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Re: [PATCH] Fix more PR80928 fallout


On Fri, 23 Jun 2017, Jeff Law wrote:

> On 06/23/2017 05:39 AM, Richard Biener wrote:
> > 
> > SLP induction vectorization runs into the issue that it remembers
> > pointers to PHI nodes in the SLP tree during analysis.  But those
> > may get invalidated by loop copying (for prologue/epilogue peeling
> > or versioning) as the low-level CFG helper copy_bbs works in the
> > way of copying individual BBs plus their outgoing edges but with
> > old destinations and at the end re-directing the edges to the
> > desired location.  In SSA this triggers the whole machinery of
> > making room for new PHI nodes -- that is undesirable because it
> > causes re-allocation of PHI nodes in the set of source blocks.
> > 
> > After much pondering I arrived at the following (least ugly) solution
> > to this "problem" (well, I define it as a problem, it's at least
> > an inefficiency and a workaround in the vectorizer would be way
> > uglier).  Namely simply do not trigger the SSA machinery for
> > blocks with BB_DUPLICATED (I skimmed all other users and they seem
> > fine).
> > 
> > In the process I also implemented some poisoning of the old PHI node
> > when we reallocate (well, free) PHI nodes.  But that triggers some
> > other issues, one fixed by the tree-ssa-phionlycoprop.c hunk below.
> > So I'm not submitting it as part of this fix.
> > 
> > Bootstrapped (with the poisoning sofar, plain patch still running)
> > on x86_64-unknown-linux-gnu, testing in progress.
> > 
> > Comments welcome, testing won't finish before I leave for the
> > weekend.
> I fully support poisoning the old PHI nodes -- I tracked down a similar
> problem just a few months back that probably would have been obvious if
> we had poisoned the old nodes (79621 which is now a missed optimization
> bug).
> 
> I wouldn't be surprised if there's others lurking and given the general
> trend of using block duplication to enable various optimizations,
> catching this stuff early would definitely be good.

I've applied this fix.  For reference, below is what passed bootstrap
and regtest minus a few PRE related testsuite fallouts (stupid PRE
simple-minded DCE is giving me a hard time here ... the interaction
between remove_dead_inserted_code and el_to_remove is quite ugly).

When looking at this I also wondered why/if the cache of allocated
PHI nodes is worth the extra trouble of duplicated API like
remove_phi_node vs. gsi_remove.  Not something I have time right now
to clean up, so I'll sit on the patch below for some more time as well.

Richard.

Index: gcc/tree-phinodes.c
===================================================================
--- gcc/tree-phinodes.c	(revision 249638)
+++ gcc/tree-phinodes.c	(working copy)
@@ -67,7 +67,7 @@ along with GCC; see the file COPYING3.
    the -2 on all the calculations below.  */
 
 #define NUM_BUCKETS 10
-static GTY ((deletable (""))) vec<gimple *, va_gc> *free_phinodes[NUM_BUCKETS - 2];
+static GTY ((deletable (""))) vec<gphi *, va_gc> *free_phinodes[NUM_BUCKETS - 2];
 static unsigned long free_phinode_count;
 
 static int ideal_phi_node_len (int);
@@ -103,10 +103,10 @@ allocate_phi_node (size_t len)
 
   /* If our free list has an element, then use it.  */
   if (bucket < NUM_BUCKETS - 2
-      && gimple_phi_capacity ((*free_phinodes[bucket])[0]) >= len)
+      && (*free_phinodes[bucket]).last()->capacity >= len)
     {
       free_phinode_count--;
-      phi = as_a <gphi *> (free_phinodes[bucket]->pop ());
+      phi = free_phinodes[bucket]->pop ();
       if (free_phinodes[bucket]->is_empty ())
 	vec_free (free_phinodes[bucket]);
       if (GATHER_STATISTICS)
@@ -208,7 +208,7 @@ make_phi_node (tree var, int len)
 /* We no longer need PHI, release it so that it may be reused.  */
 
 static void
-release_phi_node (gimple *phi)
+release_phi_node (gphi *phi)
 {
   size_t bucket;
   size_t len = gimple_phi_capacity (phi);
@@ -220,6 +220,13 @@ release_phi_node (gimple *phi)
       imm = gimple_phi_arg_imm_use_ptr (phi, x);
       delink_imm_use (imm);
     }
+  if (flag_checking)
+    {
+      memset (phi, 0xfe, (sizeof (struct gphi)
+			  - sizeof (struct phi_arg_d)
+			  + sizeof (struct phi_arg_d) * len));
+      phi->capacity = len;
+    }
 
   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
   bucket -= 2;
@@ -438,7 +445,7 @@ remove_phi_args (edge e)
 void
 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 {
-  gimple *phi = gsi_stmt (*gsi);
+  gphi *phi = as_a <gphi *> (gsi_stmt (*gsi));
 
   if (release_lhs_p)
     insert_debug_temps_for_defs (gsi);
@@ -447,9 +454,9 @@ remove_phi_node (gimple_stmt_iterator *g
 
   /* 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.  */
Index: gcc/tree-ssa-pre.c
===================================================================
--- gcc/tree-ssa-pre.c	(revision 249638)
+++ gcc/tree-ssa-pre.c	(working copy)
@@ -4240,7 +4240,9 @@ eliminate_dom_walker::before_dom_childre
 	  if (may_propagate_copy (res, sprime))
 	    {
 	      /* Mark the PHI for removal.  */
-	      el_to_remove.safe_push (phi);
+	      if (inserted_exprs
+		  && !bitmap_bit_p (inserted_exprs, SSA_NAME_VERSION (res)))
+		el_to_remove.safe_push (phi);
 	      gsi_next (&gsi);
 	      continue;
 	    }


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