Thanks Martin!
The example I showed is simplified and I had not specified that I was using
LTO, but I am. Thanks for asking me to clarify this.
The issue I have with the information in the Lattice section of the dump is
that it shows the information once the analysis has achieved a fixed point.
However, the example is a bit more complicated. There is a recursive call
in bar and some pointer manipulation which will for sure remove the static
information that the incoming parameter to bar is a multiple of 8.
bar (void *a) {
// some pointer manipulation
a = // pointer manipulation based on a, casts to (char*) arithmetic...
bar (a);
}
However, I am mostly interested in the static information available at the
callsite foo -> bar and not on the incoming static information available at
the parameter "a" of bar. Does this make sense?
Is there a place where I can look at the static information available at
the argument and not the parameter?
Thanks!
On Thu, 17 Feb 2022 at 17:25, Martin Jambor <mjam...@suse.cz> wrote:
> Hi,
>
> On Thu, Feb 17 2022, Erick Ochoa via Gcc wrote:
> > Hello,
> >
> > I'm trying to understand ipa-bit-cp/ipa-cp and how the known bits are
> > propagated to the lattice in the case of a pointer_plus_expr.
> >
> > I have a piece of code similar to the following (this being a simplified
> > example)
> >
> > int main ()
> > {
> > // a = some pointer.
> > foo (a);
> > }
> >
> > foo (void* a)
> > {
> > bar (a + 1);
> > }
> >
> > bar (void *a) {}
> >
> > I have the following ipa-cp dump:
> >
> > callsite main -> foo
> > param 0: UNKNOWN
> > value: 0x0, mask 0xfffffffffffffff8
> >
> >
> > as it is passed to bar I have the following:
> >
> > callsite foo -> bar
> > param 0: PASS THROUGH: 0, op poiner_plus_expr 8
> > value: 0x0, mask 0xffffffffffffffff
> >
> > My question is, shouldn't the mask to bar be 0xfffffffffffffff8?
> >
> > If I understand correctly the mask being 0xfffffffffffffff8 implies that
> > the value will be a multiple of 8. By adding 8 (or any multiple of 8) to
> > it, it should still be a multiple of 8. Where I believe the mask becomes
> > 0xffffffffffffffff is that during the addition there might be an overflow
> > of the masks and the operand 8. But I am still unsure. Can someone point
> > out what is happening here?
>
> If you do not use LTO, foo and bar must be static for IPA-CP to
> determine that their arguments are aligned - and correctly so, if there
> are unknown callers passing unknown values, we cannot assume anything.
> With LTO or them being static, IPA-CP can do so.
>
> But make sure you understand that different things in the dump mean
> different stuff, you will not find any propagated values in the
> jump-function dump which you quoted, those really just describe the call
> without any larger context. For results of propagation you need to look
> into the "Lattices" section of the dump.
>
> Martin
>
