lto: Cache location_ts including BLOCKs in GIMPLE streaming [PR94311]

lto: Cache location_ts including BLOCKs in GIMPLE streaming [PR94311]

As mentioned in the PR, when compiling valgrind even on fairly small
testcase where in one larger function the location keeps oscillating
between a small line number and 8000-ish line number in the same file
we very quickly run out of all possible location_t numbers and because of
that emit non-sensical line numbers in .debug_line.
There are ways how to decrease speed of depleting location_t numbers
in libcpp, but the main reason of this is that we use
stream_input_location_now for streaming in location_t for gimple_location
and phi arg locations.  libcpp strongly prefers that the locations
it is given are sorted by the different files and by line numbers in
ascending order, otherwise it depletes quickly no matter what and is much
more costly (many extra file changes etc.).
The reason for not caching those were the BLOCKs that were streamed
immediately after the location and encoded into the locations (and for PHIs
we failed to stream the BLOCKs altogether).
This patch enhances the location cache to handle also BLOCKs (but not for
everything, only for the spots we care about the BLOCKs) and also optimizes
the size of the LTO stream by emitting a single bit into a pack whether the
BLOCK changed from last case and only streaming the BLOCK tree if it
changed.

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

	PR lto/94311
	* gimple.h (gimple_location_ptr, gimple_phi_arg_location_ptr): New
	functions.
	* streamer-hooks.h (struct streamer_hooks): Add
	output_location_and_block callback.  Fix up formatting for
	output_location.
	(stream_output_location_and_block): Define.
	* lto-streamer.h (class lto_location_cache): Fix comment typo.  Add
	current_block member.
	(lto_location_cache::input_location_and_block): New method.
	(lto_location_cache::lto_location_cache): Initialize current_block.
	(lto_location_cache::cached_location): Add block member.
	(struct output_block): Add current_block member.
	(lto_output_location): Formatting fix.
	(lto_output_location_and_block): Declare.
	* lto-streamer.c (lto_streamer_hooks_init): Initialize
	streamer_hooks.output_location_and_block.
	* lto-streamer-in.c (lto_location_cache::cmp_loc): Also compare
	block members.
	(lto_location_cache::apply_location_cache): Handle blocks.
	(lto_location_cache::accept_location_cache,
	lto_location_cache::revert_location_cache): Fix up function comments.
	(lto_location_cache::input_location_and_block): New method.
	(lto_location_cache::input_location): Implement using
	input_location_and_block.
	(input_function): Invoke apply_location_cache after streaming in all
	bbs.
	* lto-streamer-out.c (clear_line_info): Set current_block.
	(lto_output_location_1): New function, moved from lto_output_location,
	added block handling.
	(lto_output_location): Implement using lto_output_location_1.
	(lto_output_location_and_block): New function.
	* gimple-streamer-in.c (input_phi): Use input_location_and_block
	to input and cache both location and block.
	(input_gimple_stmt): Likewise.
	* gimple-streamer-out.c (output_phi): Use
	stream_output_location_and_block.
	(output_gimple_stmt): Likewise.