[LLVMdev] Inferring dependencies in phi instructions

[Anirudh Sivaraman]

On Jun 29, 2015 3:16 AM, "Evgeny Astigeevich" <evgeny.astigeevich at arm.com>
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.
> What is the problem with using phi-functions in DFG? Yes they require more
> work to find out what they depend on.
>

Thank you for your response. Going by your definition, x is control
dependent on y. To extract this control dependency, do I need to maintain
path conditions for each basic block or can I do something simpler?

Also, I feel like this should be a recurring problem. Could you point me to
any code example that identifies all dependencies (control and data) for
phi instructions?

> 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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