help with PR78809 - inline strcmp for small constant strings

Segher Boessenkool segher@kernel.crashing.org
Fri Aug 4 22:30:00 GMT 2017


On Fri, Aug 04, 2017 at 08:38:11PM +0000, Wilco Dijkstra wrote:
> 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.

See rs6000-string.c; cc:ing Aaron.

> > 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).

All those things are handled there, too.  Most things that can really help
are quite target-specific, I think.


Segher



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