame because it should not call foo
with an argument function that might throw an exception? I am unsure if the=
 C++
standard actually forbids calling C library functions with argument functio=
ns
that might throw an exception.

So I think it would be safer to restrict the patch to -O3. But I really don=
't
know much about this.=