RE: [ping][patch] Fixed-point patch 2/10

["Fu, Chao-Ying" <fu@mips.com>]

Mark Mitchell wrote:
> 
> Fu, Chao-Ying wrote:
> 
> > gcc/ChangeLog
> > 2007-08-01  Chao-ying Fu  <fu@mips.com>
> > 
> >         * fixed-value.h: New file.
> >         * fixed-value.c: New file.
> >         * Makefile.in (OBJS-common): Add fixed-value.o.
> >         (fixed-value.o): New rule.
> >         (GTFILES): Add fixed-value.h.
> >         * double-int.c (double_int_scmp): Use casts of 
> unsigned HOST_WIDE_INT
> >         to compare a.low and b.low.
> >         * gengtype.c (main): Handle FIXED_VALUE_TYPE type 
> as scalar typedef.
> 
> This is OK -- except that I do not understand the change to
> double_int_scmp.  That function explicitly assumes that the double int
> is signed, as opposed to double_int_ucmp, which assumes that it is
> unsigned.  Please explain.

  I think the sign bit is in the "high" part, so we need to 
compare the "low" part as unsigned for "double_int_scmp".

Ex 1:  If A.high and B.high are 0, A and B are positive numbers.
Then, when we compare A.low and B.low, we should use unsigned compare.
A.high = 0 and A.low = 0x8000000000000000
B.high = 0 and B.low = 0x0000000000000001
=> A > B, 
because A.high == B.high and
(unsigned HOST_WIDE_INT) a.low > (unsigned HOST_WIDE_INT) b.low

Ex 2: (A and B are negative. )
A.high = 0xffffffffffffffff and A.low = 0x8000000000000000
B.high = 0xffffffffffffffff and B.low = 0x0000000000000001
=> A > B, 
because A.high == B.high and
(unsigned HOST_WIDE_INT) a.low > (unsigned HOST_WIDE_INT) b.low

  Thanks!

Regards,
Chao-ying