optimize basic_string

Add __builtin_unreachable conditionls to declare value ranges of
basic_string::length().  FIx max_size() to return actual max size
using logic similar to std::vector. Aviod use of size() in empty()
to save some compile time overhead.

As disucced, max_size() change is technically ABI breaking, but
hopefully this does not really matter in practice.

Change of length() breaks empty-loop testcase where we now optimize the
loop only after inlining, so template is updated to check cddce3 instead
of cddce2. This is PR117764.

With these chages we now optimize out unused strings as tested in
string-1.C

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string::size(),
	basic_string::length(), basic_string::capacity()): Add
	__builtin_unreachable to declare value ranges.
	(basic_string::empty()): Implement directly
	(basic_string::max_size()): Account correctly the terminating 0
	and limits implied by ptrdiff_t.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/empty-loop.C: xfail optimization at cddce2 and check
	it happens at cddce3.
	* g++.dg/tree-ssa/string-1.C: New test.

0 files changed, 0 insertions, 0 deletions