[polly] r246441 - Always use the branch instructions to model the PHI-node writes

[Johannes Doerfert via llvm-commits]

I was pretty sure this worked at some point. Why else do we have the
code below? It will create a dependence from the Instruction that is
incoming to the PHI to the predecessor block of the PHI.

On 08/31, Tobias Grosser wrote:
> On 08/31/2015 03:44 PM, Johannes Doerfert via llvm-commits wrote:
> >I don't believe this change was necessary because we have this part:
> >
> >     Instruction *OpI = dyn_cast<Instruction>(Op);
> >     if (OpI) {
> >       BasicBlock *OpIBB = OpI->getParent();
> >       // As we pretend there is a use (or more precise a write) of OpI in OpBB
> >       // we have to insert a scalar dependence from the definition of OpI to
> >       // OpBB if the definition is not in OpBB.
> >       if (OpIBB != OpBB) {
> >         IRAccess ScalarRead(IRAccess::READ, OpI, ZeroOffset, 1, true, OpI);
> >         AccFuncMap[OpBB].push_back(std::make_pair(ScalarRead, PHI));
> >         IRAccess ScalarWrite(IRAccess::MUST_WRITE, OpI, ZeroOffset, 1, true,
> >                               OpI);
> >         AccFuncMap[OpIBB].push_back(std::make_pair(ScalarWrite, OpI));
> >       }
> >     }
> >
> >Please tell me where I am mistaken?
> 
> Input
> =====
> loop:
>   %indvar = phi i64 [0, %entry], [%indvar.next, %backedge]
>   %val0 = fadd float 1.0, 2.0
>   %val1 = fadd float 1.0, 2.0
>   %val2 = fadd float 1.0, 2.0
>   br i1 %cond0, label %branch1, label %backedge
> 
> branch1:
>   br i1 %cond1, label %branch2, label %backedge
> 
> branch2:
>   br label %backedge
> 
> backedge:
>   %merge = phi float [%val0, %loop], [%val1, %branch1], [%val2, %branch2]
> 
> 
> Before the change
> =================
> 
> polly.stmt.loop:                                  ; preds = %polly.stmt.loop.entry
>   %polly.subregion.iv = phi i32 [ 0, %polly.stmt.loop.entry ]
>   %p_val0 = fadd float 1.000000e+00, 2.000000e+00
>   %p_val1 = fadd float 1.000000e+00, 2.000000e+00
>   %p_val2 = fadd float 1.000000e+00, 2.000000e+00
>   store float %p_val0, float* %merge.phiops
>   store float %p_val1, float* %val1.s2a
>   store float %p_val2, float* %val2.s2a
>   store float %p_val1, float* %merge.phiops
>   store float %p_val2, float* %merge.phiops
>   %polly.subregion.iv.inc = add i32 %polly.subregion.iv, 1
>   br i1 %cond0, label %polly.stmt.branch1, label %polly.stmt.backedge.exit
> 
> polly.stmt.branch1:                               ; preds = %polly.stmt.loop
>   br i1 %cond1, label %polly.stmt.branch2, label %polly.stmt.backedge.exit
> 
> polly.stmt.branch2:                               ; preds = %polly.stmt.branch1
>   br label %polly.stmt.backedge.exit
> 
> polly.stmt.backedge.exit:                         ; preds = %polly.stmt.branch2, %polly.stmt.branch1, %polly.stmt.loop
>   br label %polly.stmt.backedge
> 
> polly.stmt.backedge:                              ; preds = %polly.stmt.backedge.exit
>   %merge.phiops.reload = load float, float* %merge.phiops
> 
> 
> After the change
> ================
> 
> polly.stmt.loop:                                  ; preds = %polly.stmt.loop.entry
>   %polly.subregion.iv = phi i32 [ 0, %polly.stmt.loop.entry ]
>   %p_val0 = fadd float 1.000000e+00, 2.000000e+00
>   %p_val1 = fadd float 1.000000e+00, 2.000000e+00
>   %p_val2 = fadd float 1.000000e+00, 2.000000e+00
>   store float %p_val0, float* %merge.phiops
>   store float %p_val1, float* %val1.s2a
>   store float %p_val2, float* %val2.s2a
>   %polly.subregion.iv.inc = add i32 %polly.subregion.iv, 1
>   br i1 %cond0, label %polly.stmt.branch1, label %polly.stmt.backedge.exit
> 
> polly.stmt.branch1:                               ; preds = %polly.stmt.loop
>   store float %p_val1, float* %merge.phiops
>   br i1 %cond1, label %polly.stmt.branch2, label %polly.stmt.backedge.exit
> 
> polly.stmt.branch2:                               ; preds = %polly.stmt.branch1
>   store float %p_val2, float* %merge.phiops
>   br label %polly.stmt.backedge.exit
> 
> polly.stmt.backedge.exit:                         ; preds = %polly.stmt.branch2, %polly.stmt.branch1, %polly.stmt.loop
>   br label %polly.stmt.backedge
> 
> polly.stmt.backedge:                              ; preds = %polly.stmt.backedge.exit
>   %merge.phiops.reload = load float, float* %merge.phiops
> 
> 
> Before the change, all writes to %merge.phiops have been placed in polly.stmt.loop.
> As a result, the last write - in this case 'store float %p_val2, float* %merge.phiops' -
> defines the value read by the PHI node independently of any control flow decisions.
> After the change, the writes happen in the basic-blocks through which the backedge
> block is reached and depending on which control flow is taken, different values will
> be written in our phi location.
> 
> Best,
> Tobias

-- 

Johannes Doerfert
Researcher / PhD Student

Compiler Design Lab (Prof. Hack)
Saarland University, Computer Science
Building E1.3, Room 4.31

Tel. +49 (0)681 302-57521 : doerfert at cs.uni-saarland.de
Fax. +49 (0)681 302-3065  : http://www.cdl.uni-saarland.de/people/doerfert
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