RE: [PATCH] Fix bug on soft emulation of float point in gcc/longlong.c

["Liu Yu" <Yu dot Liu at freescale dot com>]

Hi Ian,

Thanks a lot for your detailed explanation.

> -----Original Message-----
> From: Ian Lance Taylor [mailto:iant@google.com] 
> Sent: Thursday, March 13, 2008 1:45 PM
> To: Liu Yu
> Cc: gcc@gcc.gnu.org
> Subject: Re: [PATCH] Fix bug on soft emulation of float point 
> in gcc/longlong.c
> 
> Liu Yu <Yu.Liu@freescale.com> writes:
> 
> > There are 2 bugs existing in __udiv_qrnnd_c:
> >
> > 1. remainder could not be counted correctly
> > __r1 and __r0 are not adding enough, so I use do..while to replace 
> > former if
> >
> > 2. the case of overflow of __m are not considered so I add 
> a couple of 
> > lines to handle it
> >
> > I found the bugs from Linux kernel,
> > for PowerPC math-emu use __udiv_qrnnd_c to emulate float div.
> 
> Is the kernel code testing UDIV_NEEDS_NORMALIZATION and 
> implementing the normalization required before calling __udiv_qrnnd_c?
> 
> 
> > I don't know how to trigger __udiv_qrnnd_c in gcc.
> 
> gcc will use __udiv_qrnnd_c in -lgcc for division and modulos 
> of 64-bit values on processors which can't do that directly 
> and for which there is no alternate implementation in longlong.h.
> 
> 
> > but if the argument of __udiv_qrnnd_c are like
> > 0x07fffff8 0x07fffff8 0x00210fff 0xc0000000 0x07fffff8, the 
> bug will 
> > be reproduced.
> 
> Those arguments to __udiv_qrnnd_c are not valid.  For correct 
> operation, the most significant bit of the last argument is 
> required to be 1.  UDIV_NEEDS_NORMALIZATION signals this 
> fact.  In gcc the required normalization is implemented 
> before calling __udiv_qrnnd_c, in __udivmoddi4 in libgcc2.c.
> 
> This normalization approach is taken, rather than introducing 
> while loops as you suggest, because the while loops can run 
> for quite a long time on small numbers.
> 
> Ian
>