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Re: help with PR78809 - inline strcmp for small constant strings
- From: Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>
- To: Richard Henderson <richard dot henderson at linaro dot org>, Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Cc: nd <nd at arm dot com>
- Date: Fri, 4 Aug 2017 20:38:11 +0000
- Subject: Re: help with PR78809 - inline strcmp for small constant strings
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Richard Henderson wrote:
> On 08/04/2017 05:59 AM, Prathamesh Kulkarni wrote:
> > 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.
I'd suggest to do it as a target independent way, this is not a target specific
optimization and shouldn't be done in the target unless there are special
strcmp instructions.
> 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];
> }
If there is already code that does something similar (see comment #1 in PR78809),
it could be easily adapted to handle more cases.
> if (memcmp(s, "ab", 3) != 0)
>
> to be implemented with cmp+ccmp+ccmp and one branch.
Even better would be wider loads if you either know the alignment of s or it's max size
(although given the overhead of creating the return value that works best for equality).
Wilco