On 6/13/19 5:50 PM, Martin Sebor wrote:
> While integrating the strlen and sprintf passes and investigating
> optimization opportunities that it opens up I noticed a few related
> to a strcmp optimization implemented in GCC 9. One is to take
> advantage of the fact that a nul-terminated string of a known
> length cannot be equal to a string whose length is greater, even
> if it isn't known exactly. For example, the equality below must
> be false:
>
> int f (char * restrict a, char * restrict b)
> {
> memcpy (a, "1234", 4); // length >= 4
> strcpy (b, "123"); // length == 3
>
> return strcmp (a, b) == 0; // must be false
> }
>
> The attached patch enhances the existing optimization to exploit
> this opportunity.
>
> Tested on x86_64-linux.
>
> Martin
>
> PS There are several more improvements to be made here. I've been
> raising bugs for them and I plan to submit patches for them in
> the near future. (Incidentally, they are all in the spirit of
> the suggestion made back in 2012 in pr52171.)
>
> gcc-90626.diff
>
> PR tree-optimization/90626 - fold strcmp(a, b) == 0 to zero when one string
> length is exact and the other is unequal
>
> gcc/ChangeLog:
>
> PR tree-optimization/90626
> * tree-ssa-strlen.c (strxcmp_unequal): New function.
> (handle_builtin_string_cmp): Call it.
>
> gcc/testsuite/ChangeLog:
>
> PR tree-optimization/90626
> * gcc.dg/strlenopt-65.c: New test.
> * gcc.dg/strlenopt-66.c: New test.
> * gcc.dg/strlenopt.h (strcmp, strncmp): Declare.
OK. Good to see we're able to extend Qing's work to handle more cases
without major surgery.
jeff
