[VPlan] Include IV phi and backedge cost in VPlan cost computation.

In WebAssembly, costs != 0 are assigned to be backedge and induction
phis, so make sure we include those costs in the VPlan-based cost model.

This fixes a downstream crash with WebAssembly after 242cc20
(#92555)

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7325,38 +7325,31 @@ InstructionCost LoopVectorizationPlanner::cost(VPlan &Plan,
   // cost model. Note that we do this as pre-processing; the VPlan may not have
   // any recipes associated with the original induction increment instruction
   // and may replace truncates with VPWidenIntOrFpInductionRecipe. We precompute
-  // the cost of both induction increment instructions that are represented by
-  // recipes and those that are not, to avoid distinguishing between them here,
-  // and skip all recipes that represent induction increments (the former case)
-  // later on, if they exist, to avoid counting them twice. Similarly we
-  // pre-compute the cost of any optimized truncates.
+  // the cost of induction phis and increments (both that are represented by
+  // recipes and those that are not), to avoid distinguishing between them here,
+  // and skip all recipes that represent induction phis and increments (the
+  // former case) later on, if they exist, to avoid counting them twice.
+  // Similarly we pre-compute the cost of any optimized truncates.
   // TODO: Switch to more accurate costing based on VPlan.
   for (const auto &[IV, IndDesc] : Legal->getInductionVars()) {
     Instruction *IVInc = cast<Instruction>(
         IV->getIncomingValueForBlock(OrigLoop->getLoopLatch()));
-    if (CostCtx.SkipCostComputation.insert(IVInc).second) {
-      InstructionCost InductionCost = CostCtx.getLegacyCost(IVInc, VF);
-      LLVM_DEBUG({
-        dbgs() << "Cost of " << InductionCost << " for VF " << VF
-               << ":\n induction increment " << *IVInc << "\n";
-        IVInc->dump();
-      });
-      Cost += InductionCost;
-    }
+    SmallVector<Instruction *> IVInsts = {IV, IVInc};
     for (User *U : IV->users()) {
       auto *CI = cast<Instruction>(U);
       if (!CostCtx.CM.isOptimizableIVTruncate(CI, VF))
         continue;
-      assert(!CostCtx.SkipCostComputation.contains(CI) &&
-             "Same cast for multiple inductions?");
-      CostCtx.SkipCostComputation.insert(CI);
-      InstructionCost CastCost = CostCtx.getLegacyCost(CI, VF);
+      IVInsts.push_back(CI);
+    }
+    for (Instruction *IVInst : IVInsts) {
+      if (!CostCtx.SkipCostComputation.insert(IVInst).second)
+        continue;
+      InstructionCost InductionCost = CostCtx.getLegacyCost(IVInst, VF);
       LLVM_DEBUG({
-        dbgs() << "Cost of " << CastCost << " for VF " << VF
-               << ":\n induction cast " << *CI << "\n";
-        CI->dump();
+        dbgs() << "Cost of " << InductionCost << " for VF " << VF
+               << ": induction instruction " << *IVInst << "\n";
       });
-      Cost += CastCost;
+      Cost += InductionCost;
     }
   }
 

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -764,6 +764,11 @@ InstructionCost VPRegionBlock::cost(ElementCount VF, VPCostContext &Ctx) {
     InstructionCost Cost = 0;
     for (VPBlockBase *Block : vp_depth_first_shallow(getEntry()))
       Cost += Block->cost(VF, Ctx);
+    InstructionCost BackedgeCost =
+        Ctx.TTI.getCFInstrCost(Instruction::Br, TTI::TCK_RecipThroughput);
+    LLVM_DEBUG(dbgs() << "Cost of " << BackedgeCost << " for VF " << VF
+                      << ": vector loop backedge\n");
+    Cost += BackedgeCost;
     return Cost;
   }
 

llvm/test/Transforms/LoopVectorize/WebAssembly/induction-branch-cost.ll

@@ -0,0 +1,77 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -p loop-vectorize -S %s | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-f128:64-n32:64-S128-ni:1:10:20"
+target triple = "wasm32-unknown-emscripten"
+
+define void @induction_phi_and_branch_cost(ptr %end, ptr %start.1, ptr %start.2) #0 {
+; CHECK-LABEL: define void @induction_phi_and_branch_cost(
+; CHECK-SAME: ptr [[END:%.*]], ptr [[START_1:%.*]], ptr [[START_2:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    [[END2:%.*]] = ptrtoint ptr [[END]] to i32
+; CHECK-NEXT:    [[START_11:%.*]] = ptrtoint ptr [[START_1]] to i32
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i32 [[START_11]], [[END2]]
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr i32 [[TMP0]], 2
+; CHECK-NEXT:    [[TMP2:%.*]] = add nuw nsw i32 [[TMP1]], 1
+; CHECK-NEXT:    [[MIN_ITERS_CHECK:%.*]] = icmp ult i32 [[TMP2]], 4
+; CHECK-NEXT:    br i1 [[MIN_ITERS_CHECK]], label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
+; CHECK:       [[VECTOR_PH]]:
+; CHECK-NEXT:    [[N_MOD_VF:%.*]] = urem i32 [[TMP2]], 4
+; CHECK-NEXT:    [[N_VEC:%.*]] = sub i32 [[TMP2]], [[N_MOD_VF]]
+; CHECK-NEXT:    [[TMP3:%.*]] = mul i32 [[N_VEC]], -4
+; CHECK-NEXT:    [[IND_END:%.*]] = getelementptr i8, ptr [[START_1]], i32 [[TMP3]]
+; CHECK-NEXT:    [[TMP4:%.*]] = mul i32 [[N_VEC]], -4
+; CHECK-NEXT:    [[IND_END3:%.*]] = getelementptr i8, ptr [[START_2]], i32 [[TMP4]]
+; CHECK-NEXT:    br label %[[VECTOR_BODY:.*]]
+; CHECK:       [[VECTOR_BODY]]:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[OFFSET_IDX:%.*]] = mul i32 [[INDEX]], -4
+; CHECK-NEXT:    [[TMP5:%.*]] = add i32 [[OFFSET_IDX]], 0
+; CHECK-NEXT:    [[NEXT_GEP:%.*]] = getelementptr i8, ptr [[START_2]], i32 [[TMP5]]
+; CHECK-NEXT:    [[TMP6:%.*]] = getelementptr i32, ptr [[NEXT_GEP]], i32 0
+; CHECK-NEXT:    [[TMP7:%.*]] = getelementptr i32, ptr [[TMP6]], i32 -3
+; CHECK-NEXT:    store <4 x i32> zeroinitializer, ptr [[TMP7]], align 4
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 4
+; CHECK-NEXT:    [[TMP8:%.*]] = icmp eq i32 [[INDEX_NEXT]], [[N_VEC]]
+; CHECK-NEXT:    br i1 [[TMP8]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
+; CHECK:       [[MIDDLE_BLOCK]]:
+; CHECK-NEXT:    [[CMP_N:%.*]] = icmp eq i32 [[TMP2]], [[N_VEC]]
+; CHECK-NEXT:    br i1 [[CMP_N]], label %[[EXIT:.*]], label %[[SCALAR_PH]]
+; CHECK:       [[SCALAR_PH]]:
+; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi ptr [ [[IND_END]], %[[MIDDLE_BLOCK]] ], [ [[START_1]], %[[ENTRY]] ]
+; CHECK-NEXT:    [[BC_RESUME_VAL4:%.*]] = phi ptr [ [[IND_END3]], %[[MIDDLE_BLOCK]] ], [ [[START_2]], %[[ENTRY]] ]
+; CHECK-NEXT:    br label %[[LOOP:.*]]
+; CHECK:       [[LOOP]]:
+; CHECK-NEXT:    [[PTR_IV:%.*]] = phi ptr [ [[BC_RESUME_VAL]], %[[SCALAR_PH]] ], [ [[PTR_IV_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[PTR_IV_2:%.*]] = phi ptr [ [[BC_RESUME_VAL4]], %[[SCALAR_PH]] ], [ [[PTR_IV_2_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[PTR_IV_NEXT]] = getelementptr nusw i8, ptr [[PTR_IV]], i32 -4
+; CHECK-NEXT:    [[PTR_IV_2_NEXT]] = getelementptr i8, ptr [[PTR_IV_2]], i32 -4
+; CHECK-NEXT:    store i32 0, ptr [[PTR_IV_2]], align 4
+; CHECK-NEXT:    [[EC:%.*]] = icmp eq ptr [[PTR_IV]], [[END]]
+; CHECK-NEXT:    br i1 [[EC]], label %[[EXIT]], label %[[LOOP]], !llvm.loop [[LOOP3:![0-9]+]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:
+  %ptr.iv = phi ptr [ %start.1, %entry ], [ %ptr.iv.next, %loop ]
+  %ptr.iv.2 = phi ptr [ %start.2, %entry ], [ %ptr.iv.2.next, %loop ]
+  %ptr.iv.next = getelementptr nusw i8, ptr %ptr.iv, i32 -4
+  %ptr.iv.2.next = getelementptr i8, ptr %ptr.iv.2, i32 -4
+  store i32 0, ptr %ptr.iv.2, align 4
+  %ec = icmp eq ptr %ptr.iv, %end
+  br i1 %ec, label %exit, label %loop
+
+exit:
+  ret void
+}
+
+attributes #0 = { "target-features"="+simd128" }
+;.
+; CHECK: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
+; CHECK: [[META1]] = !{!"llvm.loop.isvectorized", i32 1}
+; CHECK: [[META2]] = !{!"llvm.loop.unroll.runtime.disable"}
+; CHECK: [[LOOP3]] = distinct !{[[LOOP3]], [[META2]], [[META1]]}
+;.

llvm/test/Transforms/LoopVectorize/WebAssembly/lit.local.cfg

@@ -0,0 +1,2 @@
+if not "WebAssembly" in config.root.targets:
+    config.unsupported = True