[llvm] [VPlan] Introduce generic variable-length step support (PR #177114)

[Shih-Po Hung via llvm-commits]

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@@ -3240,89 +3224,135 @@ void VPlanTransforms::addExplicitVectorLength(
   removeDeadRecipes(Plan);
 
   // Replace all uses of VPCanonicalIVPHIRecipe by
-  // VPEVLBasedIVPHIRecipe except for the canonical IV increment.
-  CanonicalIVPHI->replaceAllUsesWith(EVLPhi);
+  // VPCumulativeIVPHIRecipe except for the canonical IV increment.
+  CanonicalIVPHI->replaceAllUsesWith(CumulativeIVPhi);
   CanonicalIVIncrement->setOperand(0, CanonicalIVPHI);
+
   // TODO: support unroll factor > 1.
   Plan.setUF(1);
+
+  // Switch the loop from up-counting to down counting.
+  // convert (branch-on-count (CanonicalInc, VTC)
+  // -> (branch-on-count (sub VTC, CanonicalIVInc), 0)
+  VPBasicBlock *LatchVPBB = LoopRegion->getExitingBasicBlock();
+  auto *LatchExitingBranch = cast<VPInstruction>(LatchVPBB->getTerminator());
+  if (match(LatchExitingBranch, m_BranchOnCond(m_True())))
+    return;
+  assert(match(LatchExitingBranch,
+               m_BranchOnCount(m_Specific(CanonicalIVIncrement),
+                               m_Specific(&Plan.getVectorTripCount()))) &&
+         "Unexpected terminator");
+  Builder.setInsertPoint(LatchExitingBranch);
+  VPValue *RemainElementCount = Builder.createOverflowingOp(
+      Instruction::Sub, {&Plan.getVectorTripCount(), CanonicalIVIncrement},
+      {/*hasNUW=*/true, /*hasNSW=*/false}, DebugLoc::getCompilerGenerated(),
+      "remain.element.count");
+  auto *Zero = Plan.getOrAddLiveIn(ConstantInt::get(CanIVTy, 0));
+  LatchExitingBranch->setOperand(0, RemainElementCount);
+  LatchExitingBranch->setOperand(1, Zero);
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arcbbb wrote:

Thanks for #178181! That makes this an NFC now.

https://github.com/llvm/llvm-project/pull/177114