tree: Improve skip_simple_arithmetic [PR119183]

tree: Improve skip_simple_arithmetic [PR119183]

The following testcase takes very long time to compile, because
skip_simple_arithmetic decides to first call tree_invariant_p on
the second argument (and indirectly recurse there).  I think before
canonicalization of operands for commutative binary expressions
(and for non-commutative ones always) it is pretty common that the
first operand is a constant, something which tree_invariant_p handles
immediately, so the following patch special cases that; I've added
there a tree_invariant_p call too after the checks, while it is not
really needed currently, tree_invariant_p has the same checks, I wanted
to be prepared in case tree_invariant_p changes.  But if you think
I should avoid it, I can drop it too.

This is just a partial fix, I think one can certainly construct a testcase
which will still have horrible compile time complexity (but I've tried and
haven't managed to do so), so perhaps we should just limit the recursion
depth through skip_simple_arithmetic/tree_invariant_p with some defaulted
argument.

2025-03-11  Jakub Jelinek  <jakub@redhat.com>

	PR c/119183
	* tree.cc (skip_simple_arithmetic): If first operand of binary
	expr is TREE_CONSTANT or TREE_READONLY with no side-effects, call
	tree_invariant_p on that operand first instead of on the second.

	* gcc.dg/pr119183.c: New test.