Re: [PATCH] more problems with newlib/libc/machine/m68k/memcpy.S

[Maxim Kuvyrkov <maxim at codesourcery dot com>]

On 2/9/10 5:15 PM, Josef Wolf wrote:
> Hello folks,
>
> I have done a closer look at newlib/libc/machine/m68k/memcpy.S and I think
> there are a couple of problems with this code:
>
> 1. When the destination pointer is mis-aligned, up to three bytes are copied
>     before getting into the optimized main loop. But d1, which contains the
>     number of bytes to copy, is never adjusted to account for this fact. As a
>     consequence, up to 3 byte too much can be copied by memcpy().

When destination is mis-aligned, D1 is properly adjusted here:

	and.l	#3,d0		| look for the lower two only
	beq	2f		| is aligned?
	sub.l	d0,d1

Or am I missing something?

> 2. When MISALIGNED_OK==0, no attempt is made to get the pointers to aligned
>     positions. This results in much slower code than the old code for such
>     CPUs. IMHO, the cause of the problem reported in the thread starting with
>     http://sourceware.org/ml/newlib/2009/msg01079.html was mis-interpreted.
>     The problem was _not_ the attempt to align dest. The problem was that
>     even after dest was aligned, src was still not aligned. Thus, disabling
>     alignment completely is the wrong response to that problem.

The problem was not misinterpreted; and it is true that memcpy can be 
optimized for CPUs without alignment module by using 2-byte copies.  It 
is not straightforward to implement a single memcpy routine that will be 
optimal for both CPUs that support unaligned accesses and those which 
don't -- that is why I posted a simple byte-copy fix.

It seems that your patch lengthens the critical path for CPUs which 
support unaligned accesses.

> 3. -mcpu32 seems to imply -mc68020. So the check for alignment capabilities
>     gives a wrong result for cpu32. BTW: alignment capabilities depend not
>     only on the CPU. It is also dependant on bus width and how the memory is
>     connected.

Thank you for pointing this out.

Regards,

--
Maxim Kuvyrkov
CodeSourcery
maxim@codesourcery.com
(650) 331-3385 x724