[LoopUnroll] Make UnrollMaxUpperBound to be overridable by target (#76029)

The UnrollMaxUpperBound should be target dependent, since different
chips provide different register set which brings different ability of
storing more temporary values of a program. So I add a MaxUpperBound
value in UnrollingPreference which can be override by targets. All uses
of UnrollMaxUpperBound are replaced with UP.MaxUpperBound.
 
The default value is still 8 and the command line argument
'--unroll-max-upperbound' takes final effect if provided.

Author: boxu-zhang

llvm/include/llvm/Analysis/TargetTransformInfo.h

@@ -560,6 +560,10 @@ class TargetTransformInfo {
     // (set to UINT_MAX to disable). This does not apply in cases where the
     // loop is being fully unrolled.
     unsigned MaxCount;
+    /// Set the maximum upper bound of trip count. Allowing the MaxUpperBound
+    /// to be overrided by a target gives more flexiblity on certain cases.
+    /// By default, MaxUpperBound uses UnrollMaxUpperBound which value is 8.
+    unsigned MaxUpperBound;
     /// Set the maximum unrolling factor for full unrolling. Like MaxCount, but
     /// applies even if full unrolling is selected. This allows a target to fall
     /// back to Partial unrolling if full unrolling is above FullUnrollMaxCount.

llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp

@@ -200,6 +200,7 @@ TargetTransformInfo::UnrollingPreferences llvm::gatherUnrollingPreferences(
   UP.Count = 0;
   UP.DefaultUnrollRuntimeCount = 8;
   UP.MaxCount = std::numeric_limits<unsigned>::max();
+  UP.MaxUpperBound = UnrollMaxUpperBound;
   UP.FullUnrollMaxCount = std::numeric_limits<unsigned>::max();
   UP.BEInsns = 2;
   UP.Partial = false;
@@ -237,6 +238,8 @@ TargetTransformInfo::UnrollingPreferences llvm::gatherUnrollingPreferences(
     UP.MaxPercentThresholdBoost = UnrollMaxPercentThresholdBoost;
   if (UnrollMaxCount.getNumOccurrences() > 0)
     UP.MaxCount = UnrollMaxCount;
+  if (UnrollMaxUpperBound.getNumOccurrences() > 0)
+    UP.MaxUpperBound = UnrollMaxUpperBound;
   if (UnrollFullMaxCount.getNumOccurrences() > 0)
     UP.FullUnrollMaxCount = UnrollFullMaxCount;
   if (UnrollAllowPartial.getNumOccurrences() > 0)
@@ -777,7 +780,7 @@ shouldPragmaUnroll(Loop *L, const PragmaInfo &PInfo,
     return TripCount;
 
   if (PInfo.PragmaEnableUnroll && !TripCount && MaxTripCount &&
-      MaxTripCount <= UnrollMaxUpperBound)
+      MaxTripCount <= UP.MaxUpperBound)
     return MaxTripCount;
 
   // if didn't return until here, should continue to other priorties
@@ -952,7 +955,7 @@ bool llvm::computeUnrollCount(
   // cost of exact full unrolling.  As such, if we have an exact count and
   // found it unprofitable, we'll never chose to bounded unroll.
   if (!TripCount && MaxTripCount && (UP.UpperBound || MaxOrZero) &&
-      MaxTripCount <= UnrollMaxUpperBound) {
+      MaxTripCount <= UP.MaxUpperBound) {
     UP.Count = MaxTripCount;
     if (auto UnrollFactor = shouldFullUnroll(L, TTI, DT, SE, EphValues,
                                              MaxTripCount, UCE, UP)) {
@@ -1026,7 +1029,7 @@ bool llvm::computeUnrollCount(
   }
 
   // Don't unroll a small upper bound loop unless user or TTI asked to do so.
-  if (MaxTripCount && !UP.Force && MaxTripCount < UnrollMaxUpperBound) {
+  if (MaxTripCount && !UP.Force && MaxTripCount < UP.MaxUpperBound) {
     UP.Count = 0;
     return false;
   }