[LLVMdev] Inferring dependencies in phi instructions

[John Criswell]

On 6/29/15 5:16 AM, Evgeny Astigeevich wrote:
> Hi Anirudh,
>
> 'x' has a control dependency on 'y' because the value assigned to 'x'
> depends on a path selected. This dependency can be converted into a data
> dependency by means of a 'select' instruction because the control flow is
> simple.

Just an FYI, there is an optimization called "If-Conversion" which 
transforms control dependencies to data dependencies; it is discussed in 
the Allen and Kennedy book ("Optimizing Compilers for Modern 
Architectures" in Chapter 6 or 7,  I think).

If you need to find control dependencies, you can find them easily using 
LLVM's ReverseDominanceFrontier pass.

Regards,

John Criswell

> What is the problem with using phi-functions in DFG? Yes they require more
> work to find out what they depend on.
>
> Kind regards,
> Evgeny Astigeevich
>
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
> Behalf Of Anirudh Sivaraman
> Sent: 29 June 2015 07:25
> To: LLVM Developers Mailing List
> Subject: [LLVMdev] Inferring dependencies in phi instructions
>
> I am trying to infer data dependencies using LLVM's def-use chains and I am
> having trouble dealing with 'phi' instructions. Specifically,
>
> If I am given the code snippet:
>
> int foo() {
>    int y = 1;
>    int x;
>    if (y) {
>      x = 2;
>    } else {
>      x = 3;
>    }
>    return x;
> }
>
> Here, x has a data dependence on y (not control because x is assigned in
> both halves), but LLVM expresses 'x' as: %x.0 = phi i32 [ 2, %2 ], [ 3, %3 ]
> using phi-nodes, omitting the dependence on y.
>
> If I write the function as:
>
> int foo() {
>    int y = 1;
>    int x = y ? 2 : 3;
>    return x;
> },
>
> the dependencies work out alright because LLVM now uses a select
> instruction, where the dependence on y is explicit:
>
> %y = select i1 %x, i32 2, i32 3
>
> In general, I don't think I can convert phi nodes into select statements
> (because a phi node can refer to values from more than two paths in general,
> while select statements permit only two options, one each for a true and a
> false branch). Any thoughts on how this is handled in practice would be very
> helpful.
>
> Anirudh
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-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell