[LLVMdev] mem2reg Question

[David Greene]

On Friday 19 September 2008 17:17, Eli Friedman wrote:
> On Fri, Sep 19, 2008 at 11:08 AM, David Greene <dag at cray.com> wrote:
> > \Why is this only prevented when NOT storing
> > to global values?
>
> It's not a question of storing *to* a global value, but storing a
> global value into the alloca.  If there's only a single store to an

Ah, ok.

> alloca, there are only two possible values that can be extracted from
> the alloca: the value stored, and undef.  Assuming it's well-defined,
> it's always correct to use the value stored, because undef can be
> anything.

I agree.

> The reason for the check is question is that the stored value might
> not dominate the use.  This is only possible, however, if the stored

Yes, that's the case I am seeing.

> value is an instruction; dominance doesn't make sense if
> StoringGlobalVal is true.

Ok, so you're saying that because we're storing a global value, by
definition it DOES dominate the use because it's a global (and
thus live-in to the Function).  Is that right?

In the case I'm seeing, the "global value" is a constant.  So I guess it's
ok to forward that constant value to the use even though the store does
not dominate it (as you say, undef can be anything).

All right, this makes sense to me.

But why is it any different if the stored value is from an instruction?  We're
storiung some value into an alloca (under a condition in this case) and the
alloca later gets used in a place not dominated by the store (outside the
condition in this case).  Who cares where the value being stored came from?

                                         -Dave