[LoopVectorize] Refine runtime memory check costs when there is an outer loop

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1957,6 +1957,8 @@ class GeneratedRTChecks {
   bool CostTooHigh = false;
   const bool AddBranchWeights;
 
+  Loop *OuterLoop = nullptr;
+
 public:
   GeneratedRTChecks(ScalarEvolution &SE, DominatorTree *DT, LoopInfo *LI,
                     TargetTransformInfo *TTI, const DataLayout &DL,
@@ -2053,6 +2055,9 @@ class GeneratedRTChecks {
       DT->eraseNode(SCEVCheckBlock);
       LI->removeBlock(SCEVCheckBlock);
     }
+
+    // Outer loop is used as part of the later cost calculations.
+    OuterLoop = L->getParentLoop();
   }
 
   InstructionCost getCost() {
@@ -2076,16 +2081,61 @@ class GeneratedRTChecks {
         LLVM_DEBUG(dbgs() << "  " << C << "  for " << I << "\n");
         RTCheckCost += C;
       }
-    if (MemCheckBlock)
+    if (MemCheckBlock) {
+      InstructionCost MemCheckCost = 0;
       for (Instruction &I : *MemCheckBlock) {
         if (MemCheckBlock->getTerminator() == &I)
           continue;
         InstructionCost C =
             TTI->getInstructionCost(&I, TTI::TCK_RecipThroughput);
         LLVM_DEBUG(dbgs() << "  " << C << "  for " << I << "\n");
-        RTCheckCost += C;
+        MemCheckCost += C;
       }
 
+      // If the runtime memory checks are being created inside an outer loop
+      // we should find out if these checks are outer loop invariant. If so,
+      // the checks will likely be hoisted out and so the effective cost will
+      // reduce according to the outer loop trip count.
+      if (OuterLoop) {
+        ScalarEvolution *SE = MemCheckExp.getSE();
+        // TODO: If profitable, we could refine this further by analysing every
+        // individual memory check, since there could be a mixture of loop
+        // variant and invariant checks that mean the final condition is
+        // variant.
+        const SCEV *Cond = SE->getSCEV(MemRuntimeCheckCond);
+        if (SE->isLoopInvariant(Cond, OuterLoop)) {
+          // It seems reasonable to assume that we can reduce the effective
+          // cost of the checks even when we know nothing about the trip
+          // count. Assume that the outer loop executes at least twice.
+          unsigned BestTripCount = 2;
+
+          // If exact trip count is known use that.
+          if (unsigned SmallTC = SE->getSmallConstantTripCount(OuterLoop))
+            BestTripCount = SmallTC;
+          else if (LoopVectorizeWithBlockFrequency) {
+            // Else use profile data if available.
+            if (auto EstimatedTC = getLoopEstimatedTripCount(OuterLoop))
+              BestTripCount = *EstimatedTC;
+          }
+
+          InstructionCost NewMemCheckCost = MemCheckCost / BestTripCount;
+
+          // Let's ensure the cost is always at least 1.
+          NewMemCheckCost = std::max(*NewMemCheckCost.getValue(),
+                                     (InstructionCost::CostType)1);
+
+          LLVM_DEBUG(dbgs()
+                     << "We expect runtime memory checks to be hoisted "
+                     << "out of the outer loop. Cost reduced from "
+                     << MemCheckCost << " to " << NewMemCheckCost << '\n');
+
+          MemCheckCost = NewMemCheckCost;
+        }
+      }
+
+      RTCheckCost += MemCheckCost;
+    }
+
     if (SCEVCheckBlock || MemCheckBlock)
       LLVM_DEBUG(dbgs() << "Total cost of runtime checks: " << RTCheckCost
                         << "\n");
@@ -2144,8 +2194,8 @@ class GeneratedRTChecks {
 
     BranchInst::Create(LoopVectorPreHeader, SCEVCheckBlock);
     // Create new preheader for vector loop.
-    if (auto *PL = LI->getLoopFor(LoopVectorPreHeader))
-      PL->addBasicBlockToLoop(SCEVCheckBlock, *LI);
+    if (OuterLoop)
+      OuterLoop->addBasicBlockToLoop(SCEVCheckBlock, *LI);
 
     SCEVCheckBlock->getTerminator()->eraseFromParent();
     SCEVCheckBlock->moveBefore(LoopVectorPreHeader);
@@ -2179,8 +2229,8 @@ class GeneratedRTChecks {
     DT->changeImmediateDominator(LoopVectorPreHeader, MemCheckBlock);
     MemCheckBlock->moveBefore(LoopVectorPreHeader);
 
-    if (auto *PL = LI->getLoopFor(LoopVectorPreHeader))
-      PL->addBasicBlockToLoop(MemCheckBlock, *LI);
+    if (OuterLoop)
+      OuterLoop->addBasicBlockToLoop(MemCheckBlock, *LI);
 
     BranchInst &BI =
         *BranchInst::Create(Bypass, LoopVectorPreHeader, MemRuntimeCheckCond);

llvm/test/Transforms/LoopVectorize/AArch64/low_trip_memcheck_cost.ll

@@ -0,0 +1,217 @@
+; REQUIRES: asserts
+; RUN: opt -p loop-vectorize -debug-only=loop-vectorize -S -disable-output < %s 2>&1 | FileCheck %s
+
+target triple = "aarch64-unknown-linux-gnu"
+
+define void @no_outer_loop(ptr nocapture noundef %a, ptr nocapture noundef readonly %b, i64 noundef %off, i64 noundef %n) {
+; CHECK-LABEL: LV: Checking a loop in 'no_outer_loop'
+; CHECK:      Calculating cost of runtime checks:
+; CHECK-NOT:  We expect runtime memory checks to be hoisted out of the outer loop.
+; CHECK:      Total cost of runtime checks: 4
+; CHECK-NEXT: LV: Minimum required TC for runtime checks to be profitable:16
+entry:
+  br label %inner.loop
+
+inner.loop:
+  %inner.iv = phi i64 [ 0, %entry ], [ %inner.iv.next, %inner.loop ]
+  %add.us = add nuw nsw i64 %inner.iv, %off
+  %arrayidx.us = getelementptr inbounds i8, ptr %b, i64 %add.us
+  %0 = load i8, ptr %arrayidx.us, align 1
+  %arrayidx7.us = getelementptr inbounds i8, ptr %a, i64 %add.us
+  %1 = load i8, ptr %arrayidx7.us, align 1
+  %add9.us = add i8 %1, %0
+  store i8 %add9.us, ptr %arrayidx7.us, align 1
+  %inner.iv.next = add nuw nsw i64 %inner.iv, 1
+  %exitcond.not = icmp eq i64 %inner.iv.next, %n
+  br i1 %exitcond.not, label %inner.exit, label %inner.loop
+
+inner.exit:
+  ret void
+}
+
+define void @outer_no_tc(ptr nocapture noundef %a, ptr nocapture noundef readonly %b, i64 noundef %m, i64 noundef %n) {
+; CHECK-LABEL: LV: Checking a loop in 'outer_no_tc'
+; CHECK:      Calculating cost of runtime checks:
+; CHECK:      We expect runtime memory checks to be hoisted out of the outer loop. Cost reduced from 6 to 3
+; CHECK:      Total cost of runtime checks: 3
+; CHECK-NEXT: LV: Minimum required TC for runtime checks to be profitable:16
+entry:
+  br label %outer.loop
+
+outer.loop:
+  %outer.iv = phi i64 [ %outer.iv.next, %inner.exit ], [ 0, %entry ]
+  %mul.us = mul nsw i64 %outer.iv, %n
+  br label %inner.loop
+
+inner.loop:
+  %inner.iv = phi i64 [ 0, %outer.loop ], [ %inner.iv.next, %inner.loop ]
+  %add.us = add nuw nsw i64 %inner.iv, %mul.us
+  %arrayidx.us = getelementptr inbounds i8, ptr %b, i64 %add.us
+  %0 = load i8, ptr %arrayidx.us, align 1
+  %arrayidx7.us = getelementptr inbounds i8, ptr %a, i64 %add.us
+  %1 = load i8, ptr %arrayidx7.us, align 1
+  %add9.us = add i8 %1, %0
+  store i8 %add9.us, ptr %arrayidx7.us, align 1
+  %inner.iv.next = add nuw nsw i64 %inner.iv, 1
+  %exitcond.not = icmp eq i64 %inner.iv.next, %n
+  br i1 %exitcond.not, label %inner.exit, label %inner.loop
+
+inner.exit:
+  %outer.iv.next = add nuw nsw i64 %outer.iv, 1
+  %exitcond27.not = icmp eq i64 %outer.iv.next, %m
+  br i1 %exitcond27.not, label %outer.exit, label %outer.loop
+
+outer.exit:
+  ret void
+}
+
+
+define void @outer_known_tc3(ptr nocapture noundef %a, ptr nocapture noundef readonly %b, i64 noundef %n) {
+; CHECK-LABEL: LV: Checking a loop in 'outer_known_tc3'
+; CHECK:      Calculating cost of runtime checks:
+; CHECK:      We expect runtime memory checks to be hoisted out of the outer loop. Cost reduced from 6 to 2
+; CHECK:      Total cost of runtime checks: 2
+; CHECK-NEXT: LV: Minimum required TC for runtime checks to be profitable:16
+entry:
+  br label %outer.loop
+
+outer.loop:
+  %outer.iv = phi i64 [ %outer.iv.next, %inner.exit ], [ 0, %entry ]
+  %mul.us = mul nsw i64 %outer.iv, %n
+  br label %inner.loop
+
+inner.loop:
+  %inner.iv = phi i64 [ 0, %outer.loop ], [ %inner.iv.next, %inner.loop ]
+  %add.us = add nuw nsw i64 %inner.iv, %mul.us
+  %arrayidx.us = getelementptr inbounds i8, ptr %b, i64 %add.us
+  %0 = load i8, ptr %arrayidx.us, align 1
+  %arrayidx7.us = getelementptr inbounds i8, ptr %a, i64 %add.us
+  %1 = load i8, ptr %arrayidx7.us, align 1
+  %add9.us = add i8 %1, %0
+  store i8 %add9.us, ptr %arrayidx7.us, align 1
+  %inner.iv.next = add nuw nsw i64 %inner.iv, 1
+  %exitcond.not = icmp eq i64 %inner.iv.next, %n
+  br i1 %exitcond.not, label %inner.exit, label %inner.loop
+
+inner.exit:
+  %outer.iv.next = add nuw nsw i64 %outer.iv, 1
+  %exitcond26.not = icmp eq i64 %outer.iv.next, 3
+  br i1 %exitcond26.not, label %outer.exit, label %outer.loop
+
+outer.exit:
+  ret void
+}
+
+
+define void @outer_known_tc64(ptr nocapture noundef %a, ptr nocapture noundef readonly %b, i64 noundef %n) {
+; CHECK-LABEL: LV: Checking a loop in 'outer_known_tc64'
+; CHECK:      Calculating cost of runtime checks:
+; CHECK:      We expect runtime memory checks to be hoisted out of the outer loop. Cost reduced from 6 to 1
+; CHECK:      Total cost of runtime checks: 1
+; CHECK-NEXT: LV: Minimum required TC for runtime checks to be profitable:16
+entry:
+  br label %outer.loop
+
+outer.loop:
+  %outer.iv = phi i64 [ %outer.iv.next, %inner.exit ], [ 0, %entry ]
+  %mul.us = mul nsw i64 %outer.iv, %n
+  br label %inner.loop
+
+inner.loop:
+  %inner.iv = phi i64 [ 0, %outer.loop ], [ %inner.iv.next, %inner.loop ]
+  %add.us = add nuw nsw i64 %inner.iv, %mul.us
+  %arrayidx.us = getelementptr inbounds i8, ptr %b, i64 %add.us
+  %0 = load i8, ptr %arrayidx.us, align 1
+  %arrayidx7.us = getelementptr inbounds i8, ptr %a, i64 %add.us
+  %1 = load i8, ptr %arrayidx7.us, align 1
+  %add9.us = add i8 %1, %0
+  store i8 %add9.us, ptr %arrayidx7.us, align 1
+  %inner.iv.next = add nuw nsw i64 %inner.iv, 1
+  %exitcond.not = icmp eq i64 %inner.iv.next, %n
+  br i1 %exitcond.not, label %inner.exit, label %inner.loop
+
+inner.exit:
+  %outer.iv.next = add nuw nsw i64 %outer.iv, 1
+  %exitcond26.not = icmp eq i64 %outer.iv.next, 64
+  br i1 %exitcond26.not, label %outer.exit, label %outer.loop
+
+outer.exit:
+  ret void
+}
+
+
+define void @outer_pgo_3(ptr nocapture noundef %a, ptr nocapture noundef readonly %b, i64 noundef %m, i64 noundef %n) {
+; CHECK-LABEL: LV: Checking a loop in 'outer_pgo_3'
+; CHECK:      Calculating cost of runtime checks:
+; CHECK:      We expect runtime memory checks to be hoisted out of the outer loop. Cost reduced from 6 to 2
+; CHECK:      Total cost of runtime checks: 2
+; CHECK-NEXT: LV: Minimum required TC for runtime checks to be profitable:16
+entry:
+  br label %outer.loop
+
+outer.loop:
+  %outer.iv = phi i64 [ %outer.iv.next, %inner.exit ], [ 0, %entry ]
+  %mul.us = mul nsw i64 %outer.iv, %n
+  br label %inner.loop
+
+inner.loop:
+  %inner.iv = phi i64 [ 0, %outer.loop ], [ %inner.iv.next, %inner.loop ]
+  %add.us = add nuw nsw i64 %inner.iv, %mul.us
+  %arrayidx.us = getelementptr inbounds i8, ptr %b, i64 %add.us
+  %0 = load i8, ptr %arrayidx.us, align 1
+  %arrayidx7.us = getelementptr inbounds i8, ptr %a, i64 %add.us
+  %1 = load i8, ptr %arrayidx7.us, align 1
+  %add9.us = add i8 %1, %0
+  store i8 %add9.us, ptr %arrayidx7.us, align 1
+  %inner.iv.next = add nuw nsw i64 %inner.iv, 1
+  %exitcond.not = icmp eq i64 %inner.iv.next, %n
+  br i1 %exitcond.not, label %inner.exit, label %inner.loop
+
+inner.exit:
+  %outer.iv.next = add nuw nsw i64 %outer.iv, 1
+  %exitcond26.not = icmp eq i64 %outer.iv.next, %m
+  br i1 %exitcond26.not, label %outer.exit, label %outer.loop, !prof !0
+
+outer.exit:
+  ret void
+}
+
+
+define void @outer_known_tc3_full_range_checks(ptr nocapture noundef %dst, ptr nocapture noundef readonly %src, i64 noundef %n) {
+; CHECK-LABEL: LV: Checking a loop in 'outer_known_tc3_full_range_checks'
+; CHECK:      Calculating cost of runtime checks:
+; CHECK:      We expect runtime memory checks to be hoisted out of the outer loop. Cost reduced from 6 to 2
+; CHECK:      Total cost of runtime checks: 2
+; CHECK-NEXT: LV: Minimum required TC for runtime checks to be profitable:4
+entry:
+  br label %outer.loop
+
+outer.loop:
+  %outer.iv = phi i64 [ 0, %entry ], [ %outer.iv.next, %inner.exit ]
+  %0 = mul nsw i64 %outer.iv, %n
+  br label %inner.loop
+
+inner.loop:
+  %iv.inner = phi i64 [ 0, %outer.loop ], [ %iv.inner.next, %inner.loop ]
+  %1 = add nuw nsw i64 %iv.inner, %0
+  %arrayidx.us = getelementptr inbounds i32, ptr %src, i64 %1
+  %2 = load i32, ptr %arrayidx.us, align 4
+  %arrayidx8.us = getelementptr inbounds i32, ptr %dst, i64 %1
+  %3 = load i32, ptr %arrayidx8.us, align 4
+  %add9.us = add nsw i32 %3, %2
+  store i32 %add9.us, ptr %arrayidx8.us, align 4
+  %iv.inner.next = add nuw nsw i64 %iv.inner, 1
+  %inner.exit.cond = icmp eq i64 %iv.inner.next, %n
+  br i1 %inner.exit.cond, label %inner.exit, label %inner.loop
+
+inner.exit:
+  %outer.iv.next = add nuw nsw i64 %outer.iv, 1
+  %outer.exit.cond = icmp eq i64 %outer.iv.next, 3
+  br i1 %outer.exit.cond, label %outer.exit, label %outer.loop
+
+outer.exit:
+  ret void
+}
+
+
+!0 = !{!"branch_weights", i32 10, i32 20}