[llvm-dev] Issue with __attribute__((constructor)) and -Os -fno-common

[Jerome Forissier via llvm-dev]

Hi,

I think that Clang erroneously discards a function annotated with
__attribute__((constructor)) when flags -Os -fno-common are given. Test
case below.

What do you think?

Thanks.

----8<--------8<--------8<--------8<--------8<--------8<--------
$ cat ctor.c
int val;

static void __attribute__((constructor)) init_fn(void)
{
        val = 1;
}

int main(int argc, char *argv[])
{
        return val;
}
----8<--------8<--------8<--------8<--------8<--------8<--------

Here is what I observed:

- Clang (10.0.0-4ubuntu1) with -Os -fno-common: function init_fn() is
NOT emitted,
- Clang (10.0.0-4ubuntu1) with no flag, or only -Os or -fno-common:
init_fn() is present as expected,
- GCC (Ubuntu 9.3.0-10ubuntu1) with the same flags: init_fn() is present
too,
- Since https://reviews.llvm.org/D75056, -fno-common is the default and
therefore -Os is enough to cause the issue.

----8<--------8<--------8<--------8<--------8<--------8<--------
$ clang --target=arm-linux-gnueabihf -Os -fno-common -S ctor.c \
  -o /dev/stdout | grep init_fn
$ clang --target=arm-linux-gnueabihf -Os -S ctor.c \
  -o /dev/stdout | grep init_fn
        .p2align        2               @ -- Begin function init_fn
        .type   init_fn,%function
        .code   32                      @ @init_fn
init_fn:
        .size   init_fn, .Lfunc_end0-init_fn
        .long   init_fn(target1)
        .addrsig_sym init_fn
$ clang --target=arm-linux-gnueabihf -fno-common -S ctor.c \
  -o /dev/stdout | grep init_fn
        .p2align        2               @ -- Begin function init_fn
        .type   init_fn,%function
        .code   32                      @ @init_fn
init_fn:
        .size   init_fn, .Lfunc_end0-init_fn
        .long   init_fn(target1)
        .addrsig_sym init_fn
$ arm-linux-gnueabihf-gcc -Os -fno-common -S ctor.c \
  -o /dev/stdout | grep init_fn
        .type   init_fn, %function
init_fn:
        .size   init_fn, .-init_fn
        .word   init_fn(target1)
----8<--------8<--------8<--------8<--------8<--------8<--------

-- 
Jerome