https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70825
ktkachov at gcc dot gnu.org changed:
What |Removed |Added
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CC| |ktkachov at gcc dot gnu.org
--- Comment #5 from ktkachov at gcc dot gnu.org ---
I've looked at RTL dce a bit and the reason it doesn't remove the store is
because the MEM rtx used in the atomic instruction pattern is volatile andalso
has the alias set ALIAS_SET_MEMORY_BARRIER associated with it.
When the dse pass sees either of those it inserts a "wild read" into its
calculations indicating that a memory happened from potentially any location,
thus the stack store is potentially not dead and can't be eliminated.
I've confirmed this by hacking get_builtin_sync_mem from builtins.c (that
creates that mem rtx) to not set MEM_VOLATILE and the alias set to not
ALIAS_SET_MEMORY_BARRIER. With those changes I see the stack store being
eliminated by dse.
So the same information that's used to prevent the compiler from moving memory
instructions across these atomic operations prevents it from eliminating the
preceding stack store.
