Re: [PATCH v1 1/2] LoongArch: Define barrier_before_unreachable() as empty

[Xi Ruoyao]

On Wed, 2024-08-21 at 15:37 +0800, Huacai Chen wrote:
> > I am not sure whether the GCC bug has been fixed, I can not find the
> > fixup in the link https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365
> > and in the GCC repo. So I am not sure whether it is time and proper
> > to remove this workaround in the common header totally, just remove
> > it in the arch specified header when compiling kernel with a newer
> > GCC version (for example GCC 12.1 or higher on LoongArch) at least.

> What's your opinion? From my point of view, this GCC bug hasn't been
> fixed. So there may still be potential problems.

I'm pretty sure it isn't fixed.  Using the test case from the bug
report:

struct i2c_board_info {
        char type[20];
        char pad[100];
};

#ifdef NONORETURN
void fortify_panic();
#else
void fortify_panic() __attribute__((noreturn));
#endif


int f(int a)
{
  if (a)
    fortify_panic();
}


void i2c_new_device(struct i2c_board_info *);
int em28xx_dvb_init(int model, int a, int b, int c, int d)
{
        switch (model) {
        case 1:{
                        struct i2c_board_info info = {};
                        f(a);
                        i2c_new_device(&info);
                        break;
                }
        case 2:{
                        struct i2c_board_info info = {};
                        f(b);
                        i2c_new_device(&info);
                        break;
                }
        case 3:{
                        struct i2c_board_info info = { };
                        f(c);
                        i2c_new_device(&info);
                        break;
                }
        case 4:{
                        struct i2c_board_info info = { };
                        f(d);
                        i2c_new_device(&info);
                        break;
                }
        }
        return 0;
}

$ cc -v
Using built-in specs.
COLLECT_GCC=cc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/loongarch64-unknown-linux-gnu/14.2.0/lto-wrapper
Target: loongarch64-unknown-linux-gnu
Configured with: ../configure --prefix=/usr LD=ld --enable-languages=c,c++ --enable-default-pie --enable-default-ssp --disable-multilib --with-build-config=bootstrap-lto --disable-fixincludes --with-system-zlib --enable-host-pie
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 14.2.0 (GCC) 
$ cc t.c -S -Wframe-larger-than=1 -DNONORETURN -O2
t.c: In function 'em28xx_dvb_init':
t.c:50:1: warning: the frame size of 144 bytes is larger than 1 bytes [-Wframe-larger-than=]
   50 | }
      | ^
$ cc t.c -S -Wframe-larger-than=1 -O2            
t.c: In function 'em28xx_dvb_init':
t.c:50:1: warning: the frame size of 512 bytes is larger than 1 bytes [-Wframe-larger-than=]
   50 | }
      | ^

And I'm puzzled why "unreachable instruction" is not a problem on x86?

-- 
Xi Ruoyao <xry111@xxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University