[LoopVectorize] Further improve cost model for early exit loops (#126235)

david-arm · web-flow · commit 26ecf978951b · 2025-03-11T11:48:55.000Z
Following on from #125058, this patch takes into account the work done in the vector early exit block when assessing the profitability of vectorising the loop. I have renamed areRuntimeChecksProfitable to isOutsideLoopWorkProfitable and we now pass in the early exit costs. As part of this, I have added the ExtractFirstActive opcode to VPInstruction::computeCost. It's worth pointing out that when we assess profitability of the loop we calculate a minimum trip count and compare that against the *maximum* trip count. However, since the loop has an early exit the runtime trip count can still end up being less than the minimum. Alternatively, we may never take the early exit at all at runtime and so we have the opposite problem of over-estimating the cost of the loop. The loop vectoriser cannot simultaneously take two contradictory positions and so I feel the only sensible thing to do is be conservative and assume the loop will be more expensive than loops without early exits. We may find in future that we need to adjust the cost according to the probability of taking the early exit. This will become even more important once we support multiple early exits. However, we have to start somewhere and we can always revisit this later.
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -10171,19 +10171,56 @@ static void checkMixedPrecision(Loop *L, OptimizationRemarkEmitter *ORE) {
   }
 }
 
-static bool areRuntimeChecksProfitable(GeneratedRTChecks &Checks,
-                                       VectorizationFactor &VF, Loop *L,
-                                       PredicatedScalarEvolution &PSE,
-                                       ScalarEpilogueLowering SEL,
-                                       std::optional<unsigned> VScale) {
-  InstructionCost CheckCost = Checks.getCost();
-  if (!CheckCost.isValid())
+/// For loops with uncountable early exits, find the cost of doing work when
+/// exiting the loop early, such as calculating the final exit values of
+/// variables used outside the loop.
+/// TODO: This is currently overly pessimistic because the loop may not take
+/// the early exit, but better to keep this conservative for now. In future,
+/// it might be possible to relax this by using branch probabilities.
+static InstructionCost calculateEarlyExitCost(VPCostContext &CostCtx,
+                                              VPlan &Plan, ElementCount VF) {
+  InstructionCost Cost = 0;
+  for (auto *ExitVPBB : Plan.getExitBlocks()) {
+    for (auto *PredVPBB : ExitVPBB->getPredecessors()) {
+      // If the predecessor is not the middle.block, then it must be the
+      // vector.early.exit block, which may contain work to calculate the exit
+      // values of variables used outside the loop.
+      if (PredVPBB != Plan.getMiddleBlock()) {
+        LLVM_DEBUG(dbgs() << "Calculating cost of work in exit block "
+                          << PredVPBB->getName() << ":\n");
+        Cost += PredVPBB->cost(VF, CostCtx);
+      }
+    }
+  }
+  return Cost;
+}
+
+/// This function determines whether or not it's still profitable to vectorize
+/// the loop given the extra work we have to do outside of the loop:
+///  1. Perform the runtime checks before entering the loop to ensure it's safe
+///     to vectorize.
+///  2. In the case of loops with uncountable early exits, we may have to do
+///     extra work when exiting the loop early, such as calculating the final
+///     exit values of variables used outside the loop.
+static bool isOutsideLoopWorkProfitable(GeneratedRTChecks &Checks,
+                                        VectorizationFactor &VF, Loop *L,
+                                        PredicatedScalarEvolution &PSE,
+                                        VPCostContext &CostCtx, VPlan &Plan,
+                                        ScalarEpilogueLowering SEL,
+                                        std::optional<unsigned> VScale) {
+  InstructionCost TotalCost = Checks.getCost();
+  if (!TotalCost.isValid())
     return false;
 
+  // Add on the cost of any work required in the vector early exit block, if
+  // one exists.
+  TotalCost += calculateEarlyExitCost(CostCtx, Plan, VF.Width);
+
   // When interleaving only scalar and vector cost will be equal, which in turn
   // would lead to a divide by 0. Fall back to hard threshold.
   if (VF.Width.isScalar()) {
-    if (CheckCost > VectorizeMemoryCheckThreshold) {
+    // TODO: Should we rename VectorizeMemoryCheckThreshold?
+    if (TotalCost > VectorizeMemoryCheckThreshold) {
       LLVM_DEBUG(
           dbgs()
           << "LV: Interleaving only is not profitable due to runtime checks\n");
@@ -10209,7 +10246,9 @@ static bool areRuntimeChecksProfitable(GeneratedRTChecks &Checks,
   //  The total cost of the vector loop is
   //    RtC + VecC * (TC / VF) + EpiC
   //  where
-  //  * RtC is the cost of the generated runtime checks
+  //  * RtC is the cost of the generated runtime checks plus the cost of
+  //    performing any additional work in the vector.early.exit block for loops
+  //    with uncountable early exits.
   //  * VecC is the cost of a single vector iteration.
   //  * TC is the actual trip count of the loop
   //  * VF is the vectorization factor
@@ -10227,7 +10266,7 @@ static bool areRuntimeChecksProfitable(GeneratedRTChecks &Checks,
   // the computations are performed on doubles, not integers and the result
   // is rounded up, hence we get an upper estimate of the TC.
   unsigned IntVF = getEstimatedRuntimeVF(VF.Width, VScale);
-  uint64_t RtC = *CheckCost.getValue();
+  uint64_t RtC = *TotalCost.getValue();
   uint64_t Div = ScalarC * IntVF - *VF.Cost.getValue();
   uint64_t MinTC1 = Div == 0 ? 0 : divideCeil(RtC * IntVF, Div);
 
@@ -10555,8 +10594,8 @@ bool LoopVectorizePass::processLoop(Loop *L) {
         // iteration count is low. However, setting the epilogue policy to
         // `CM_ScalarEpilogueNotAllowedLowTripLoop` prevents vectorizing loops
         // with runtime checks. It's more effective to let
-        // `areRuntimeChecksProfitable` determine if vectorization is beneficial
-        // for the loop.
+        // `isOutsideLoopWorkProfitable` determine if vectorization is
+        // beneficial for the loop.
         if (SEL != CM_ScalarEpilogueNotNeededUsePredicate)
           SEL = CM_ScalarEpilogueNotAllowedLowTripLoop;
       } else {
@@ -10654,9 +10693,12 @@ bool LoopVectorizePass::processLoop(Loop *L) {
     // Check if it is profitable to vectorize with runtime checks.
     bool ForceVectorization =
         Hints.getForce() == LoopVectorizeHints::FK_Enabled;
+    VPCostContext CostCtx(CM.TTI, *CM.TLI, CM.Legal->getWidestInductionType(),
+                          CM, CM.CostKind);
     if (!ForceVectorization &&
-        !areRuntimeChecksProfitable(Checks, VF, L, PSE, SEL,
-                                    CM.getVScaleForTuning())) {
+        !isOutsideLoopWorkProfitable(Checks, VF, L, PSE, CostCtx,
+                                     LVP.getPlanFor(VF.Width), SEL,
+                                     CM.getVScaleForTuning())) {
       ORE->emit([&]() {
         return OptimizationRemarkAnalysisAliasing(
                    DEBUG_TYPE, "CantReorderMemOps", L->getStartLoc(),
diff --git a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
@@ -742,6 +742,18 @@ InstructionCost VPInstruction::computeCost(ElementCount VF,
     return Ctx.TTI.getArithmeticReductionCost(
         Instruction::Or, cast<VectorType>(VecTy), std::nullopt, Ctx.CostKind);
   }
+  case VPInstruction::ExtractFirstActive: {
+    // Calculate the cost of determining the lane index.
+    auto *PredTy = toVectorTy(Ctx.Types.inferScalarType(getOperand(1)), VF);
+    IntrinsicCostAttributes Attrs(Intrinsic::experimental_cttz_elts,
+                                  Type::getInt64Ty(Ctx.LLVMCtx),
+                                  {PredTy, Type::getInt1Ty(Ctx.LLVMCtx)});
+    InstructionCost Cost = Ctx.TTI.getIntrinsicInstrCost(Attrs, Ctx.CostKind);
+    // Add on the cost of extracting the element.
+    auto *VecTy = toVectorTy(Ctx.Types.inferScalarType(getOperand(0)), VF);
+    return Cost + Ctx.TTI.getVectorInstrCost(Instruction::ExtractElement, VecTy,
+                                             Ctx.CostKind);
+  }
   default:
     // TODO: Compute cost other VPInstructions once the legacy cost model has
     // been retired.
diff --git a/llvm/test/Transforms/LoopVectorize/AArch64/early_exit_costs.ll b/llvm/test/Transforms/LoopVectorize/AArch64/early_exit_costs.ll
@@ -0,0 +1,86 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; REQUIRES: asserts
+; RUN: opt -S < %s -p loop-vectorize -enable-early-exit-vectorization -disable-output \
+; RUN:   -debug-only=loop-vectorize 2>&1 | FileCheck %s --check-prefixes=CHECK
+
+target triple = "aarch64-unknown-linux-gnu"
+
+declare void @init_mem(ptr, i64);
+
+define i64 @same_exit_block_pre_inc_use1_sve() #1 {
+; CHECK-LABEL: LV: Checking a loop in 'same_exit_block_pre_inc_use1_sve'
+; CHECK: LV: Selecting VF: vscale x 16
+; CHECK: Calculating cost of work in exit block vector.early.exit
+; CHECK-NEXT: Cost of 6 for VF vscale x 16: EMIT vp<{{.*}}> = extract-first-active
+; CHECK-NEXT: Cost of 6 for VF vscale x 16: EMIT vp<{{.*}}> = extract-first-active
+; CHECK: LV: Minimum required TC for runtime checks to be profitable:32
+entry:
+  %p1 = alloca [1024 x i8]
+  %p2 = alloca [1024 x i8]
+  call void @init_mem(ptr %p1, i64 1024)
+  call void @init_mem(ptr %p2, i64 1024)
+  br label %loop
+
+loop:
+  %index = phi i64 [ %index.next, %loop.inc ], [ 3, %entry ]
+  %index2 = phi i64 [ %index2.next, %loop.inc ], [ 15, %entry ]
+  %arrayidx = getelementptr inbounds i8, ptr %p1, i64 %index
+  %ld1 = load i8, ptr %arrayidx, align 1
+  %arrayidx1 = getelementptr inbounds i8, ptr %p2, i64 %index
+  %ld2 = load i8, ptr %arrayidx1, align 1
+  %cmp3 = icmp eq i8 %ld1, %ld2
+  br i1 %cmp3, label %loop.inc, label %loop.end
+
+loop.inc:
+  %index.next = add i64 %index, 1
+  %index2.next = add i64 %index2, 2
+  %exitcond = icmp ne i64 %index.next, 67
+  br i1 %exitcond, label %loop, label %loop.end
+
+loop.end:
+  %val1 = phi i64 [ %index, %loop ], [ 67, %loop.inc ]
+  %val2 = phi i64 [ %index2, %loop ], [ 98, %loop.inc ]
+  %retval = add i64 %val1, %val2
+  ret i64 %retval
+}
+
+define i64 @same_exit_block_pre_inc_use1_nosve() {
+; CHECK-LABEL: LV: Checking a loop in 'same_exit_block_pre_inc_use1_nosve'
+; CHECK: LV: Selecting VF: 16
+; CHECK: Calculating cost of work in exit block vector.early.exit
+; CHECK-NEXT: Cost of 50 for VF 16: EMIT vp<{{.*}}> = extract-first-active
+; CHECK-NEXT: Cost of 50 for VF 16: EMIT vp<{{.*}}> = extract-first-active
+; CHECK: LV: Minimum required TC for runtime checks to be profitable:176
+; CHECK-NEXT: LV: Vectorization is not beneficial: expected trip count < minimum profitable VF (64 < 176)
+; CHECK-NEXT: LV: Too many memory checks needed.
+entry:
+  %p1 = alloca [1024 x i8]
+  %p2 = alloca [1024 x i8]
+  call void @init_mem(ptr %p1, i64 1024)
+  call void @init_mem(ptr %p2, i64 1024)
+  br label %loop
+
+loop:
+  %index = phi i64 [ %index.next, %loop.inc ], [ 3, %entry ]
+  %index2 = phi i64 [ %index2.next, %loop.inc ], [ 15, %entry ]
+  %arrayidx = getelementptr inbounds i8, ptr %p1, i64 %index
+  %ld1 = load i8, ptr %arrayidx, align 1
+  %arrayidx1 = getelementptr inbounds i8, ptr %p2, i64 %index
+  %ld2 = load i8, ptr %arrayidx1, align 1
+  %cmp3 = icmp eq i8 %ld1, %ld2
+  br i1 %cmp3, label %loop.inc, label %loop.end
+
+loop.inc:
+  %index.next = add i64 %index, 1
+  %index2.next = add i64 %index2, 2
+  %exitcond = icmp ne i64 %index.next, 67
+  br i1 %exitcond, label %loop, label %loop.end
+
+loop.end:
+  %val1 = phi i64 [ %index, %loop ], [ 67, %loop.inc ]
+  %val2 = phi i64 [ %index2, %loop ], [ 98, %loop.inc ]
+  %retval = add i64 %val1, %val2
+  ret i64 %retval
+}
+
+attributes #1 = { "target-features"="+sve" vscale_range(1,16) }
diff --git a/llvm/test/Transforms/LoopVectorize/AArch64/simple_early_exit.ll b/llvm/test/Transforms/LoopVectorize/AArch64/simple_early_exit.ll
@@ -274,6 +274,7 @@ define i64 @loop_contains_safe_div() #1 {
 ; CHECK-NEXT:    call void @init_mem(ptr [[P2]], i64 1024)
 ; CHECK-NEXT:    [[TMP11:%.*]] = call i64 @llvm.vscale.i64()
 ; CHECK-NEXT:    [[TMP12:%.*]] = mul i64 [[TMP11]], 4
+; CHECK-NEXT:    [[TMP18:%.*]] = call i64 @llvm.umax.i64(i64 8, i64 [[TMP12]])
 ; CHECK-NEXT:    br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
 ; CHECK:       vector.ph:
 ; CHECK-NEXT:    [[TMP10:%.*]] = call i64 @llvm.vscale.i64()
