Re: V4L2: __ucmpdi2 undefined on ppc

Re: V4L2: __ucmpdi2 undefined on ppc

[David Woodhouse]

On Thu, 2006-12-14 at 14:58 -0500, Kyle McMartin wrote:
> I posted a patch to Paul this week to fix this, 

Hm, I didn't see it on linuxppc-dev.

> Since ppc32 can't do a 64bit comparison on its own it seems, gcc
> will generate a call to a helper function from libgcc. What other
> arches do is link libgcc.a into libs-y, and export the symbol
> they want from it.

You still get to 'accidentally' do 64-bit arithmetic in-kernel that way
though. Might be better just to provide __ucmpdi2, just as we have for
the other functions which are required from libgcc 

It'd be better just to fix the compiler though -- which is in fact what
they've done: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25724
              http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21237

I've applied this as a temporary hack to the Fedora kernel until the
compiler is updated there...

--- linux-2.6.19.ppc/arch/powerpc/kernel/misc_32.S~	2006-11-29 21:57:37.000000000 +0000
+++ linux-2.6.19.ppc/arch/powerpc/kernel/misc_32.S	2006-12-17 12:19:48.000000000 +0000
@@ -728,6 +728,27 @@ _GLOBAL(__lshrdi3)
 	or	r4,r4,r7	# LSW |= t2
 	blr
 
+/*
+ * __ucmpdi2: 64-bit comparison
+ *
+ * R3/R4 has 64 bit value A
+ * R5/R6 has 64 bit value B
+ * result in R3: 0 for A < B
+ *		 1 for A == B
+ *		 2 for A > B
+ */
+_GLOBAL(__ucmpdi2)
+	cmplw	r7,r3,r5	# compare high words
+	li	r3,0
+	blt	r7,2f		# a < b ... return 0
+	bgt	r7,1f		# a > b ... return 2
+	cmplw	r6,r4,r6	# compare low words
+	blt	r6,2f		# a < b ... return 0
+	li	r3,1
+	ble	r6,2f		# a = b ... return 1
+1:	li	r3,2
+2:	blr
+
 _GLOBAL(abs)
 	srawi	r4,r3,31
 	xor	r3,r3,r4
--- linux-2.6.19.ppc/arch/powerpc/kernel/ppc_ksyms.c~	2006-12-15 17:19:56.000000000 +0000
+++ linux-2.6.19.ppc/arch/powerpc/kernel/ppc_ksyms.c	2006-12-17 12:16:54.000000000 +0000
@@ -161,9 +161,11 @@ EXPORT_SYMBOL(to_tm);
 long long __ashrdi3(long long, int);
 long long __ashldi3(long long, int);
 long long __lshrdi3(long long, int);
+int __ucmpdi2(uint64_t, uint64_t);
 EXPORT_SYMBOL(__ashrdi3);
 EXPORT_SYMBOL(__ashldi3);
 EXPORT_SYMBOL(__lshrdi3);
+EXPORT_SYMBOL(__ucmpdi2);
 #endif
 
 EXPORT_SYMBOL(memcpy);

-- 
dwmw2

Re: V4L2: __ucmpdi2 undefined on ppc

[Stephane Marchesin]

On 12/17/06, David Woodhouse <dwmw2@infradead.org> wrote:
>
> You still get to 'accidentally' do 64-bit arithmetic in-kernel that way
> though. Might be better just to provide __ucmpdi2, just as we have for
> the other functions which are required from libgcc
>
> It'd be better just to fix the compiler though -- which is in fact what
> they've done: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25724
>               http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21237
>
> I've applied this as a temporary hack to the Fedora kernel until the
> compiler is updated there...
>

Hello,

We're hitting this i nouveau as well (http://nouveau.freedesktop.org),
since we make extensive use ot 64 bit ints. Over time, we've had a
number of reports on this issue, and at one point I read that it
should be fixed in gcc. But recently, a nouveau user on PPC32 (Henrik
in CC:) reported the issue again with gcc 4.2.3. Others have it on gcc
4.2.2 too:
http://bugs.freedesktop.org/show_bug.cgi?id=10547

So, the point of this email is to ask about the possibility of merging
in one of the __ucmpdi2 patches, like David's which is kept below for
reference. Most distros seem to ship with such a patch already, and it
seems that other drivers hit this as well.

Thanks,
Stephane


> --- linux-2.6.19.ppc/arch/powerpc/kernel/misc_32.S~     2006-11-29 21:57:37.000000000 +0000
> +++ linux-2.6.19.ppc/arch/powerpc/kernel/misc_32.S      2006-12-17 12:19:48.000000000 +0000
> @@ -728,6 +728,27 @@ _GLOBAL(__lshrdi3)
>         or      r4,r4,r7        # LSW |= t2
>         blr
>
> +/*
> + * __ucmpdi2: 64-bit comparison
> + *
> + * R3/R4 has 64 bit value A
> + * R5/R6 has 64 bit value B
> + * result in R3: 0 for A < B
> + *              1 for A == B
> + *              2 for A > B
> + */
> +_GLOBAL(__ucmpdi2)
> +       cmplw   r7,r3,r5        # compare high words
> +       li      r3,0
> +       blt     r7,2f           # a < b ... return 0
> +       bgt     r7,1f           # a > b ... return 2
> +       cmplw   r6,r4,r6        # compare low words
> +       blt     r6,2f           # a < b ... return 0
> +       li      r3,1
> +       ble     r6,2f           # a = b ... return 1
> +1:     li      r3,2
> +2:     blr
> +
>  _GLOBAL(abs)
>         srawi   r4,r3,31
>         xor     r3,r3,r4
> --- linux-2.6.19.ppc/arch/powerpc/kernel/ppc_ksyms.c~   2006-12-15 17:19:56.000000000 +0000
> +++ linux-2.6.19.ppc/arch/powerpc/kernel/ppc_ksyms.c    2006-12-17 12:16:54.000000000 +0000
> @@ -161,9 +161,11 @@ EXPORT_SYMBOL(to_tm);
>  long long __ashrdi3(long long, int);
>  long long __ashldi3(long long, int);
>  long long __lshrdi3(long long, int);
> +int __ucmpdi2(uint64_t, uint64_t);
>  EXPORT_SYMBOL(__ashrdi3);
>  EXPORT_SYMBOL(__ashldi3);
>  EXPORT_SYMBOL(__lshrdi3);
> +EXPORT_SYMBOL(__ucmpdi2);
>  #endif
>
>  EXPORT_SYMBOL(memcpy);
>
> --
> dwmw2
>

Re: V4L2: __ucmpdi2 undefined on ppc

[Stephane Marchesin]

On 2/6/08, Stephane Marchesin <marchesin@icps.u-strasbg.fr> wrote:
>
>  We're hitting this i nouveau as well (http://nouveau.freedesktop.org),
>  since we make extensive use ot 64 bit ints. Over time, we've had a
>  number of reports on this issue, and at one point I read that it
>  should be fixed in gcc. But recently, a nouveau user on PPC32 (Henrik
>  in CC:) reported the issue again with gcc 4.2.3. Others have it on gcc
>  4.2.2 too:
>  http://bugs.freedesktop.org/show_bug.cgi?id=10547
>
>  So, the point of this email is to ask about the possibility of merging
>  in one of the __ucmpdi2 patches, like David's which is kept below for
>  reference. Most distros seem to ship with such a patch already, and it
>  seems that other drivers hit this as well.
>

So, could we have that thing in main tree ? It's not like it's
untested, most distros carry that, and a couple of arches provide
their own ucmpdi2 implementation already. It's also such a small
function...

Stephane

Re: V4L2: __ucmpdi2 undefined on ppc

[Segher Boessenkool]

>> +/*
>> + * __ucmpdi2: 64-bit comparison
>> + *
>> + * R3/R4 has 64 bit value A
>> + * R5/R6 has 64 bit value B
>> + * result in R3: 0 for A < B
>> + *              1 for A == B
>> + *              2 for A > B
>> + */
>> +_GLOBAL(__ucmpdi2)
>> +       cmplw   r7,r3,r5        # compare high words
>> +       li      r3,0
>> +       blt     r7,2f           # a < b ... return 0
>> +       bgt     r7,1f           # a > b ... return 2
>> +       cmplw   r6,r4,r6        # compare low words
>> +       blt     r6,2f           # a < b ... return 0
>> +       li      r3,1
>> +       ble     r6,2f           # a = b ... return 1
>> +1:     li      r3,2
>> +2:     blr

Every occurrence of r7 here is wrong (and some of the r6).  Is there
any reason to do this in assembler code at all?


Segher

Re: V4L2: __ucmpdi2 undefined on ppc

[David Woodhouse]

On Sun, 2008-03-02 at 22:53 +0100, Segher Boessenkool wrote:
> Every occurrence of r7 here is wrong (and some of the r6).

Can you elucidate?

>   Is there any reason to do this in assembler code at all?

Is there any particular reason not to?

-- 
dwmw2

Re: V4L2: __ucmpdi2 undefined on ppc

[Scott Wood]

On Tue, Mar 04, 2008 at 04:37:28PM +0000, David Woodhouse wrote:
> 
> On Sun, 2008-03-02 at 22:53 +0100, Segher Boessenkool wrote:
> > Every occurrence of r7 here is wrong (and some of the r6).
> 
> Can you elucidate?

They were being used as condition registers rather than GPRs.

> >   Is there any reason to do this in assembler code at all?
> 
> Is there any particular reason not to?

Same reason most of the rest of the kernel is in C rather than assembly.

Of course, a better question is why we don't just link libgcc...

-Scott

Re: V4L2: __ucmpdi2 undefined on ppc

[Segher Boessenkool]

>> Every occurrence of r7 here is wrong (and some of the r6).
>
> Can you elucidate?

Sure!  It should read either "7" or "cr7".

>>   Is there any reason to do this in assembler code at all?
>
> Is there any particular reason not to?

1) If written in assembler, it needs to be written for every 
architecture
separately, and sometimes even for every architecture variant (32 vs. 64
bit is only the tip of the iceberg).

2) As shown here as well as in the recent strncmp() patch, it is a lot 
harder
to write correct assembler code than it is to write correct C code(*), 
and
the C code isn't less efficient either.


Segher

(*) Well, the generic C strncmp() code in the kernel is broken too, bad
example perhaps :-)

Re: V4L2: __ucmpdi2 undefined on ppc

[Paul Mackerras]

Segher Boessenkool writes:

> Every occurrence of r7 here is wrong (and some of the r6).  Is there
> any reason to do this in assembler code at all?

The fact that the obvious C code generates ... a call to __ucmpdi2? :)

Paul.

Re: V4L2: __ucmpdi2 undefined on ppc

[Scott Wood]

Paul Mackerras wrote:
> Segher Boessenkool writes:
> 
>> Every occurrence of r7 here is wrong (and some of the r6).  Is there
>> any reason to do this in assembler code at all?
> 
> The fact that the obvious C code generates ... a call to __ucmpdi2? :)

So use the correct C code, not the obvious C code. :-)

-Scott

Re: V4L2: __ucmpdi2 undefined on ppc

[Segher Boessenkool]

>> Every occurrence of r7 here is wrong (and some of the r6).  Is there
>> any reason to do this in assembler code at all?
>
> The fact that the obvious C code generates ... a call to __ucmpdi2? :)

Hrm?  Here's the "obvious" C code, portable to all architectures
(modulo the specific types used, this isn't a proposed patch):


unsigned int ucmpdi2(unsigned long long a, unsigned long long b)
{
         unsigned long ahi, bhi, alo, blo;

         ahi = a >> 32;
         bhi = b >> 32;
         if (ahi < bhi)
                 return 0;
         if (ahi > bhi)
                 return 2;

         alo = a;
         blo = b;
         if (alo < blo)
                 return 0;
         if (alo > blo)
                 return 2;

         return 1;
}


(libgcc does it a bit differently, with unions and stuff).


Segher