dwarf2out: Fix up dwarf2out_next_real_insn caching [PR96729]

dwarf2out: Fix up dwarf2out_next_real_insn caching [PR96729]

The addition of NOTE_INSN_BEGIN_STMT and NOTE_INSN_INLINE_ENTRY notes
reintroduced quadratic behavior into dwarf2out_var_location.
This function needs to know the next real instruction to which the var
location note applies, but the way final_scan_insn is called outside of
final.c main loop doesn't make it easy to look up the next real insn in
there (and for non-dwarf it is even useless).  Usually next real insn is
only a few notes away, but we can have hundreds of thousands of consecutive
notes only followed by a real insn.  dwarf2out_var_location to avoid the
quadratic behavior contains a cache, it remembers the next note and when it
is called again on that loc_note, it can use the previously computed
dwarf2out_next_real_insn result, rather than walking the insn chain once
again.  But, for NOTE_INSN_{BEGIN_STMT,INLINE_ENTRY} dwarf2out_var_location
is not called while the code puts into the cache those notes, which means if
we have e.g. in the worst case NOTE_INSN_VAR_LOCATION and
NOTE_INSN_BEGIN_STMT notes alternating, the cache is not really used.

The following patch fixes it by looking up the next NOTE_INSN_VAR_LOCATION
if any.  While the lookup could be perhaps done together with looking for
the next real insn once (e.g. in dwarf2out_next_real_insn or its copy),
there are other dwarf2out_next_real_insn callers which don't need/want that
behavior and if there are more than two NOTE_INSN_VAR_LOCATION notes
followed by the same real insn, we need to do that "find next
NOTE_INSN_VAR_LOCATION" walk anyway.

On the testcase from the PR this patch speeds it 2.8times, from 0m0.674s
to 0m0.236s (why it takes for the reporter more than 60s is unknown).

2020-08-26  Jakub Jelinek  <jakub@redhat.com>

	PR debug/96729
	* dwarf2out.c (dwarf2out_next_real_insn): Adjust function comment.
	(dwarf2out_var_location): Look for next_note only if next_real is
	non-NULL, in that case look for the first non-deleted
	NOTE_INSN_VAR_LOCATION between loc_note and next_real, if any.