[LLVMdev] [PATCH]: Allow PRE to insert no-cost phi nodes

[Daniel Berlin]

On Wed Jan 28 2015 at 9:51:02 PM Hal Finkel <hfinkel at anl.gov> wrote:

> [moving to llvm-commits for patch review]
>
> Hi Danny,
>
> This seems reasonable to me.
>
> You're patch has tabs in it, which you'll need to remove ;)
>

Sorry, i haven't had emacs set up for llvm in a while :)
I'll make sure it happens


>
> +; <label>:6                                       ; preds = %4, %2
> +; CHECK: pre-phi
> +; CHECK-NEXT: ret i32 %.pre-phi
> +  %7 = add nsw i32 %a, %b
> +  ret i32 %7
>
> I'd like to see this test case be more explicit about what it is
> expecting. Can you please check for the new phi you're expecting with its
> value, and then check that it is returned?
>

Will do.


>
> Thanks again,
> Hal
>
> ----- Original Message -----
> > From: "Daniel Berlin" <dberlin at dberlin.org>
> > To: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> > Sent: Wednesday, January 28, 2015 5:15:20 PM
> > Subject: [LLVMdev] [PATCH]: Allow PRE to insert no-cost phi nodes
> >
> >
> >
> > Current PRE tries to not increase code size.
> > It does this with a check whether or not the number of places it
> > would have to insert is 1 or not.
> >
> >
> > The problem with this is that the answer could also be "we have to
> > insert in zero places" in the case it's available in all
> > predecessors.
> > :)
> >
> >
> > Normal dominator based elimination in GVN will not catch these cases
> > because the branches do not dominate the merge point.
> >
> >
> > A simple example
> > int c;
> >
> > int d;
> > int pre(int foo, int a, int b)
> > {
> > int g;
> > if (foo) {
> > c = a+b;
> > } else {
> > d = a+ b;
> > }
> > g = a+b;
> > return g;
> > }
> >
> >
> > Without this patch, PRE (and GVN) will not eliminate g = a+b.
> > With this patch, PRE will create phi(c, d) and eliminate g = a + b.
> >
> >
> > (It is tremendously more difficult to make current GVN understand the
> > problem, and GCC solves this the same way i'm about to).
> >
> >
> > The patch looks large because of code movement, but it basically all
> > boils down to changing this check:
> >
> >
> > if (NumWithout != 1 || NumWith == 0)
> >
> >
> > to
> > if (NumWithout > 1 || NumWith == 0)
> >
> >
> > and skipping insertion when NumWithout == 0
> >
> >
> > testcase included.
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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