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

[James Y Knight via llvm-dev]

The global constructor was removed by setting the initial value of "val" to
1 instead of 0. So, the behavior of this program is preserved. Doesn't look
like erroneous behavior.

On Thu, Jun 11, 2020 at 10:12 AM Jerome Forissier via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
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>
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