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

[Hal Finkel]

[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 ;)

+; <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?

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.
> 
> 
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> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory