Re: PATCH: PR target/42542: Vectorizer produces incorrect results on max of signed intergers

[Paolo Bonzini <bonzini at gnu dot org>]

On 01/05/2010 05:11 PM, Uros Bizjak wrote:
> On 01/05/2010 04:55 PM, Paolo Bonzini wrote:
> > > > I see no other way to fixup the underflow. Some comments below:
> >
> > You can always subtract (or XOR) the sign bit to turn an unsigned
> > value to the corresponding signed value. The attached patch (on top of
> > the code before H.J.'s patch) fixes the C testcase in the PR, but I
> > didn't test it further and I don't have time unfortunately.
>
> It will work until you change the sign bit of 0x80000000, either of an
> input operand or an intermediate result of subtraction. For the
> reference of interested readers, this problem is described in [1].

No, that's a different thing.  The problems with 0x8000000 arise when 
you have 0x80000000 as a signed number, while here you have 0x80000000 
as an unsigned number.

Subtracting 0x80000000 from a number in the range 0 to 0xFFFFFFFF will 
produce a result in the range -0x80000000 to 0x7FFFFFFF which is exactly 
two's complement.  XOR just happens to give the same result.

It's a very well known trick IMNSHO.  Please provide a counterexample if 
you think there is one.

Paolo