libstdc++: optimize bit iterators assuming normalization [PR110807]

libstdc++: optimize bit iterators assuming normalization [PR110807]

The representation of bit iterators, using a pointer into an array of
words, and an unsigned bit offset into that word, makes for some
optimization challenges: because the compiler doesn't know that the
offset is always in a certain narrow range, beginning at zero and
ending before the word bitwidth, when a function loads an offset that
it hasn't normalized itself, it may fail to derive certain reasonable
conclusions, even to the point of retaining useless calls that elicit
incorrect warnings.

Case at hand: The 110807.cc testcase for bit vectors assigns a 1-bit
list to a global bit vector variable.  Based on the compile-time
constant length of the list, we decide in _M_insert_range whether to
use the existing storage or to allocate new storage for the vector.
After allocation, we decide in _M_copy_aligned how to copy any
preexisting portions of the vector to the newly-allocated storage.
When copying two or more words, we use __builtin_memmove.

However, because we compute the available room using bit offsets
without range information, even comparing them with constants, we fail
to infer ranges for the preexisting vector depending on word size, and
may thus retain the memmove call despite knowing we've only allocated
one word.

Other parts of the compiler then detect the mismatch between the
constant allocation size and the much larger range that could
theoretically be copied into the newly-allocated storage if we could
reach the call.

Ensuring the compiler is aware of the constraints on the offset range
enables it to do a much better job at optimizing.  Using attribute
assume (_M_offset <= ...) didn't work, because gimple lowered that to
something that vrp could only use to ensure 'this' was non-NULL.
Exposing _M_offset as an automatic variable/gimple register outside
the unevaluated assume operand enabled the optimizer to do its job.

Rather than placing such load-then-assume constructs all over, I
introduced an always-inline member function in bit iterators that does
the job of conveying to the compiler the information that the
assumption is supposed to hold, and various calls throughout functions
pertaining to bit iterators that might not otherwise know that the
offsets have to be in range, so that the compiler no longer needs to
make conservative assumptions that prevent optimizations.

With the explicit assumptions, the compiler can correlate the test for
available storage in the vector with the test for how much storage
might need to be copied, and determine that, if we're not asking for
enough room for two or more words, we can omit entirely the code to
copy two or more words, without any runtime overhead whatsoever: no
traces remain of the undefined behavior or of the tests that inform
the compiler about the assumptions that must hold.

for  libstdc++-v3/ChangeLog

	PR libstdc++/110807
	* include/bits/stl_bvector.h (_Bit_iterator_base): Add
	_M_assume_normalized member function.  Call it in _M_bump_up,
	_M_bump_down, _M_incr, operator==, operator<=>, operator<, and
	operator-.
	(_Bit_iterator): Also call it in operator*.
	(_Bit_const_iterator): Likewise.