tree-ssa-strlen.c looks for cases in which a string is built up using
operations like:

    memcpy (a, "foo", 4);
    memcpy (a + 3, "bar", 4);
    int x = strlen (a);

As a side-effect, it optimises the non-final memcpys so that they don't
include the nul terminator.

However, after removing some "& ~0x1"s from tree-ssa-dse.c, the DSE pass
does this optimisation itself (because it can tell that later memcpys
overwrite the terminators).  The strlen pass wasn't able to handle these
pre-optimised calls in the same way as the unoptimised ones.

This patch adds support for tracking unterminated strings.

[Based on the code ARM contributed in branches/ARM/sve-branch@246236]

2017-07-02  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-ssa-strlen.c (strinfo): Rename the length field to
	nonzero_chars.  Add a full_string_p field.
	(compare_nonzero_chars, zero_length_string_p): New functions.
	(get_addr_stridx): Add an offset_out parameter.
	Use compare_nonzero_chars.
	(get_stridx): Update accordingly.  Use compare_nonzero_chars.
	(new_strinfo): Update after above changes to strinfo.
	(set_endptr_and_length): Set full_string_p.
	(get_string_length): Update after above changes to strinfo.
	(unshare_strinfo): Update call to new_strinfo.
	(maybe_invalidate): Likewise.
	(get_stridx_plus_constant): Change off to unsigned HOST_WIDE_INT.
	Use compare_nonzero_chars and zero_string_p.  Treat nonzero_chars
	as a uhwi instead of an shwi.  Update after above changes to
	strinfo and new_strinfo.
	(zero_length_string): Assert that chainsi contains full strings.
	Use zero_length_string_p.  Update call to new_strinfo.
	(adjust_related_strinfos): Update after above changes to strinfo.
	Copy full_string_p from origsi.
	(adjust_last_stmt): Use zero_length_string_p.
	(handle_builtin_strlen): Update after above changes to strinfo and
	new_strinfo.  Install the lhs as the string length if the previous
	entry didn't describe a full string.
	(handle_builtin_strchr): Update after above changes to strinfo
	and new_strinfo.
	(handle_builtin_strcpy): Likewise.
	(handle_builtin_strcat): Likewise.
	(handle_builtin_malloc): Likewise.
	(handle_pointer_plus): Likewise.
	(handle_builtin_memcpy): Likewise.  Track nonzero characters
	that aren't necessarily followed by a nul terminator.
	(handle_char_store): Likewise.

gcc/testsuite/
	* gcc.dg/strlenopt-32.c: New testcase.
	* gcc.dg/strlenopt-33.c: Likewise.
	* gcc.dg/strlenopt-33g.c: Likewise.
	* gcc.dg/strlenopt-34.c: Likewise.
	* gcc.dg/strlenopt-35.c: Likewise.

From-SVN: r249880