[PATCH] tree-optimization/114557 - reduce ehcleanup peak memory use

[Richard Biener <rguenther@suse.de>]

The following reduces peak memory use for the PR114480 testcase at -O1
which is almost exclusively spent by the ehcleanup pass in allocating
PHI nodes.  The free_phinodes cache we maintain isn't very effective
since it has effectively two slots, one for 4 and one for 9 argument
PHIs and it is only ever used for allocations up to 9 arguments but
we put all larger PHIs in the 9 argument bucket.  This proves
uneffective resulting in much garbage to be kept when incrementally
growing PHI nodes by edge redirection.

The mitigation is to rely on the GC freelist for larger sizes and
thus immediately return all larger bucket sized PHIs to it via ggc_free.

This reduces the peak memory use from 19.8GB to 11.3GB and compile-time
from 359s to 168s.

Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.

OK for trunk?  I'll leave more surgery for stage1.

Thanks,
Richard.

	PR tree-optimization/114557
	PR tree-optimization/114480
	* tree-phinodes.cc (release_phi_node): Return PHIs from
	allocation buckets not covered by free_phinodes to GC.
	(remove_phi_node): Release the PHI LHS before freeing the
	PHI node.
---
 gcc/tree-phinodes.cc | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/gcc/tree-phinodes.cc b/gcc/tree-phinodes.cc
index ddd731323e1..5a7e4a94e57 100644
--- a/gcc/tree-phinodes.cc
+++ b/gcc/tree-phinodes.cc
@@ -223,6 +223,14 @@ release_phi_node (gimple *phi)
       delink_imm_use (imm);
     }
 
+  /* Immediately return the memory to the allocator when we would
+     only ever re-use it for a smaller size allocation.  */
+  if (len - 2 >= NUM_BUCKETS - 2)
+    {
+      ggc_free (phi);
+      return;
+    }
+
   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
   bucket -= 2;
   vec_safe_push (free_phinodes[bucket], phi);
@@ -445,9 +453,9 @@ remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 
   /* If we are deleting the PHI node, then we should release the
      SSA_NAME node so that it can be reused.  */
-  release_phi_node (phi);
   if (release_lhs_p)
     release_ssa_name (gimple_phi_result (phi));
+  release_phi_node (phi);
 }
 
 /* Remove all the phi nodes from BB.  */
-- 
2.35.3