Re: Try to update dominance info in tree-call-cdce.c

[Richard Sandiford <richard dot sandiford at arm dot com>]

Steven Bosscher <stevenb.gcc@gmail.com> writes:
> On Fri, Oct 30, 2015 at 2:11 PM, Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>> Is the split_block change really so bad?
>
> IMHO: Yes.

Fair enough :-)

> split_block just splits some basic block B into two blocks B1/B2
> somewhere in the middle of B. The dominance relations between B1 and
> B2 are obvious and intuitive. The new flag to split_block is not. The
> dominance info is not updated but the loops are? Confusing, if you ask
> me...
>
> Your situation, where a pass knows the dominance relations will
> change, is unusual. The extra work to fix up the ET tree twice should
> be negligible anyway.

I just thought that the cfg was in practice never going to stay in the
state that it is on return from split_block.  The caller is bound to
add (at least) another edge somewhere.

Here's a version that works with split_block rather than against it.
Tested on x86_64-linux-gnu, arm-linux-gnueabi and aarch64-linux-gnu.
OK to install?

Thanks,
Richard

    
gcc/
    	* tree-call-cdce.c (shrink_wrap_one_built_in_call): Try to update
    	the dominance info; free it if we can't.
    	(pass_call_cdce::execute): Don't free the dominance info here.

diff --git a/gcc/tree-call-cdce.c b/gcc/tree-call-cdce.c
index ffc1c4e..a5f38ce 100644
--- a/gcc/tree-call-cdce.c
+++ b/gcc/tree-call-cdce.c
@@ -731,6 +731,32 @@ shrink_wrap_one_built_in_call (gcall *bi_call)
   if (nconds == 0)
     return false;
 
+  /* The cfg we want to create looks like this:
+
+	   [guard n-1]         <- guard_bb (old block)
+	     |    \
+	     | [guard n-2]                   }
+	     |    / \                        }
+	     |   /  ...                      } new blocks
+	     |  /  [guard 0]                 }
+	     | /    /   |                    }
+	    [ call ]    |     <- bi_call_bb  }
+	     | \        |
+	     |  \       |
+	     |   [ join ]     <- join_tgt_bb (old iff call must end bb)
+	     |
+	 possible EH edges (only if [join] is old)
+
+     When [join] is new, the immediate dominators for these blocks are:
+
+     1. [guard n-1]: unchanged
+     2. [call]: [guard n-1]
+     3. [guard m]: [guard m+1] for 0 <= m <= n-2
+     4. [join]: [guard n-1]
+
+     We punt for the more complex case case of [join] being old and
+     simply free the dominance info.  We also punt on postdominators,
+     which aren't expected to be available at this point anyway.  */
   bi_call_bb = gimple_bb (bi_call);
 
   /* Now find the join target bb -- split bi_call_bb if needed.  */
@@ -741,6 +767,7 @@ shrink_wrap_one_built_in_call (gcall *bi_call)
       join_tgt_in_edge_from_call = find_fallthru_edge (bi_call_bb->succs);
       if (join_tgt_in_edge_from_call == NULL)
         return false;
+      free_dominance_info (CDI_DOMINATORS);
     }
   else
     join_tgt_in_edge_from_call = split_block (bi_call_bb, bi_call);
@@ -820,6 +847,15 @@ shrink_wrap_one_built_in_call (gcall *bi_call)
       guard_bb_in_edge->count = guard_bb->count - bi_call_in_edge->count;
     }
 
+  if (dom_info_available_p (CDI_DOMINATORS))
+    {
+      /* The split_blocks leave [guard 0] as the immediate dominator
+	 of [call] and [call] as the immediate dominator of [join].
+	 Fix them up.  */
+      set_immediate_dominator (CDI_DOMINATORS, bi_call_bb, guard_bb);
+      set_immediate_dominator (CDI_DOMINATORS, join_tgt_bb, guard_bb);
+    }
+
   if (dump_file && (dump_flags & TDF_DETAILS))
     {
       location_t loc;
@@ -927,7 +963,6 @@ pass_call_cdce::execute (function *fun)
 
   if (something_changed)
     {
-      free_dominance_info (CDI_DOMINATORS);
       free_dominance_info (CDI_POST_DOMINATORS);
       /* As we introduced new control-flow we need to insert PHI-nodes
          for the call-clobbers of the remaining call.  */