| Message ID | 20170110225510.GE21933@tucnak |
|---|---|
| State | New |
| Headers | show |
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 <jakub@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 > >
--- 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; +}