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SubjectRe: [PATCH] add strncmp to PowerPC
On Fri, Feb 29, 2008 at 10:56:45PM -0500, Steven Rostedt wrote:
>
> On Sat, 1 Mar 2008, Benjamin Herrenschmidt wrote:
> >
> > Do we have any indication that it performs better than the C one ?
>
> See below.
>
> >
> > Ben.
> >
>
> > >
> > > +_GLOBAL(strncmp)
> > > + mtctr r5
> > > + addi r5,r3,-1
> > > + addi r4,r4,-1
> > > +1: lbzu r3,1(r5)
> > > + cmpwi 1,r3,0
> > > + lbzu r0,1(r4)
> > > + subf. r3,r0,r3
> > > + beqlr 1
> > > + bdnzt eq,1b
> > > + blr
> > > +
>
>
> And here's the objdump of the C version:
>
> 0000000000000080 <.strncmp>:
> 80: fb e1 ff f0 std r31,-16(r1)
> 84: f8 21 ff c1 stdu r1,-64(r1)
> 88: 7c 69 1b 78 mr r9,r3
> 8c: 7c a0 2b 79 mr. r0,r5
> 90: 38 60 00 00 li r3,0
> 94: 7c 09 03 a6 mtctr r0
> 98: 7c 3f 0b 78 mr r31,r1
> 9c: 41 82 00 68 beq- 104 <.strncmp+0x84>
> a0: 89 69 00 00 lbz r11,0(r9)
> a4: 88 04 00 00 lbz r0,0(r4)
> a8: 7c 00 58 50 subf r0,r0,r11
> ac: 78 00 06 20 clrldi r0,r0,56
> b0: 2f a0 00 00 cmpdi cr7,r0,0
> b4: 7c 00 07 74 extsb r0,r0
> b8: 7c 03 03 78 mr r3,r0
> bc: 40 9e 00 48 bne- cr7,104 <.strncmp+0x84>
> c0: 2f ab 00 00 cmpdi cr7,r11,0
> c4: 41 9e 00 40 beq- cr7,104 <.strncmp+0x84>
> c8: 38 84 00 01 addi r4,r4,1
> cc: 38 69 00 01 addi r3,r9,1
> d0: 42 40 00 30 bdz- 100 <.strncmp+0x80>
> d4: 88 03 00 00 lbz r0,0(r3)
> d8: 89 24 00 00 lbz r9,0(r4)
> dc: 38 63 00 01 addi r3,r3,1
> e0: 38 84 00 01 addi r4,r4,1
> e4: 2f 20 00 00 cmpdi cr6,r0,0
> e8: 7c 09 00 50 subf r0,r9,r0
> ec: 78 00 06 20 clrldi r0,r0,56
> f0: 2f a0 00 00 cmpdi cr7,r0,0
> f4: 7c 00 07 74 extsb r0,r0
> f8: 40 9e 00 08 bne- cr7,100 <.strncmp+0x80>
> fc: 40 9a ff d4 bne+ cr6,d0 <.strncmp+0x50>
> 100: 7c 03 03 78 mr r3,r0
> 104: e8 21 00 00 ld r1,0(r1)
> 108: eb e1 ff f0 ld r31,-16(r1)
> 10c: 4e 80 00 20 blr
>
>
> I'll let you decide ;-)
>
> Even if it was logically faster (which I still doubt) it's a hell of a lot
> of cache lines to waste.

Indeed, but there are some corner cases that the C code handles. Like
a length of 0 which may lead to infinite loop in the asm code.

OTOH, I'm a bit surprised by the extsb instructions in the compiler generated
code. We don't compile with -fsigned-char, do we? The clrldi
instructions are also extremely stupid.

Now that I think a bit more about it, I believe that the C version is
incorrect: the clrldi/extsb dance takes a value between -255 and +255
and collapses it into the -128 to 127 range, meaning that the return
value may be wrong if we rely on the sign of the result. So unless I
miss something, the problem is much more serious than just stupid code
(I had just a look at the libc version in C and characters are cast to
unsigned char before the comparison).

Regards,
Gabriel


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