[llvm] [VPlan] Add initial CFG simplification, removing BranchOnCond true. (PR #106748)

[via llvm-commits]

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@@ -1467,6 +1467,49 @@ void VPlanTransforms::truncateToMinimalBitwidths(
          "some entries in MinBWs haven't been processed");
 }
 
+/// Remove BranchOnCond recipes with true conditions together with removing
+/// dead edges to their successors.
+static void simplifyCFG(VPlan &Plan) {
+  using namespace llvm::VPlanPatternMatch;
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
+           vp_depth_first_deep(Plan.getEntry()))) {
+    if (VPBB->getNumSuccessors() != 2 ||
+        !match(&VPBB->back(), m_BranchOnCond(m_True())))
+      continue;
+
+    VPBasicBlock *RemovedSucc = cast<VPBasicBlock>(VPBB->getSuccessors()[1]);
+    const auto &Preds = RemovedSucc->getPredecessors();
+    unsigned DeadIdx = std::distance(Preds.begin(), find(Preds, VPBB));
+
+    // Remove values coming from VPBB from phi-like recipes in RemovedSucc.
+    for (VPRecipeBase &R : make_early_inc_range(*RemovedSucc)) {
+      assert((!isa<VPIRInstruction>(&R) ||
+              !isa<PHINode>(cast<VPIRInstruction>(&R)->getInstruction())) &&
+             !isa<VPHeaderPHIRecipe>(&R) &&
+             "Cannot update VPIRInstructions wrapping phis or header phis yet");
+      auto *VPI = dyn_cast<VPInstruction>(&R);
+      if (!VPI || VPI->getOpcode() != VPInstruction::ResumePhi)
+        break;
+      VPBuilder B(VPI);
+      SmallVector<VPValue *> NewOps;
+      // Create new operand list, with the dead incoming value filtered out.
+      for (const auto &[Idx, Op] : enumerate(VPI->operands())) {
+        if (Idx == DeadIdx)
+          continue;
+        NewOps.push_back(Op);
+      }
+      VPI->replaceAllUsesWith(B.createNaryOp(VPInstruction::ResumePhi, NewOps,
+                                             VPI->getDebugLoc(),
+                                             VPI->getName()));
+      VPI->eraseFromParent();
+    }
+    // Disconnect blocks and remove the terminator. RemovedSucc will be deleted
+    // automatically on VPlan destruction.
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ayalz wrote:

```suggestion
    // automatically on VPlan destruction if it becomes unreachable.
```
If RemovedSucc becomes unreachable, i.e., Preds consists of VPBB only, do the resumePhis need to be cleared, or better bail out early?

https://github.com/llvm/llvm-project/pull/106748