Re: [PATCH] fold strings as long as the array they're stored in (PR 86428)

[Richard Biener <richard dot guenther at gmail dot com>]

On Sat, Jul 7, 2018 at 11:47 PM Martin Sebor <msebor@gmail.com> wrote:
>
> While working on other string folding improvements (PR 77357)
> I came across another distinct case where GCC could do better:
> it doesn't consider as candidates strings that have as many
> elements as the size of the array they are stored in, even if
> their length is within the bounds of the array.  For instance,
> only the first strlen() call below is folded even though
> the arguments to both are valid NUL-terminated strings.
>
>     const char a[4] = "123";
>     int f (void)
>     {
>       return strlen (a);   // folded
>     }
>
>     const char b[4] = "123\0";
>     int g (void)
>     {
>       return strlen (b);   // not folded
>     }
>
> The attached tiny patch adjusts the the string_constant() function
> to recognize as valid string constants all those whose length (as
> returned by strlen()) is less than the size of the array they are
> stored in.
>
> Tested on x86_64-linux.
>
> Testing of an earlier version of the patch exposed what I think
> is a deficiency in the (very) early strlen folding: c_strlen()
> folds expressions of the form strlen(A + I) to sizeof(A) - I when
> A is an array of known size and I a non-const variable.  This not
> only prevents -Warray-bounds from warning on invalid indices but
> also defeats other, possibly more profitable optimizations based
> on the range of the result of the strlen() call.  The logs show
> that the code dates at least as far back as 1992.  With VRP and
> other data flow based optimizations I think we can do better
> today.  I opened bug 86434 to improve things.

I think that

+      unsigned HOST_WIDE_INT length = strlen (TREE_STRING_POINTER (init));

is dangerous and you should use strnlen (TREE_STRING_POINTER (init),
TREE_STRING_LENGTH (init))
instead.

OK with that change.

Richard.

> Martin
>