https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64477
--- Comment #3 from Vladimir Makarov <vmakarov at gcc dot gnu.org> --- I investigated this problem too. IRA in GCC-4.9 allocates memory to the pseudo in insn 2, then the memory slot is reloaded for insn 8. Post-reload optimization changes stack slot by register. GCC-4.8 IRA allocates a register to the pseudo. So the behaviour difference starts in IRA. I believe that DSE after RA should remove the store. But the DSE optimization algorithm is too simple. It does not work for example with stack-pointer addressed memory (only with frame-pointer addressed memory). And x86-64 uses a stack-based addresses for stack slots by default. Even if DSE were able to work with stack-based addresses, I guess it also can not handle stack slot sharing (it means we need CFG-sensitive global DSE). This particular problem could be solved in IRA or in DSE. I'll try to investigate what can I do to fix it in IRA. Still I belive we need much better RTL-level DSE (dealing with stack-addressed memory slots, global and CFG-sensitive DSE).
