If you have x == 1 || x == 2 || x == 3 || x == 4, the pass considered the set
{x==1, x==2, x==3, x==4}
and then also
{x==2, x==3, x==4}
{x==3, x==4}
Since the comparison is itself linear in the size of the set, this was overall
taking time quadratic in the length of the || or && sequence.
Worse, if it found duplicates, they'd be reported a quadratic number of times.
This CL cuts the time and output to linear by avoiding already-checked
subexpressions. This cuts the time spent analyzing cmd/compile/internal/ssa
(with all passes enabled, not just this one) by 20%.
Fixesgolang/go#28086.
Change-Id: I812f64bd5a44fea995c9ab0c4fa2fbefb44037ce
Reviewed-on: https://go-review.googlesource.com/c/tools/+/176457
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
d81a07b7e5