[DTrace-devel] [DTrace] DTrace userspace branch dm/2.0-branch-dev-aggs updated. 2ffdae6376cbe280267577edbfc5dc93a776398f

[Eugene Loh]

----- kris.van.hees at oracle.com wrote:

> On Wed, Nov 25, 2020 at 09:41:33AM -0800, Eugene Loh wrote:
> > > +
> > > +	// Change to a positive value
> > > +	BPFCODE(BPF_NEG_REG(idxreg))
> > 
> > That changes the magnitude.  Probably need BPF_MUL_IMM(regfoo, -1) or 
> > something.  Note that x and -x are not 2s complements.  If x and y are 
> > 2s complements, they add up to -1.  x and -x add up to 0.
> 
> If the purpose of this code is to turn a vlaue that is negative into one that
> is positive, i.e. the value is a int64_t, then why would BPF_NEG_REG(reg) be
> wrong?  It is actually implemented as: regs[reg] = -regs[reg]

Yup.  You're right.  My bad.  I was thinking bitwise NOT.