diff mbox series

[committed] More gimple const/copy propagation opportunities

Message ID 6baf42b9-0534-dc81-7a54-11317c732a68@ventanamicro.com
State New
Headers show
Series [committed] More gimple const/copy propagation opportunities | expand

Commit Message

Jeff Law Sept. 30, 2022, 11:32 p.m. UTC 
While investigating a benchmark for optimization opportunities I came 
across single block loop which either iterates precisely once or 
forever.    This is an interesting scenario as we can ignore the 
infinite looping path and treat any PHI nodes as degenerates. So more 
concretely let's consider this trivial testcase:

volatile void abort (void);

void
foo(int a)
{
  int b = 0;

  while (1)
    {
      if (!a)
        break;
      b = 1;
    }

  if (b != 0)
    abort ();
}




Quick analysis shows that b's initial value is 0 and its value only 
changes if we enter an infinite loop.  So if we get to the test b != 0, 
the only possible value b could have would be 0 and the test and its 
true arm can be eliminated.


The DOM3 dump looks something like this:

;;   basic block 2, loop depth 0, count 118111600 (estimated locally), 
maybe hot
;;    prev block 0, next block 3, flags: (NEW, VISITED)
;;    pred:       ENTRY [always]  count:118111600 (estimated locally) 
(FALLTHRU,EXECUTABLE)
;;    succ:       3 [always]  count:118111600 (estimated locally) 
(FALLTHRU,EXECUTABLE)

;;   basic block 3, loop depth 1, count 1073741824 (estimated locally), 
maybe hot
;;    prev block 2, next block 4, flags: (NEW, VISITED)
;;    pred:       2 [always]  count:118111600 (estimated locally) 
(FALLTHRU,EXECUTABLE)
;;                3 [89.0% (guessed)]  count:955630224 (estimated 
locally) (FALSE_VALUE,EXECUTABLE)
   # b_1 = PHI <0(2), 1(3)>
   if (a_3(D) == 0)
     goto <bb 4>; [11.00%]
   else
     goto <bb 3>; [89.00%]
;;    succ:       4 [11.0% (guessed)]  count:118111600 (estimated 
locally) (TRUE_VALUE,EXECUTABLE)
;;                3 [89.0% (guessed)]  count:955630224 (estimated 
locally) (FALSE_VALUE,EXECUTABLE)

;;   basic block 4, loop depth 0, count 118111600 (estimated locally), 
maybe hot
;;    prev block 3, next block 5, flags: (NEW, VISITED)
;;    pred:       3 [11.0% (guessed)]  count:118111600 (estimated 
locally) (TRUE_VALUE,EXECUTABLE)
   if (b_1 != 0)
     goto <bb 5>; [0.00%]
   else
     goto <bb 6>; [100.00%]
;;    succ:       5 [never]  count:0 (precise) (TRUE_VALUE,EXECUTABLE)
;;                6 [always]  count:118111600 (estimated locally) 
(FALSE_VALUE,EXECUTABLE)

This is a good representative of what the benchmark code looks like.


The primary effect we want to capture is to realize that the test if 
(b_1 != 0) is always false and optimize it accordingly.


In the benchmark, this opportunity is well hidden until after the loop 
optimizers have completed, so the first chance to capture this case is 
in DOM3.  Furthermore, DOM wants loops normalized with latch 
blocks/edges.  So instead of bb3 looping back to itself, there's an 
intermediate empty block during DOM.

I originally thought this was likely to only affect the benchmark.  But 
when I instrumented the optimization and bootstrapped GCC, much to my 
surprise there were several hundred similar cases identified in GCC 
itself.  So it's not as benchmark specific as I'd initially feared.


Anyway, detecting this in DOM is pretty simple.   We detect the infinite 
loop, including the latch block.  Once we've done that, we walk the PHI 
nodes and attach equivalences to the appropriate outgoing edge.   That's 
all we need to do as the rest of DOM is already prepared to handle 
equivalences on edges.


Pushed to the trunk,


Jeff
commit 1214196da79aabbe5c14ed36e5a28012e141f04c
Author: Jeff Law <jeffreyalaw@gmail.com>
Date:   Fri Sep 30 19:26:31 2022 -0400

    More gimple const/copy propagation opportunities
    
    While investigating a benchmark for optimization opportunities I came across single block loop which either iterates precisely once or forever.    This is an interesting scenario as we can ignore the infinite looping path and treat any PHI nodes as degenerates.  So more concretely let's consider this trivial testcase:
    
    volatile void abort (void);
    
    void
    foo(int a)
    {
     int b = 0;
    
     while (1)
       {
         if (!a)
           break;
         b = 1;
       }
    
     if (b != 0)
       abort ();
    }
    
    Quick analysis shows that b's initial value is 0 and its value only changes if we enter an infinite loop.  So if we get to the test b != 0, the only possible value b could have would be 0 and the test and its true arm can be eliminated.
    
    The DOM3 dump looks something like this:
    
    ;;   basic block 2, loop depth 0, count 118111600 (estimated locally), maybe hot
    ;;    prev block 0, next block 3, flags: (NEW, VISITED)
    ;;    pred:       ENTRY [always]  count:118111600 (estimated locally) (FALLTHRU,EXECUTABLE)
    ;;    succ:       3 [always]  count:118111600 (estimated locally) (FALLTHRU,EXECUTABLE)
    
    ;;   basic block 3, loop depth 1, count 1073741824 (estimated locally), maybe hot
    ;;    prev block 2, next block 4, flags: (NEW, VISITED)
    ;;    pred:       2 [always]  count:118111600 (estimated locally) (FALLTHRU,EXECUTABLE)
    ;;                3 [89.0% (guessed)]  count:955630224 (estimated locally) (FALSE_VALUE,EXECUTABLE)
      # b_1 = PHI <0(2), 1(3)>
      if (a_3(D) == 0)
        goto <bb 4>; [11.00%]
      else
        goto <bb 3>; [89.00%]
    ;;    succ:       4 [11.0% (guessed)]  count:118111600 (estimated locally) (TRUE_VALUE,EXECUTABLE)
    ;;                3 [89.0% (guessed)]  count:955630224 (estimated locally) (FALSE_VALUE,EXECUTABLE)
    
    ;;   basic block 4, loop depth 0, count 118111600 (estimated locally), maybe hot
    ;;    prev block 3, next block 5, flags: (NEW, VISITED)
    ;;    pred:       3 [11.0% (guessed)]  count:118111600 (estimated locally) (TRUE_VALUE,EXECUTABLE)
      if (b_1 != 0)
        goto <bb 5>; [0.00%]
      else
        goto <bb 6>; [100.00%]
    ;;    succ:       5 [never]  count:0 (precise) (TRUE_VALUE,EXECUTABLE)
    ;;                6 [always]  count:118111600 (estimated locally) (FALSE_VALUE,EXECUTABLE)
    
    This is a good representative of what the benchmark code looks like.
    
    The primary effect we want to capture is to realize that the test if (b_1 != 0) is always false and optimize it accordingly.
    
    In the benchmark, this opportunity is well hidden until after the loop optimizers have completed, so the first chance to capture this case is in DOM3.  Furthermore, DOM wants loops normalized with latch blocks/edges.  So instead of bb3 looping back to itself, there's an intermediate empty block during DOM.
    
    I originally thought this was likely to only affect the benchmark.  But when I instrumented the optimization and bootstrapped GCC, much to my surprise there were several hundred similar cases identified in GCC itself.  So it's not as benchmark specific as I'd initially feared.
    
    Anyway, detecting this in DOM is pretty simple.   We detect the infinite loop, including the latch block.  Once we've done that, we walk the PHI nodes and attach equivalences to the appropriate outgoing edge.   That's all we need to do as the rest of DOM is already prepared to handle equivalences on edges.
    
    gcc/
            * tree-ssa-dom.cc (single_block_loop_p): New function.
            (record_edge_info): Also record equivalences for the outgoing
            edge of a single block loop where the condition is an invariant.
    
    gcc/testsuite/
            * gcc.dg/infinite-loop.c: New test.

Comments

Bernhard Reutner-Fischer Oct. 1, 2022, 6:55 p.m. UTC | #1 
On Fri, 30 Sep 2022 17:32:34 -0600
Jeff Law <jlaw@ventanamicro.com> wrote:

> +  /* This looks good from a CFG standpoint.  Now look at the guts
> +     of PRED.  Basically we want to verify there are no PHI nodes
> +     and no real statements.  */
> +  if (! gimple_seq_empty_p (phi_nodes (pred)))
> +    return false;

So, given the below, neither DEBUG nor labels do count towards an
empty seq [coming in from any PHI that is, otherwise it's a different
thing], which is a bit surprising but well, ok. It looks at PHI IL, so
probably yes. Allegedly that's what it is. Neat if that's true.

> +
> +  gimple_stmt_iterator gsi;
> +  for (gsi = gsi_last_bb (pred); !gsi_end_p (gsi); gsi_prev (&gsi))
> +    {
> +      gimple *stmt = gsi_stmt (gsi);
> +
> +      switch (gimple_code (stmt))
> +	{
> +	  case GIMPLE_LABEL:
> +	    if (DECL_NONLOCAL (gimple_label_label (as_a <glabel *> (stmt))))
> +	      return false;
> +	    break;
> +
> +	  case GIMPLE_DEBUG:
> +	    break;
> +
> +	  default:
> +	    return false;

don't like, sounds odd. Are we sure there's no other garbage that can
manifest here? int meow=42;, and meow unused won't survive?, pragmas
neither or stuff ?

> +	}
> +    }
> +
> +  return true;
> +}
> +
>  /* We have finished optimizing BB, record any information implied by
>     taking a specific outgoing edge from BB.  */
>  

> @@ -583,6 +656,62 @@ record_edge_info (basic_block bb)
>                if (can_infer_simple_equiv && TREE_CODE (inverted) == EQ_EXPR)
>  		edge_info->record_simple_equiv (op0, op1);
>              }
> +
> +	  /* If this block is a single block loop, then we may be able to
> +	     record some equivalences on the loop's exit edge.  */
> +	  if (single_block_loop_p (bb))
> +	    {
> +	      /* We know it's a single block loop.  Now look at the loop
> +		 exit condition.  What we're looking for is whether or not
> +		 the exit condition is loop invariant which we can detect
> +		 by checking if all the SSA_NAMEs referenced are defined
> +		 outside the loop.  */
> +	      if ((TREE_CODE (op0) != SSA_NAME
> +		   || gimple_bb (SSA_NAME_DEF_STMT (op0)) != bb)
> +		  && (TREE_CODE (op1) != SSA_NAME
> +		      || gimple_bb (SSA_NAME_DEF_STMT (op1)) != bb))
> +		{
> +		  /* At this point we know the exit condition is loop
> +		     invariant.  The only way to get out of the loop is
> +		     if never traverses the backedge to begin with.  This

s/if /if it /

> +		     implies that any PHI nodes create equivalances we can

that any threw me off asking for "that if any". Would have been nicer,
i think?

> +		     attach to the loop exit edge.  */

attach it to

> +		  int alternative

bool

> +		    = (EDGE_PRED (bb, 0)->flags & EDGE_DFS_BACK) ? 1 : 0;
> +
> +		  gphi_iterator gsi;
> +		  for (gsi = gsi_start_phis (bb);
> +		       !gsi_end_p (gsi);
> +		       gsi_next (&gsi))
> +		    {
> +		      /* If the other alternative is the same as the result,
> +			 then this is a degenerate and can be ignored.  */
> +		      if (dst == PHI_ARG_DEF (phi, !alternative))
> +			continue;
> +
> +		      /* Now get the EDGE_INFO class so we can append
> +			 it to our list.  We want the successor edge
> +			 where the destination is not the source of
> +			 an incoming edge.  */
> +		      gphi *phi = gsi.phi ();
> +		      tree src = PHI_ARG_DEF (phi, alternative);
> +		      tree dst = PHI_RESULT (phi);
> +
> +		      if (EDGE_SUCC (bb, 0)->dest
> +			  != EDGE_PRED (bb, !alternative)->src)

by now, alternative would be easier to grok if it would have been spelled
from_backedge_p or something. IMHO.
thanks,

> +			edge_info = (class edge_info *)EDGE_SUCC (bb, 0)->aux;
> +		      else
> +			edge_info = (class edge_info *)EDGE_SUCC (bb, 1)->aux;
> +
> +		      /* Note that since this processing is done independently
> +			 of other edge equivalency processing, we may not
> +			 have an EDGE_INFO structure set up yet.  */
> +		      if (edge_info == NULL)
> +			edge_info = new class edge_info (false_edge);
> +		      edge_info->record_simple_equiv (dst, src);
> +		    }
> +		}
> +	    }
>          }
>      }
>  }
Jeff Law Oct. 31, 2022, 10:43 p.m. UTC | #2 
On 10/1/22 12:55, Bernhard Reutner-Fischer wrote:
> On Fri, 30 Sep 2022 17:32:34 -0600
> Jeff Law <jlaw@ventanamicro.com> wrote:
>
>> +  /* This looks good from a CFG standpoint.  Now look at the guts
>> +     of PRED.  Basically we want to verify there are no PHI nodes
>> +     and no real statements.  */
>> +  if (! gimple_seq_empty_p (phi_nodes (pred)))
>> +    return false;
> So, given the below, neither DEBUG nor labels do count towards an
> empty seq [coming in from any PHI that is, otherwise it's a different
> thing], which is a bit surprising but well, ok. It looks at PHI IL, so
> probably yes. Allegedly that's what it is. Neat if that's true.

Right.  A forwarder block is allowed to have local labels, but not 
labels for nonlocal gotos, so we can and should ignore local labels.  
Debug statements must be ignored or you can end up with differing code 
generation based on whether or not -g is enabled or not.

In some contexts PHIs are allowed in forwarders, in other contexts they 
are not.  In this specific case I doubt it matters because of the 
restrictions we put on the CFG, the predecessor block is restricted to a 
single incoming edge.  The only way that'll have a PHI is if the PHI 
became a degenerate during DOM.




>
>> +
>> +  gimple_stmt_iterator gsi;
>> +  for (gsi = gsi_last_bb (pred); !gsi_end_p (gsi); gsi_prev (&gsi))
>> +    {
>> +      gimple *stmt = gsi_stmt (gsi);
>> +
>> +      switch (gimple_code (stmt))
>> +	{
>> +	  case GIMPLE_LABEL:
>> +	    if (DECL_NONLOCAL (gimple_label_label (as_a <glabel *> (stmt))))
>> +	      return false;
>> +	    break;
>> +
>> +	  case GIMPLE_DEBUG:
>> +	    break;
>> +
>> +	  default:
>> +	    return false;
> don't like, sounds odd. Are we sure there's no other garbage that can
> manifest here? int meow=42;, and meow unused won't survive?, pragmas
> neither or stuff ?

That would generate a real statement and would thus be rejected. If 
there's anything other than a local label or debug statements in the 
block, then it's rejected.



>> @@ -583,6 +656,62 @@ record_edge_info (basic_block bb)
>>                 if (can_infer_simple_equiv && TREE_CODE (inverted) == EQ_EXPR)
>>   		edge_info->record_simple_equiv (op0, op1);
>>               }
>> +
>> +	  /* If this block is a single block loop, then we may be able to
>> +	     record some equivalences on the loop's exit edge.  */
>> +	  if (single_block_loop_p (bb))
>> +	    {
>> +	      /* We know it's a single block loop.  Now look at the loop
>> +		 exit condition.  What we're looking for is whether or not
>> +		 the exit condition is loop invariant which we can detect
>> +		 by checking if all the SSA_NAMEs referenced are defined
>> +		 outside the loop.  */
>> +	      if ((TREE_CODE (op0) != SSA_NAME
>> +		   || gimple_bb (SSA_NAME_DEF_STMT (op0)) != bb)
>> +		  && (TREE_CODE (op1) != SSA_NAME
>> +		      || gimple_bb (SSA_NAME_DEF_STMT (op1)) != bb))
>> +		{
>> +		  /* At this point we know the exit condition is loop
>> +		     invariant.  The only way to get out of the loop is
>> +		     if never traverses the backedge to begin with.  This
> s/if /if it /

Will fix.  THanks.


>
>> +		     implies that any PHI nodes create equivalances we can
> that any threw me off asking for "that if any". Would have been nicer,
> i think?

All PHIs at the target of the loop backedge in this case create an 
equivalence.  That's the whole point of the patch, to prove a set of 
circumstances that ultimately require all the PHIs on the loop backedge 
to create an equivalence on the loop exit.



>
>> +		     attach to the loop exit edge.  */
> attach it to

Updated, slightly differently, but should be clearer.


>
>> +		  int alternative
> bool

Sure.



>
>> +		    = (EDGE_PRED (bb, 0)->flags & EDGE_DFS_BACK) ? 1 : 0;
>> +
>> +		  gphi_iterator gsi;
>> +		  for (gsi = gsi_start_phis (bb);
>> +		       !gsi_end_p (gsi);
>> +		       gsi_next (&gsi))
>> +		    {
>> +		      /* If the other alternative is the same as the result,
>> +			 then this is a degenerate and can be ignored.  */
>> +		      if (dst == PHI_ARG_DEF (phi, !alternative))
>> +			continue;
>> +
>> +		      /* Now get the EDGE_INFO class so we can append
>> +			 it to our list.  We want the successor edge
>> +			 where the destination is not the source of
>> +			 an incoming edge.  */
>> +		      gphi *phi = gsi.phi ();
>> +		      tree src = PHI_ARG_DEF (phi, alternative);
>> +		      tree dst = PHI_RESULT (phi);
>> +
>> +		      if (EDGE_SUCC (bb, 0)->dest
>> +			  != EDGE_PRED (bb, !alternative)->src)
> by now, alternative would be easier to grok if it would have been spelled
> from_backedge_p or something. IMHO.

Agreed it's a bit ugly.  Let me think about  how to clean this up.


Anyway, I've queued the updates.  I'll push them the next time I do a 
bootstrap & regression test.


jeff
diff mbox series

Patch

diff --git a/gcc/testsuite/gcc.dg/infinite-loop.c b/gcc/testsuite/gcc.dg/infinite-loop.c
new file mode 100644
index 00000000000..25037a2027e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/infinite-loop.c
@@ -0,0 +1,26 @@ 
+/* { dg-do link } */
+/* { dg-options "-O2" } */
+void link_error (void);
+
+void __attribute__ ((noinline,noipa))
+foo(int a)
+{
+ int b = 0;
+
+ while (1)
+   {
+     if (!a)
+       break;
+     b = 1;
+   }
+
+ if (b != 0)
+   link_error ();
+}
+
+int
+main()
+{
+  foo (0);
+}
+
diff --git a/gcc/tree-ssa-dom.cc b/gcc/tree-ssa-dom.cc
index fa43dbe6c44..8d8312ca350 100644
--- a/gcc/tree-ssa-dom.cc
+++ b/gcc/tree-ssa-dom.cc
@@ -426,6 +426,74 @@  free_all_edge_infos (void)
     }
 }
 
+/* Return TRUE if BB has precisely two preds, one of which
+   is a backedge from a forwarder block where the forwarder
+   block is a direct successor of BB.  Being a forwarder
+   block, it has no side effects other than transfer of
+   control.  Otherwise return FALSE.  */
+
+static bool
+single_block_loop_p (basic_block bb)
+{
+  /* Two preds.  */
+  if (EDGE_COUNT (bb->preds) != 2)
+    return false;
+
+  /* One and only one of the edges must be marked with
+     EDGE_DFS_BACK.  */
+  basic_block pred = NULL;
+  unsigned int count = 0;
+  if (EDGE_PRED (bb, 0)->flags & EDGE_DFS_BACK)
+    {
+      pred = EDGE_PRED (bb, 0)->src;
+      count++;
+    }
+  if (EDGE_PRED (bb, 1)->flags & EDGE_DFS_BACK)
+    {
+      pred = EDGE_PRED (bb, 1)->src;
+      count++;
+    }
+
+  if (count != 1)
+    return false;
+
+  /* Now examine PRED.  It should have a single predecessor which
+     is BB and a single successor that is also BB.  */
+  if (EDGE_COUNT (pred->preds) != 1
+      || EDGE_COUNT (pred->succs) != 1
+      || EDGE_PRED (pred, 0)->src != bb
+      || EDGE_SUCC (pred, 0)->dest != bb)
+    return false;
+
+  /* This looks good from a CFG standpoint.  Now look at the guts
+     of PRED.  Basically we want to verify there are no PHI nodes
+     and no real statements.  */
+  if (! gimple_seq_empty_p (phi_nodes (pred)))
+    return false;
+
+  gimple_stmt_iterator gsi;
+  for (gsi = gsi_last_bb (pred); !gsi_end_p (gsi); gsi_prev (&gsi))
+    {
+      gimple *stmt = gsi_stmt (gsi);
+
+      switch (gimple_code (stmt))
+	{
+	  case GIMPLE_LABEL:
+	    if (DECL_NONLOCAL (gimple_label_label (as_a <glabel *> (stmt))))
+	      return false;
+	    break;
+
+	  case GIMPLE_DEBUG:
+	    break;
+
+	  default:
+	    return false;
+	}
+    }
+
+  return true;
+}
+
 /* We have finished optimizing BB, record any information implied by
    taking a specific outgoing edge from BB.  */
 
@@ -435,6 +503,13 @@  record_edge_info (basic_block bb)
   gimple_stmt_iterator gsi = gsi_last_bb (bb);
   class edge_info *edge_info;
 
+  /* Free all the outgoing edge info data associated with
+     BB's outgoing edges.  */
+  edge e;
+  edge_iterator ei;
+  FOR_EACH_EDGE (e, ei, bb->succs)
+    free_dom_edge_info (e);
+
   if (! gsi_end_p (gsi))
     {
       gimple *stmt = gsi_stmt (gsi);
@@ -450,8 +525,6 @@  record_edge_info (basic_block bb)
 	      int i;
               int n_labels = gimple_switch_num_labels (switch_stmt);
 	      tree *info = XCNEWVEC (tree, last_basic_block_for_fn (cfun));
-	      edge e;
-	      edge_iterator ei;
 
 	      for (i = 0; i < n_labels; i++)
 		{
@@ -583,6 +656,62 @@  record_edge_info (basic_block bb)
               if (can_infer_simple_equiv && TREE_CODE (inverted) == EQ_EXPR)
 		edge_info->record_simple_equiv (op0, op1);
             }
+
+	  /* If this block is a single block loop, then we may be able to
+	     record some equivalences on the loop's exit edge.  */
+	  if (single_block_loop_p (bb))
+	    {
+	      /* We know it's a single block loop.  Now look at the loop
+		 exit condition.  What we're looking for is whether or not
+		 the exit condition is loop invariant which we can detect
+		 by checking if all the SSA_NAMEs referenced are defined
+		 outside the loop.  */
+	      if ((TREE_CODE (op0) != SSA_NAME
+		   || gimple_bb (SSA_NAME_DEF_STMT (op0)) != bb)
+		  && (TREE_CODE (op1) != SSA_NAME
+		      || gimple_bb (SSA_NAME_DEF_STMT (op1)) != bb))
+		{
+		  /* At this point we know the exit condition is loop
+		     invariant.  The only way to get out of the loop is
+		     if never traverses the backedge to begin with.  This
+		     implies that any PHI nodes create equivalances we can
+		     attach to the loop exit edge.  */
+		  int alternative
+		    = (EDGE_PRED (bb, 0)->flags & EDGE_DFS_BACK) ? 1 : 0;
+
+		  gphi_iterator gsi;
+		  for (gsi = gsi_start_phis (bb);
+		       !gsi_end_p (gsi);
+		       gsi_next (&gsi))
+		    {
+		      /* If the other alternative is the same as the result,
+			 then this is a degenerate and can be ignored.  */
+		      if (dst == PHI_ARG_DEF (phi, !alternative))
+			continue;
+
+		      /* Now get the EDGE_INFO class so we can append
+			 it to our list.  We want the successor edge
+			 where the destination is not the source of
+			 an incoming edge.  */
+		      gphi *phi = gsi.phi ();
+		      tree src = PHI_ARG_DEF (phi, alternative);
+		      tree dst = PHI_RESULT (phi);
+
+		      if (EDGE_SUCC (bb, 0)->dest
+			  != EDGE_PRED (bb, !alternative)->src)
+			edge_info = (class edge_info *)EDGE_SUCC (bb, 0)->aux;
+		      else
+			edge_info = (class edge_info *)EDGE_SUCC (bb, 1)->aux;
+
+		      /* Note that since this processing is done independently
+			 of other edge equivalency processing, we may not
+			 have an EDGE_INFO structure set up yet.  */
+		      if (edge_info == NULL)
+			edge_info = new class edge_info (false_edge);
+		      edge_info->record_simple_equiv (dst, src);
+		    }
+		}
+	    }
         }
     }
 }