[PATCH] Verify edge probability consistency in verify_flow_info

[Tom de Vries <Tom_deVries at mentor dot com>]

Hi,

I.

for target nvptx we recently ran into PR81442, an ICE in verify_flow_info:
...
error: verify_flow_info: REG_BR_PROB is set but cfg probability is not
...

We start out with a jump instruction:
...
(jump_insn 18 17 31 2 (set (pc)
        (if_then_else (ne (reg:BI 83)
                (const_int 0 [0]))
            (label_ref 31)
            (pc))) "reduction-5.c":52 104 {br_true}
     (expr_list:REG_DEAD (reg:BI 83)
        (int_list:REG_BR_PROB 10000 (nil)))
 -> 31)
...

The probability is set on the branch edge, but not on the fallthru edge.

After fixup_reorder_chain, the condition is reversed, and the 
probability reg-note update accordingly:
...
(jump_insn 18 17 32 2 (set (pc)
        (if_then_else (eq (reg:BI 83)
                (const_int 0 [0]))
            (label_ref:DI 33)
            (pc))) "reduction-5.c":52 105 {br_false}
     (expr_list:REG_DEAD (reg:BI 83)
        (int_list:REG_BR_PROB 0 (nil)))
 -> 33)
...

The branch and fallthru edge are also reversed, which means now the 
probability is set on the fallthru edge, and not on the branch edge.

This is the immediate cause for the verify_flow_info error.


II.

We've fixed the ICE in the target by setting the probability on the 
fallthru edge when we introduce it (r250422).

We've noted other places where we were forgetting to set the probability 
(fixed in r250823), but that did not trigger the ICE.


III.

I've written this patch to check for the missing probability more 
consistently. I'm not certain if we can require that the probability 
should always be set, so I'm just requiring that if it is set on one 
outgoing edge, it needs to be set on all outgoing edges.

Sofar I've build a c-only x86_64 non-bootstrap compiler and ran dg.exp. 
The only problem I ran into was in attr-simd{,-2,-4}.c. I've written a 
tentative patch for that, and will submit it as follow-up.

Is this check a good idea?

OK for trunk if bootstrap and reg-test on x86_64 succeeds?

Thanks,
- Tom

Verify edge probability consistency in verify_flow_info

2017-08-02  Tom de Vries  <tom@codesourcery.com>

	* cfghooks.c (verify_flow_info): Verify that if one outgoing edge has
	probability set, all outgoing edges have probability set.

---
 gcc/cfghooks.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/gcc/cfghooks.c b/gcc/cfghooks.c
index 18dc49a..b973651 100644
--- a/gcc/cfghooks.c
+++ b/gcc/cfghooks.c
@@ -152,6 +152,8 @@ verify_flow_info (void)
 		 bb->index, bb->frequency);
 	  err = 1;
 	}
+      bool has_prob_uninit_edges = false;
+      bool has_prob_init_edges = false;
       FOR_EACH_EDGE (e, ei, bb->succs)
 	{
 	  if (last_visited [e->dest->index] == bb)
@@ -166,6 +168,10 @@ verify_flow_info (void)
 		     e->src->index, e->dest->index);
 	      err = 1;
 	    }
+	  if (e->probability.initialized_p ())
+	    has_prob_init_edges = true;
+	  else
+	    has_prob_uninit_edges = true;
 	  if (!e->count.verify ())
 	    {
 	      error ("verify_flow_info: Wrong count of edge %i->%i",
@@ -197,6 +203,12 @@ verify_flow_info (void)
 	  error ("wrong amount of branch edges after unconditional jump %i", bb->index);
 	  err = 1;
 	}
+      if (has_prob_uninit_edges && has_prob_init_edges)
+	{
+	  error ("Missing edge probability after unconditional jump in bb %i",
+		 bb->index);
+	  err = 1;
+	}
 
       FOR_EACH_EDGE (e, ei, bb->preds)
 	{