Automerge: [VPlan] Handle FirstActiveLane when unrolling. (#145394)

Currently FirstActiveLane is not handled correctly during
 unrolling. This is currently causing mis-compiles when
 vectorizing early-exit loops with interleaving forced.

This patch updates handling of FirstActiveLane to be analogous to
computing final reduction results: during unrolling, the created copies
for its original operand are added as additional operands, and
FirstActiveLane will always produce the index of the first active lane
across all unrolled iterations.

Note that some of the generated code is still incorrect, as we also need
to handle ExtractElement with FirstActiveLane operands. I will share
patches for those soon as well.

PR: llvm/llvm-project#145394

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -966,7 +966,9 @@ class VPInstruction : public VPRecipeWithIRFlags,
     // all unrolled iterations. Unrolling will add all copies of its original
     // operand as additional operands.
     AnyOf,
-    // Calculates the first active lane index of the vector predicate operand.
+    // Calculates the first active lane index of the vector predicate operands.
+    // It produces the lane index across all unrolled iterations. Unrolling will
+    // add all copies of its original operand as additional operands.
     FirstActiveLane,
 
     // The opcodes below are used for VPInstructionWithType.

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -856,9 +856,32 @@ Value *VPInstruction::generate(VPTransformState &State) {
     return Builder.CreateOrReduce(Res);
   }
   case VPInstruction::FirstActiveLane: {
-    Value *Mask = State.get(getOperand(0));
-    return Builder.CreateCountTrailingZeroElems(Builder.getInt64Ty(), Mask,
-                                                true, Name);
+    if (getNumOperands() == 1) {
+      Value *Mask = State.get(getOperand(0));
+      return Builder.CreateCountTrailingZeroElems(Builder.getInt64Ty(), Mask,
+                                                  true, Name);
+    }
+    // If there are multiple operands, create a chain of selects to pick the
+    // first operand with an active lane and add the number of lanes of the
+    // preceding operands.
+    Value *RuntimeVF =
+        getRuntimeVF(State.Builder, State.Builder.getInt64Ty(), State.VF);
+    unsigned LastOpIdx = getNumOperands() - 1;
+    Value *Res = nullptr;
+    for (int Idx = LastOpIdx; Idx >= 0; --Idx) {
+      Value *TrailingZeros = Builder.CreateCountTrailingZeroElems(
+          Builder.getInt64Ty(), State.get(getOperand(Idx)), true, Name);
+      Value *Current = Builder.CreateAdd(
+          Builder.CreateMul(RuntimeVF, Builder.getInt64(Idx)), TrailingZeros);
+      if (Res) {
+        Value *Cmp = Builder.CreateICmpNE(TrailingZeros, RuntimeVF);
+        Res = Builder.CreateSelect(Cmp, Current, Res);
+      } else {
+        Res = Current;
+      }
+    }
+
+    return Res;
   }
   default:
     llvm_unreachable("Unsupported opcode for instruction");

llvm/lib/Transforms/Vectorize/VPlanUnroll.cpp

@@ -345,10 +345,12 @@ void UnrollState::unrollBlock(VPBlockBase *VPB) {
     if (ToSkip.contains(&R) || isa<VPIRInstruction>(&R))
       continue;
 
-    // Add all VPValues for all parts to AnyOf and Compute*Result which combine
-    // all parts to compute the final value.
+    // Add all VPValues for all parts to AnyOf, FirstActiveLaneMask and
+    // Compute*Result which combine all parts to compute the final value.
     VPValue *Op1;
     if (match(&R, m_VPInstruction<VPInstruction::AnyOf>(m_VPValue(Op1))) ||
+        match(&R, m_VPInstruction<VPInstruction::FirstActiveLane>(
+                      m_VPValue(Op1))) ||
         match(&R, m_VPInstruction<VPInstruction::ComputeAnyOfResult>(
                       m_VPValue(), m_VPValue(), m_VPValue(Op1))) ||
         match(&R, m_VPInstruction<VPInstruction::ComputeReductionResult>(

llvm/test/Transforms/LoopVectorize/AArch64/single-early-exit-interleave.ll

@@ -77,8 +77,27 @@ define i64 @same_exit_block_pre_inc_use1() #0 {
 ; CHECK-NEXT:    [[CMP_N:%.*]] = icmp eq i64 510, [[N_VEC]]
 ; CHECK-NEXT:    br i1 [[CMP_N]], label [[LOOP_END:%.*]], label [[SCALAR_PH]]
 ; CHECK:       vector.early.exit:
+; CHECK-NEXT:    [[TMP63:%.*]] = call i64 @llvm.vscale.i64()
+; CHECK-NEXT:    [[TMP42:%.*]] = mul nuw i64 [[TMP63]], 16
+; CHECK-NEXT:    [[TMP44:%.*]] = call i64 @llvm.experimental.cttz.elts.i64.nxv16i1(<vscale x 16 x i1> [[TMP32]], i1 true)
+; CHECK-NEXT:    [[TMP62:%.*]] = mul i64 [[TMP42]], 3
+; CHECK-NEXT:    [[TMP45:%.*]] = add i64 [[TMP62]], [[TMP44]]
+; CHECK-NEXT:    [[TMP46:%.*]] = call i64 @llvm.experimental.cttz.elts.i64.nxv16i1(<vscale x 16 x i1> [[TMP31]], i1 true)
+; CHECK-NEXT:    [[TMP58:%.*]] = mul i64 [[TMP42]], 2
+; CHECK-NEXT:    [[TMP50:%.*]] = add i64 [[TMP58]], [[TMP46]]
+; CHECK-NEXT:    [[TMP47:%.*]] = icmp ne i64 [[TMP46]], [[TMP42]]
+; CHECK-NEXT:    [[TMP51:%.*]] = select i1 [[TMP47]], i64 [[TMP50]], i64 [[TMP45]]
+; CHECK-NEXT:    [[TMP52:%.*]] = call i64 @llvm.experimental.cttz.elts.i64.nxv16i1(<vscale x 16 x i1> [[TMP30]], i1 true)
+; CHECK-NEXT:    [[TMP64:%.*]] = mul i64 [[TMP42]], 1
+; CHECK-NEXT:    [[TMP56:%.*]] = add i64 [[TMP64]], [[TMP52]]
+; CHECK-NEXT:    [[TMP53:%.*]] = icmp ne i64 [[TMP52]], [[TMP42]]
+; CHECK-NEXT:    [[TMP57:%.*]] = select i1 [[TMP53]], i64 [[TMP56]], i64 [[TMP51]]
 ; CHECK-NEXT:    [[TMP15:%.*]] = call i64 @llvm.experimental.cttz.elts.i64.nxv16i1(<vscale x 16 x i1> [[TMP11]], i1 true)
-; CHECK-NEXT:    [[TMP16:%.*]] = add i64 [[INDEX1]], [[TMP15]]
+; CHECK-NEXT:    [[TMP65:%.*]] = mul i64 [[TMP42]], 0
+; CHECK-NEXT:    [[TMP60:%.*]] = add i64 [[TMP65]], [[TMP15]]
+; CHECK-NEXT:    [[TMP59:%.*]] = icmp ne i64 [[TMP15]], [[TMP42]]
+; CHECK-NEXT:    [[TMP61:%.*]] = select i1 [[TMP59]], i64 [[TMP60]], i64 [[TMP57]]
+; CHECK-NEXT:    [[TMP16:%.*]] = add i64 [[INDEX1]], [[TMP61]]
 ; CHECK-NEXT:    [[TMP17:%.*]] = add i64 3, [[TMP16]]
 ; CHECK-NEXT:    br label [[LOOP_END]]
 ; CHECK:       scalar.ph: