[AArch64][LoopVectorize] Use upper bound trip count instead of the constant TC when choosing max VF (#67697)

This patch is based off of
#67543.

We are currently using the exact trip count to make decisions regarding
the maximum VF. We can instead use the upper bound TC, which will be the
same as the constant trip count when that is known.

Author: Rin

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1695,14 +1695,14 @@ class LoopVectorizationCostModel {
   /// elements is a power-of-2 larger than zero. If scalable vectorization is
   /// disabled or unsupported, then the scalable part will be equal to
   /// ElementCount::getScalable(0).
-  FixedScalableVFPair computeFeasibleMaxVF(unsigned ConstTripCount,
+  FixedScalableVFPair computeFeasibleMaxVF(unsigned MaxTripCount,
                                            ElementCount UserVF,
                                            bool FoldTailByMasking);
 
   /// \return the maximized element count based on the targets vector
   /// registers and the loop trip-count, but limited to a maximum safe VF.
   /// This is a helper function of computeFeasibleMaxVF.
-  ElementCount getMaximizedVFForTarget(unsigned ConstTripCount,
+  ElementCount getMaximizedVFForTarget(unsigned MaxTripCount,
                                        unsigned SmallestType,
                                        unsigned WidestType,
                                        ElementCount MaxSafeVF,
@@ -4809,7 +4809,7 @@ LoopVectorizationCostModel::getMaxLegalScalableVF(unsigned MaxSafeElements) {
 }
 
 FixedScalableVFPair LoopVectorizationCostModel::computeFeasibleMaxVF(
-    unsigned ConstTripCount, ElementCount UserVF, bool FoldTailByMasking) {
+    unsigned MaxTripCount, ElementCount UserVF, bool FoldTailByMasking) {
   MinBWs = computeMinimumValueSizes(TheLoop->getBlocks(), *DB, &TTI);
   unsigned SmallestType, WidestType;
   std::tie(SmallestType, WidestType) = getSmallestAndWidestTypes();
@@ -4897,12 +4897,12 @@ FixedScalableVFPair LoopVectorizationCostModel::computeFeasibleMaxVF(
   FixedScalableVFPair Result(ElementCount::getFixed(1),
                              ElementCount::getScalable(0));
   if (auto MaxVF =
-          getMaximizedVFForTarget(ConstTripCount, SmallestType, WidestType,
+          getMaximizedVFForTarget(MaxTripCount, SmallestType, WidestType,
                                   MaxSafeFixedVF, FoldTailByMasking))
     Result.FixedVF = MaxVF;
 
   if (auto MaxVF =
-          getMaximizedVFForTarget(ConstTripCount, SmallestType, WidestType,
+          getMaximizedVFForTarget(MaxTripCount, SmallestType, WidestType,
                                   MaxSafeScalableVF, FoldTailByMasking))
     if (MaxVF.isScalable()) {
       Result.ScalableVF = MaxVF;
@@ -4926,6 +4926,7 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
   }
 
   unsigned TC = PSE.getSE()->getSmallConstantTripCount(TheLoop);
+  unsigned MaxTC = PSE.getSE()->getSmallConstantMaxTripCount(TheLoop);
   LLVM_DEBUG(dbgs() << "LV: Found trip count: " << TC << '\n');
   if (TC == 1) {
     reportVectorizationFailure("Single iteration (non) loop",
@@ -4936,7 +4937,7 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
 
   switch (ScalarEpilogueStatus) {
   case CM_ScalarEpilogueAllowed:
-    return computeFeasibleMaxVF(TC, UserVF, false);
+    return computeFeasibleMaxVF(MaxTC, UserVF, false);
   case CM_ScalarEpilogueNotAllowedUsePredicate:
     [[fallthrough]];
   case CM_ScalarEpilogueNotNeededUsePredicate:
@@ -4974,7 +4975,7 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
       LLVM_DEBUG(dbgs() << "LV: Cannot fold tail by masking: vectorize with a "
                            "scalar epilogue instead.\n");
       ScalarEpilogueStatus = CM_ScalarEpilogueAllowed;
-      return computeFeasibleMaxVF(TC, UserVF, false);
+      return computeFeasibleMaxVF(MaxTC, UserVF, false);
     }
     return FixedScalableVFPair::getNone();
   }
@@ -4991,7 +4992,7 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
     InterleaveInfo.invalidateGroupsRequiringScalarEpilogue();
   }
 
-  FixedScalableVFPair MaxFactors = computeFeasibleMaxVF(TC, UserVF, true);
+  FixedScalableVFPair MaxFactors = computeFeasibleMaxVF(MaxTC, UserVF, true);
 
   // Avoid tail folding if the trip count is known to be a multiple of any VF
   // we choose.
@@ -5067,7 +5068,7 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
 }
 
 ElementCount LoopVectorizationCostModel::getMaximizedVFForTarget(
-    unsigned ConstTripCount, unsigned SmallestType, unsigned WidestType,
+    unsigned MaxTripCount, unsigned SmallestType, unsigned WidestType,
     ElementCount MaxSafeVF, bool FoldTailByMasking) {
   bool ComputeScalableMaxVF = MaxSafeVF.isScalable();
   const TypeSize WidestRegister = TTI.getRegisterBitWidth(
@@ -5106,24 +5107,24 @@ ElementCount LoopVectorizationCostModel::getMaximizedVFForTarget(
   }
 
   // When a scalar epilogue is required, at least one iteration of the scalar
-  // loop has to execute. Adjust ConstTripCount accordingly to avoid picking a
+  // loop has to execute. Adjust MaxTripCount accordingly to avoid picking a
   // max VF that results in a dead vector loop.
-  if (ConstTripCount > 0 && requiresScalarEpilogue(true))
-    ConstTripCount -= 1;
-
-  if (ConstTripCount && ConstTripCount <= WidestRegisterMinEC &&
-      (!FoldTailByMasking || isPowerOf2_32(ConstTripCount))) {
-    // If loop trip count (TC) is known at compile time there is no point in
-    // choosing VF greater than TC (as done in the loop below). Select maximum
-    // power of two which doesn't exceed TC.
-    // If MaxVectorElementCount is scalable, we only fall back on a fixed VF
-    // when the TC is less than or equal to the known number of lanes.
-    auto ClampedConstTripCount = llvm::bit_floor(ConstTripCount);
+  if (MaxTripCount > 0 && requiresScalarEpilogue(true))
+    MaxTripCount -= 1;
+
+  if (MaxTripCount && MaxTripCount <= WidestRegisterMinEC &&
+      (!FoldTailByMasking || isPowerOf2_32(MaxTripCount))) {
+    // If upper bound loop trip count (TC) is known at compile time there is no
+    // point in choosing VF greater than TC (as done in the loop below). Select
+    // maximum power of two which doesn't exceed TC. If MaxVectorElementCount is
+    // scalable, we only fall back on a fixed VF when the TC is less than or
+    // equal to the known number of lanes.
+    auto ClampedUpperTripCount = llvm::bit_floor(MaxTripCount);
     LLVM_DEBUG(dbgs() << "LV: Clamping the MaxVF to maximum power of two not "
                          "exceeding the constant trip count: "
-                      << ClampedConstTripCount << "\n");
+                      << ClampedUpperTripCount << "\n");
     return ElementCount::get(
-        ClampedConstTripCount,
+        ClampedUpperTripCount,
         FoldTailByMasking ? MaxVectorElementCount.isScalable() : false);
   }
 

llvm/test/Transforms/LoopVectorize/AArch64/clamped-trip-count.ll

@@ -21,3 +21,34 @@ for.body:                                         ; preds = %entry, %for.body
 for.cond.cleanup:                                 ; preds = %for.body
   ret void
 }
+
+define void @clamped_tc_max_8(ptr nocapture %dst, i32 %n, i64 %val){
+; CHECK-LABEL: define void @clamped_tc_max_8(
+; CHECK: call void @llvm.masked.store.nxv8i8.p0(<vscale x 8 x i8> {{.*}}, ptr {{.*}}, i32 1, <vscale x 8 x i1> {{.*}})
+
+entry:
+  %rem = and i32 %n, 63
+  %cmp8.not = icmp eq i32 %rem, 0
+  br i1 %cmp8.not, label %for.cond.cleanup, label %for.body.preheader
+
+for.body.preheader:                               ; preds = %entry
+  %add = add nuw nsw i32 %rem, 7
+  %shr = lshr i32 %add, 3
+  %wide.trip.count = zext i32 %shr to i64
+  br label %for.body
+
+for.body:                                         ; preds = %for.body.preheader, %for.body
+  %indvars.iv = phi i64 [ 0, %for.body.preheader ], [ %indvars.iv.next, %for.body ]
+  %p_out_tail.09 = phi ptr [ %dst, %for.body.preheader ], [ %incdec.ptr, %for.body ]
+  %0 = shl nuw nsw i64 %indvars.iv, 3
+  %shr3 = lshr i64 %val, %0
+  %conv4 = trunc i64 %shr3 to i8
+  store i8 %conv4, ptr %p_out_tail.09, align 1
+  %incdec.ptr = getelementptr inbounds i8, ptr %p_out_tail.09, i64 1
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond.not = icmp eq i64 %indvars.iv.next, %wide.trip.count
+  br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.body
+  ret void
+}