tree-optimization/38474 - fix store-merging compile-time regression

tree-optimization/38474 - fix store-merging compile-time regression

The following puts a limit on the number of alias tests we do in
terminate_all_aliasing_chains which is quadratic in the number of
overall stores currentrly tracked.  There is already a limit in
place on the maximum number of stores in a single chain so the
following adds a limit on the number of chains tracked.  The
worst number of overall stores tracked from the defaults (64 and 64)
is then 4096 which when imposed as the sole limit for the testcase
still causes

 store merging                      :  71.65 ( 56%)

because the testcase is somewhat degenerate with most chains
consisting only of a single store (and 25% of exactly three stores).
The single stores are all CLOBBERs at the point variables go out of
scope.  Note unpatched we have

 store merging                      : 308.60 ( 84%)

Limiting the number of chains to 64 brings this down to

 store merging                      :   1.52 (  3%)

which is more reasonable.  There are ideas on how to make
terminate_all_aliasing_chains cheaper but for this degenerate case
they would not have any effect so I'll defer for GCC 12 for those.

I'm not sure we want to have both --params, just keeping the
more to-the-point max-stores-to-track works but makes the
degenerate case above slower.
I made the current default 1024 which for the testcasse
(without limiting chains) results in 25% compile time and 20s
putting it in the same ballpart as the next offender (which is PTA).

This is a regression on trunk and the GCC 10 branch btw.

2021-02-11  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/38474
	* params.opt (-param=max-store-chains-to-track=): New param.
	(-param=max-stores-to-track=): Likewise.
	* doc/invoke.texi (max-store-chains-to-track): Document.
	(max-stores-to-track): Likewise.
	* gimple-ssa-store-merging.c (pass_store_merging::m_n_chains):
	New.
	(pass_store_merging::m_n_stores): Likewise.
	(pass_store_merging::terminate_and_process_chain): Update
	m_n_stores and m_n_chains.
	(pass_store_merging::process_store): Likewise.   Terminate
	oldest chains if the number of stores or chains get too large.
	(imm_store_chain_info::terminate_and_process_chain): Dump
	chain length.