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

[Liu Yu]

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.

I sent the patch to the maintainers of kernel, but they replied to me
that
this code came from gcc and should better keep coincident.
So I think I should send patch to here.

I don't know how to trigger __udiv_qrnnd_c in gcc.
but if the argument of __udiv_qrnnd_c are like
0x07fffff8 0x07fffff8 0x00210fff 0xc0000000 0x07fffff8,
the bug will be reproduced.

Also, you can try the program below, I have test it on Linux kernel
math-emu but haven't test on gcc.

---
#include <stdio.h>

union fu {
        unsigned int u;
        float f;
};

union fu fdiv(union fu a, union fu b, union fu expected)
{
        union fu z;
        z.f = a.f / b.f;

        printf("fdiv %08x / %08x = %08x expected %08x %s\n",
                        a.u, b.u, z.u, expected.u,
                        (z.u == expected.u) ? "(PASS)" : "(FAIL)");
        printf("     %e / %e = %e\n", a.f, b.f, z.f);
}

int main()
{
        union fu fa,fb,fe;

        fa.u = 0xc0843fff;
        fb.u = 0x80ffffff;
        fe.u = 0x7f044000;
        fdiv(fa,fb,fe);
}
--


---
 gcc/longlong.h |   18 ++++++++++++------
 1 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/gcc/longlong.h b/gcc/longlong.h
index 6c24564..256399b 100644
--- a/gcc/longlong.h
+++ b/gcc/longlong.h
@@ -1368,12 +1368,15 @@ UDItype __umulsidi3 (USItype, USItype);
     __q1 = (n1) / __d1;
\
     __m = (UWtype) __q1 * __d0;
\
     __r1 = __r1 * __ll_B | __ll_highpart (n0);
\
+    if (__d0 && __q1 > __m / __d0)
\
+       do {
\
+           __q1--, __r1 += (d);
\
+       } while (__r1 >= (d));
\
     if (__r1 < __m)
\
       {
\
-       __q1--, __r1 += (d);
\
-       if (__r1 >= (d)) /* i.e. we didn't get carry when adding to __r1
*/\
-         if (__r1 < __m)
\
+       do {
\
            __q1--, __r1 += (d);
\
+       } while (__r1 >= (d) && __r1 < __m);
\
       }
\
     __r1 -= __m;
\
 
\
@@ -1381,12 +1384,15 @@ UDItype __umulsidi3 (USItype, USItype);
     __q0 = __r1 / __d1;
\
     __m = (UWtype) __q0 * __d0;
\
     __r0 = __r0 * __ll_B | __ll_lowpart (n0);
\
+    if (__d0 && __q0 > __m / __d0)
\
+       do {
\
+           __q0--, __r0 += (d);
\
+       } while (__r0 >= (d));
\
     if (__r0 < __m)
\
       {
\
-       __q0--, __r0 += (d);
\
-       if (__r0 >= (d))
\
-         if (__r0 < __m)
\
+       do {
\
            __q0--, __r0 += (d);
\
+       } while (__r0 >= (d) && __r0 < __m);
\
       }
\
     __r0 -= __m;
\
 
\
--