Re: [PATCH] Fix carry bug in 128-bit unsigned integer adding

On Jan 6, 2008, at 8:26 AM, Liu Yu wrote:

This bug exists in math emulation for powerpc.
The macro define are mainly used by multiplication.

When adding two unsigned operands ( r = x + y ),
the carry bit can be counted by whether r is less than x.
However, when adding three unsigned operands, this method does not  
work.

The original code below uses this method to count carry,
it apparently overlook the case of three operands.
Assume all the operands is 32-bit wide,
( r = x + y + last_carry , x = 0, y = 0xffffffff, last_carry = 1),
then r is no less than x but it actually gets a carry.

I tried to fix this bug, but this patch seems not that pretty.
Are there any better ideas?
Comments are always welcomed!

take a look at how include/math-emu/op-4.h implements __FP_FRAC_ADD_4  
& __FP_FRAC_SUB_4.  Will that fix the bug, if so we should make the  
code match how its done there.

- k