Re: [BUG] PPC math-emu multiply problem

[Greg Weeks <greg.weeks@timesys.com>]

Greg Weeks wrote:

> Dan Malek wrote:
>
>>
>> On Jul 29, 2004, at 10:06 AM, Kumar Gala wrote:
>>
>>>
>>> On Jul 29, 2004, at 8:14 AM, Greg Weeks wrote:
>>>
>>>> I'm seeing what appears to be a bug in the ppc kernel trap math
>>>> emulator. An extreme case for multiplies isn't working the way gcc
>>>> soft-float or hardware floating point is.
>>>
>>>
>>
>> I'm not surprised.  I lifted this code from Sparc, glibc, and adapted
>> it as best I could for PPC years ago for the 8xx.  I was happy when
>> it appeared to work for the general cases. :-)
>>
>> Due to its overhead, I never expected it to be _the_ solution for
>> processors that don't have floating point hardware.  Recompiling
>> the libraries with soft-float and using that option when compiling
>> is the way to go.
>
>
> OK, this patch fixes my multiply problem with the LSB test. I still 
> need to test to make sure I didn't break anything else, but it appears 
> the rounding is only used when converting back to IEEE format. Is 
> there some reason this is something really dumb to do?
>
When I actually built a kernel rather than just my test code the 
FP_ROUNDMODE is picked up from the linux/math-emu/soft-fp.h. I don't 
want to change the common definition unless I'm sure this is the correct 
solution.

Signed-off-by: Greg Weeks <greg.weeks@timesys.com> under TS0087

--- tanith-linux-2.6.6/arch/ppc/math-emu/soft-fp.h.orig    2004-07-30 
10:31:34.000000000 -0400
+++ tanith-linux-2.6.6/arch/ppc/math-emu/soft-fp.h    2004-07-30 
11:18:59.000000000 -0400
@@ -14,9 +14,8 @@
 # define FP_RND_ZERO        1
 # define FP_RND_PINF        2
 # define FP_RND_MINF        3
-#ifndef FP_ROUNDMODE
-# define FP_ROUNDMODE        FP_RND_NEAREST
-#endif
+#undef FP_ROUNDMODE
+# define FP_ROUNDMODE        FP_RND_ZERO
 #endif
 
 #define _FP_ROUND_NEAREST(wc, X)            \