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

[Daniel Berlin]

Updated patch attached

On Fri Jan 30 2015 at 1:58:04 PM Daniel Berlin <dberlin at dberlin.org> wrote:

> 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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