GMP test fails with -flto

[Vincent Lefevre]

On 2019-07-02 12:41:15 +0200, Torbjorn Granlund wrote:
> Richard Biener <richard.guenther at gmail.com> writes:
> 
>   On Mon, Jul 1, 2019 at 11:13 PM Torbjörn Granlund <tg at gmplib.org> wrote:
>   >
>   > Vincent Lefevre <vincent at vinc17.net> writes:
>   >
>   >   Yes, with LTO, the object file does not contain the structure as is.
>   >   Thus the detection from "od -b conftest.$OBJEXT" does not work.
>   >
>   > That could be solved by generating a final executable, right?
>   >
>   Yes.

I would say that this may not work due to optimization (even if the
structure is in a different file than main(), as LTO could optimize
anyway). If the structure is declared as volatile, I think that
this will always prevent related optimization, but I'm not sure.
Generating a library should always work, IMHO.

> Does the patch below make sense?
> 
> The main funcion is needed as we create an executable, its references to
> foo are needed, else LTO will remove foo altogether.
> 
> *** /tmp/extdiff.MsIr0g/gmp-main.3300fbb5d615/acinclude.m4	Mon Jun 17 00:11:50 2019
> --- /home/tege/prec/gmp-main/acinclude.m4	Tue Jul  2 12:37:53 2019
> ***************
> *** 3307,3313 ****
>     -123456789.0,
>     { '\376', '\334', '\272', '\230', '\166', '\124', '\062', '\020' },
> ! };]
>   EOF
> ! gmp_compile="$CC $CFLAGS $CPPFLAGS -c conftest.c >&AC_FD_CC 2>&1"
>   if AC_TRY_EVAL(gmp_compile); then
>   cat >conftest.awk <<\EOF
> --- 3307,3320 ----
>     -123456789.0,
>     { '\376', '\334', '\272', '\230', '\166', '\124', '\062', '\020' },
> ! };
> ! int main(){
> !   int i;
> !   for (i = 0; i < 8; i++) {
> !     printf ("%d %d %f\n", foo.before[i] + foo.after[i], foo.x);
> !   }

Here, after loop enrolling, the compiler could see that the arguments
are known and generate a fixed puts(). This kind of optimization must
not be done with a structure declared as volatile. But I think that
data in the structure could be rearranged (what matters is that when
run, the program will still do memory accesses and the behavior will
be the same); that's rather unlikely, though.

-- 
Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
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