Make tree-ssa-strlen.c handle partial unterminated strings

[Andi Kleen]

Richard Sandiford <richard.sandiford@linaro.org> writes:

> 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.

Would that be useful as a warning too? If the pass can figure out
the final string can be not null terminated when passed somewhere else,
warn, because it's likely a bug in the program.

-Andi