Hi Ian, Thanks a lot for your detailed explanation.
> -----Original Message----- > From: Ian Lance Taylor [mailto:[EMAIL PROTECTED] > 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 <[EMAIL PROTECTED]> 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 >
