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

[Tobias Grosser via llvm-commits]

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