[PATCH GCC][4/4]Better handle store-stores chain if eliminated stores only store loop invariant

[Bin Cheng <Bin dot Cheng at arm dot com>]

Hi,
This is a followup patch better handling below case:
     for (i = 0; i < n; i++)
       {
	 a[i] = 1;
	 a[i+2] = 2;
       }
Instead of generating root variables by loading from memory and propagating with PHI
nodes, like:
     t0 = a[0];
     t1 = a[1];
     for (i = 0; i < n; i++)
       {
	 a[i] = 1;
	 t2 = 2;
	 t0 = t1;
	 t1 = t2;
       }
     a[n] = t0;
     a[n+1] = t1;
We can simply store loop invariant values after loop body if we know loop iterates more
than chain->length times, like:
     for (i = 0; i < n; i++)
       {
	 a[i] = 1;
       }
     a[n] = 2;
     a[n+1] = 2;

Bootstrap(O2/O3) in patch series on x86_64 and AArch64.  Is it OK?

Thanks,
bin
2017-06-21  Bin Cheng  <bin.cheng@arm.com>

	* tree-predcom.c: (struct chain): Handle store-store chain in which
	stores for elimination only store loop invariant values.
	(execute_pred_commoning_chain): Ditto.
	(prepare_initializers_chain_store_elim): Ditto.
	(prepare_finalizers): Ditto.
	(is_inv_store_elimination_chain): New function.
	(initialize_root_vars_store_elim_1): New function.

Attachment: inv-store-elimination-20170621.txt
Description: inv-store-elimination-20170621.txt