On Tue, 10 Jan 2017, Jakub Jelinek wrote:
> Hi!
>
> LTO partitioning can actually take appart uses of STRING_CSTs or other
> constants and put those into multiple partitions. When -fno-merge-constants
> is in effect, it means those constants aren't merged by the linker and
> e.g. following testcase fails.
>
> I fail to see what -flto -fno-merge-constants would be good for,
> -fno-merge-constants can be used to decrease link time, or when producing
> assembly and parsing something in it, but both cases don't make much sense
> with LTO. So for now the patch just forces constant merging when in lto.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Ok.
Richard.
> 2017-01-10 Jakub Jelinek <ja...@redhat.com>
>
> PR middle-end/50199
> * lto-lang.c (lto_post_options): Force flag_merge_constants = 1
> if it was 0.
>
> * gcc.dg/lto/pr50199_0.c: New test.
>
> --- gcc/lto/lto-lang.c.jj 2017-01-01 12:45:47.000000000 +0100
> +++ gcc/lto/lto-lang.c 2017-01-10 14:37:20.158387918 +0100
> @@ -857,6 +857,12 @@ lto_post_options (const char **pfilename
> support. */
> flag_excess_precision_cmdline = EXCESS_PRECISION_FAST;
>
> + /* When partitioning, we can tear appart STRING_CSTs uses from the same
> + TU into multiple partitions. Without constant merging the constants
> + might not be equal at runtime. See PR50199. */
> + if (!flag_merge_constants)
> + flag_merge_constants = 1;
> +
> /* Initialize the compiler back end. */
> return false;
> }
> --- gcc/testsuite/gcc.dg/lto/pr50199_0.c.jj 2017-01-10 14:44:06.276071432
> +0100
> +++ gcc/testsuite/gcc.dg/lto/pr50199_0.c 2017-01-10 14:43:49.000000000
> +0100
> @@ -0,0 +1,17 @@
> +/* PR middle-end/50199 */
> +/* { dg-lto-options {{-O2 -flto -fno-merge-constants
> --param=lto-min-partition=1}} } */
> +
> +__attribute__ ((noinline)) const char *
> +foo (const char *x)
> +{
> + return x;
> +}
> +
> +int
> +main ()
> +{
> + const char *a = "ab";
> + if (a != foo (a))
> + __builtin_abort ();
> + return 0;
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
