alias: Punt after walking too many VALUEs during a toplevel find_base_term call ...

alias: Punt after walking too many VALUEs during a toplevel find_base_term call [PR94045]

As mentioned in the PR, on a largish C++ testcase the compile time
on i686-linux is about 16 minutes on a fast box, mostly spent in
find_base_term recursive calls dealing with very deep chains of preserved
VALUEs during var-tracking.

The following patch punts after we process many VALUEs (we already have code
to punt if we run into a VALUE cycle).

I've gathered statistics on when we punt this way (with BITS_PER_WORD, TU,
function columns piped through sort | uniq -c | sort -n):
     36 32 ../../gcc/asan.c _Z29initialize_sanitizer_builtinsv.part.0
    108 32 _first_test.go reflect_test.reflect_test..import
   1005 32 /home/jakub/src/gcc/gcc/testsuite/gcc.dg/pr85180.c foo
   1005 32 /home/jakub/src/gcc/gcc/testsuite/gcc.dg/pr87985.c foo
   1005 64 /home/jakub/src/gcc/gcc/testsuite/gcc.dg/pr85180.c foo
   1005 64 /home/jakub/src/gcc/gcc/testsuite/gcc.dg/pr87985.c foo
   2534 32 /home/jakub/src/gcc/gcc/testsuite/gcc.dg/stack-check-9.c f3
   6346 32 ../../gcc/brig/brig-lang.c brig_define_builtins
   6398 32 ../../gcc/d/d-builtins.cc d_define_builtins
   8816 32 ../../gcc/c-family/c-common.c c_common_nodes_and_builtins
   8824 32 ../../gcc/lto/lto-lang.c lto_define_builtins
  41413 32 /home/jakub/src/gcc/gcc/testsuite/gcc.dg/pr43058.c test
Additionally, for most of these (for the builtins definitions tested just
one) I've verified with a different alias.c change which didn't punt but
in the toplevel find_base_term recorded if visited_vals reached the limit
whether the return value was NULL_RTX or something different, and in all
these cases the end result was NULL_RTX, so at least in these cases it
should just shorten the time until it returns NULL.

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

	PR rtl-optimization/94045
	* params.opt (-param=max-find-base-term-values=): New option.
	* alias.c (find_base_term): Add cut-off for number of visited VALUEs
	in a single toplevel find_base_term call.