Fix Eh delivery in partitioned functions

[Richard Biener]

On Tue, 18 Jul 2017, Jan Hubicka wrote:

> Hi,
> this patch fixes wrong code issue with BB partitioning where sometimes EH
> is not delivered.  This is very old issue that affect all release branches
> with -fprofile-use, but recently surfaced as libstdc++ testsuite regression
> because we now partition functions based on static profile prediction.
> 
> The problem is that EH tables are stored as offsets from start of functions.
> In the record however value 0 is special and means that there is no landing
> pad for a given region.  Normally this is safe because landing pads never
> appear as very first label in function.  This is however no longer true with
> partitining where cold partition is actually quite likely to start by landing
> pad.
> 
> The change in except.c adds sanity check that no EH landing pads are very first
> in the insn stream.  The change in bb-reorder makes reorder to chose
> non-landing-pad BB as first trace for the cold partition. Such BB always exists
> because landing pads must be in same partition as the instruction throwing them
> and we never make BB both landing pad and reachable by normal control folow.
> However I am not thrilled by the fix as it is bit fragile in case some
> optimization happends after bb partitioning and code is moved away.  Also the
> logic can be confused by asm statement which may result in no code (again
> however the BB reachable from outside world should contain something that
> produce EH that is a real instruction).
> 
> Ideas for better fix would be welcome then.  If the assert I added triggers
> for valid reasons, we may just end up adding a NOP in the rare case we do
> not suceed arranging cold partition to not start with landing pad.

Yeah, I'd rather pad the function start with a nop if it starts with a
landing pad.  How difficult would it be to arrange for this?  I suppose
we'd need to touch each and every target to accomplish this?  Or end up
using gen_nop in generic code?

Richard.

> Bootstrapped/regtested x86_64-linux, looks sane?
> 
> Honza
> 
> 	PR middle-end/81331 
> 	* except.c (first_in_partition): New function.
> 	(dw2_output_call_site_table): Sanity check that landing pads are not
> 	very first in the partition.
> 	* bb-reorder.c (ok_to_be_first): New function.
> 	(connect_traces): Avoid traces that are !ok_to_be_first to start
> 	partitions.
> Index: except.c
> ===================================================================
> --- except.c	(revision 250226)
> +++ except.c	(working copy)
> @@ -2724,6 +2724,23 @@ sjlj_size_of_call_site_table (void)
>    return size;
>  }
>  
> +/* Return true if L will appear as very first in its partition.  */
> +
> +bool
> +first_in_partition (rtx_insn *l)
> +{
> +  while (l != NULL_RTX)
> +    {
> +      if (active_insn_p (l))
> +	return false;
> +      else if (GET_CODE (l) == NOTE
> +	       && NOTE_KIND (l) == NOTE_INSN_SWITCH_TEXT_SECTIONS)
> +	return true;
> +      l = PREV_INSN (l);
> +    }
> +  return true;
> +}
> +
>  static void
>  dw2_output_call_site_table (int cs_format, int section)
>  {
> @@ -2749,8 +2766,14 @@ dw2_output_call_site_table (int cs_forma
>        ASM_GENERATE_INTERNAL_LABEL (reg_end_lab, "LEHE", call_site_base + i);
>  
>        if (cs->landing_pad)
> -	ASM_GENERATE_INTERNAL_LABEL (landing_pad_lab, "L",
> -				     CODE_LABEL_NUMBER (cs->landing_pad));
> +	{
> +	  ASM_GENERATE_INTERNAL_LABEL (landing_pad_lab, "L",
> +				       CODE_LABEL_NUMBER (cs->landing_pad));
> +	  /* Be sure that the offset will not be 0 as that would make EH
> +	     delivery code to think that there is no landing pad.  */
> +	  gcc_checking_assert (!first_in_partition
> +				   (as_a <rtx_insn *> (cs->landing_pad)));
> +	}
>  
>        /* ??? Perhaps use insn length scaling if the assembler supports
>  	 generic arithmetic.  */
> Index: bb-reorder.c
> ===================================================================
> --- bb-reorder.c	(revision 250226)
> +++ bb-reorder.c	(working copy)
> @@ -1066,6 +1066,21 @@ connect_better_edge_p (const_edge e, boo
>    return is_better_edge;
>  }
>  
> +/* If we place EH landing pad as very first BB in the partition, its offset
> +   from start of function is 0 which is special cased by the eh table to mean
> +   no landing pad.  For this reason such BBs can not appear as very first in
> +   the partition.  */
> +static bool
> +ok_to_be_first (struct trace *t)
> +{
> +  edge e;
> +  edge_iterator ei;
> +  FOR_EACH_EDGE (e, ei, t->first->preds)
> +    if (e->flags & EDGE_EH)
> +      return false;
> +  return true;
> +}
> +
>  /* Connect traces in array TRACES, N_TRACES is the count of traces.  */
>  
>  static void
> @@ -1080,6 +1095,7 @@ connect_traces (int n_traces, struct tra
>    int freq_threshold;
>    gcov_type count_threshold;
>    bool for_size = optimize_function_for_size_p (cfun);
> +  bool first_in_partition;
>  
>    freq_threshold = max_entry_frequency * DUPLICATION_THRESHOLD / 1000;
>    if (max_entry_count.to_gcov_type () < INT_MAX / 1000)
> @@ -1092,6 +1108,7 @@ connect_traces (int n_traces, struct tra
>    current_pass = 1;
>    current_partition = BB_PARTITION (traces[0].first);
>    two_passes = false;
> +  first_in_partition = true;
>  
>    if (crtl->has_bb_partition)
>      for (i = 0; i < n_traces && !two_passes; i++)
> @@ -1116,6 +1133,7 @@ connect_traces (int n_traces, struct tra
>  	    current_partition = BB_COLD_PARTITION;
>  	  else
>  	    current_partition = BB_HOT_PARTITION;
> +	  first_in_partition = true;
>  	}
>  
>        if (connected[t])
> @@ -1125,6 +1143,9 @@ connect_traces (int n_traces, struct tra
>  	  && BB_PARTITION (traces[t].first) != current_partition)
>  	continue;
>  
> +      if (first_in_partition && !ok_to_be_first (&traces[t]))
> +	continue;
> +
>        connected[t] = true;
>  
>        /* Find the predecessor traces.  */
> @@ -1143,7 +1164,9 @@ connect_traces (int n_traces, struct tra
>  		  && bbd[si].end_of_trace >= 0
>  		  && !connected[bbd[si].end_of_trace]
>  		  && (BB_PARTITION (e->src) == current_partition)
> -		  && connect_better_edge_p (e, true, best_len, best, traces))
> +		  && connect_better_edge_p (e, true, best_len, best, traces)
> +		  && (!first_in_partition
> +		      || ok_to_be_first (&traces[bbd[si].end_of_trace])))
>  		{
>  		  best = e;
>  		  best_len = traces[bbd[si].end_of_trace].length;
> @@ -1151,6 +1174,8 @@ connect_traces (int n_traces, struct tra
>  	    }
>  	  if (best)
>  	    {
> +	      gcc_checking_assert (BB_PARTITION (best->src)
> +				   == BB_PARTITION (best->dest));
>  	      best->src->aux = best->dest;
>  	      t2 = bbd[best->src->index].end_of_trace;
>  	      connected[t2] = true;
> @@ -1166,8 +1191,13 @@ connect_traces (int n_traces, struct tra
>  	}
>  
>        if (last_trace >= 0)
> -	traces[last_trace].last->aux = traces[t2].first;
> +	{
> +	  gcc_checking_assert (!first_in_partition
> +			       || ok_to_be_first (&traces[t2]));
> +	  traces[last_trace].last->aux = traces[t2].first;
> +	}
>        last_trace = t;
> +      first_in_partition = false;
>  
>        /* Find the successor traces.  */
>        while (1)
> @@ -1226,6 +1256,8 @@ connect_traces (int n_traces, struct tra
>  		fprintf (dump_file, "Connection: %d %d\n",
>  			 best->src->index, best->dest->index);
>  
> +	      gcc_checking_assert (BB_PARTITION (best->dest)
> +				   == BB_PARTITION (best->src));
>  	      t = bbd[best->dest->index].start_of_trace;
>  	      traces[last_trace].last->aux = traces[t].first;
>  	      connected[t] = true;
> @@ -1238,6 +1270,8 @@ connect_traces (int n_traces, struct tra
>  		  fprintf (dump_file, "Connection: %d %d\n",
>  			   best->src->index, best->dest->index);
>  		}
> +	      gcc_checking_assert (BB_PARTITION (best->dest)
> +				   == BB_PARTITION (best->src));
>  	      t = bbd[best->dest->index].start_of_trace;
>  	      traces[last_trace].last->aux = traces[t].first;
>  	      connected[t] = true;
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)