help with PR78809 - inline strcmp for small constant strings

Richard Henderson
Fri Aug 4 18:18:00 GMT 2017

On 08/04/2017 05:59 AM, Prathamesh Kulkarni wrote:
> Hi,
> I was having a look at PR78809.
> For the test-case:
> int t1(const char *s) { return __builtin_strcmp (s, "a"); }
> for aarch64, trunk with -O2 generates:
> t1:
>         adrp    x1, .LANCHOR0
>         add     x1, x1, :lo12:.LANCHOR0
>         b       strcmp
> For i386, it seems strcmp is expanded inline via cmpstr optab by 
> expand_builtin_strcmp if one of the strings is constant. Could we similarly
> define cmpstr pattern for AArch64?

Certainly that's possible.

> For constant strings of small length (upto 3?), I was wondering if it'd be a
> good idea to manually unroll strcmp loop, similar to __strcmp_* macros in
> bits/string.h?>
> For eg in gimple-fold, transform
> x = __builtin_strcmp(s, "ab")
> to
> x = s[0] - 'a';
> if (x == 0)
> {
>   x = s[1] - 'b';
>   if (x == 0)
>     x = s[2];
> }

I don't know that it would be helpful to do that as early as gimple-fold.
Surely rtl-expand is early enough for this.

Indeed, if the gimple passes are able to transform

  strcmp(s, "ab")
  memcmp(s, "ab", 3)

when it can be shown that S has at least 3 bytes known to be allocated (e.g. s
is an array of known size).  This might allow

  if (memcmp(s, "ab", 3) != 0)

to be implemented with cmp+ccmp+ccmp and one branch.


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