[PATCH] lto: Ensure we force a change for file/line/column after clear_line_info

[Jakub Jelinek]

Hi!

As discussed yesterday:
On the streamer out side, we call clear_line_info
in multiple spots which resets the current_* values to something, but on the
reader side, we don't have corresponding resets in the same location, just have
the stream_* static variables that keep the current values through the
entire stream in (so across all the clear_line_info spots in a single LTO
object but also across jumping from one LTO object to another one).
Now, in an earlier version of my patch it actually broke LTO bootstrap
(and a lot of LTO testcases), so for the BLOCK case I've solved it by
clear_line_info setting current_block to something that should never appear,
which means that in the LTO stream after the clear_line_info spots including
the start of the LTO stream we force the block change bit to be set and thus
BLOCK to be streamed and therefore stream_block from earlier to be
ignored.  But for the rest I think that is not the case, so I wonder if we
don't sometimes end up with wrong line/column info because of that, or
please tell me what prevents that.
clear_line_info does:
  ob->current_file = NULL;
  ob->current_line = 0;
  ob->current_col = 0;
  ob->current_sysp = false;
while I think NULL current_file is something that should likely be different
from expanded_location (...).file (UNKNOWN_LOCATION/BUILTINS_LOCATION are
handled separately and not go through the caching), I think line number 0
can sometimes occur and especially column 0 occurs frequently if we ran out
of location_t with columns info.  But then we do:
      bp_pack_value (bp, ob->current_file != xloc.file, 1);
      bp_pack_value (bp, ob->current_line != xloc.line, 1);
      bp_pack_value (bp, ob->current_col != xloc.column, 1);
and stream the details only if the != is true.  If that happens immediately
after clear_line_info and e.g. xloc.column is 0, we would stream 0 bit and
not stream the actual value, so on read-in it would reuse whatever
stream_col etc. were before.  Shouldn't we set some ob->current_* new bit
that would signal we are immediately past clear_line_info which would force
all these != checks to non-zero?  Either by oring something into those
tests, or perhaps:
  if (ob->current_reset)
    {
      if (xloc.file == NULL)
        ob->current_file = "";
      if (xloc.line == 0)
        ob->current_line = 1;
      if (xloc.column == 0)
        ob->current_column = 1;
      ob->current_reset = false;
    }
before doing those bp_pack_value calls with a comment, effectively forcing
all 6 != comparisons to be true?

Bootstrapped/regtested on x86_64-linux and i686-linux and lto bootstrapped
on x86_64-linux, ok for trunk?

2020-09-03  Jakub Jelinek  <jakub@redhat.com>

	* lto-streamer.h (struct output_block): Add reset_locus member.
	* lto-streamer-out.c (clear_line_info): Set reset_locus to true.
	(lto_output_location_1): If reset_locus, clear it and ensure
	current_{file,line,col} is different from xloc members.

--- gcc/lto-streamer.h.jj	2020-09-03 12:50:54.990753979 +0200
+++ gcc/lto-streamer.h	2020-09-03 17:11:51.406571519 +0200
@@ -717,6 +717,7 @@ struct output_block
   int current_line;
   int current_col;
   bool current_sysp;
+  bool reset_locus;
   tree current_block;
 
   /* Cache of nodes written in this section.  */
--- gcc/lto-streamer-out.c.jj	2020-09-03 12:50:54.992753950 +0200
+++ gcc/lto-streamer-out.c	2020-09-03 17:16:44.601311584 +0200
@@ -60,6 +60,7 @@ clear_line_info (struct output_block *ob
   ob->current_line = 0;
   ob->current_col = 0;
   ob->current_sysp = false;
+  ob->reset_locus = true;
   /* Initialize to something that will never appear as block,
      so that the first location with block in a function etc.
      always streams a change_block bit and the first block.  */
@@ -195,6 +196,17 @@ lto_output_location_1 (struct output_blo
     {
       expanded_location xloc = expand_location (loc);
 
+      if (ob->reset_locus)
+	{
+	  if (xloc.file == NULL)
+	    ob->current_file = "";
+	  if (xloc.line == 0)
+	    ob->current_line = 1;
+	  if (xloc.column == 0)
+	    ob->current_col = 1;
+	  ob->reset_locus = false;
+	}
+
       bp_pack_value (bp, ob->current_file != xloc.file, 1);
       bp_pack_value (bp, ob->current_line != xloc.line, 1);
       bp_pack_value (bp, ob->current_col != xloc.column, 1);

	Jakub