[InstCombine] Make max size array combine a tunable.

Requested by Sanjoy/Hal a while ago, and forgotten by me
(r283612).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294323 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -230,7 +230,9 @@ Instruction *InstCombiner::foldCmpLoadFromIndexedGlobal(GetElementPtrInst *GEP,
     return nullptr;
 
   uint64_t ArrayElementCount = Init->getType()->getArrayNumElements();
-  if (ArrayElementCount > 1024) return nullptr; // Don't blow up on huge arrays.
+  // Don't blow up on huge arrays.
+  if (ArrayElementCount > MaxArraySizeForCombine)
+    return nullptr;
 
   // There are many forms of this optimization we can handle, for now, just do
   // the simple index into a single-dimensional array.

lib/Transforms/InstCombine/InstCombineInternal.h

@@ -508,6 +508,9 @@ public:
     return llvm::computeOverflowForUnsignedAdd(LHS, RHS, DL, &AC, CxtI, &DT);
   }
 
+  /// Maximum size of array considered when transforming.
+  int MaxArraySizeForCombine;
+
 private:
   /// \brief Performs a few simplifications for operators which are associative
   /// or commutative.

lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

@@ -609,7 +609,7 @@ static Instruction *unpackLoadToAggregate(InstCombiner &IC, LoadInst &LI) {
     // arrays of arbitrary size but this has a terrible impact on compile time.
     // The threshold here is chosen arbitrarily, maybe needs a little bit of
     // tuning.
-    if (NumElements > 1024)
+    if (NumElements > IC.MaxArraySizeForCombine)
       return nullptr;
 
     const DataLayout &DL = IC.getDataLayout();
@@ -1114,7 +1114,7 @@ static bool unpackStoreToAggregate(InstCombiner &IC, StoreInst &SI) {
     // arrays of arbitrary size but this has a terrible impact on compile time.
     // The threshold here is chosen arbitrarily, maybe needs a little bit of
     // tuning.
-    if (NumElements > 1024)
+    if (NumElements > IC.MaxArraySizeForCombine)
       return false;
 
     const DataLayout &DL = IC.getDataLayout();

lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -82,6 +82,10 @@ static cl::opt<bool>
 EnableExpensiveCombines("expensive-combines",
                         cl::desc("Enable expensive instruction combines"));
 
+static cl::opt<unsigned>
+MaxArraySize("instcombine-maxarray-size", cl::init(1024),
+             cl::desc("Maximum array size considered when doing a combine"));
+
 Value *InstCombiner::EmitGEPOffset(User *GEP) {
   return llvm::EmitGEPOffset(Builder, DL, GEP);
 }
@@ -3148,6 +3152,7 @@ combineInstructionsOverFunction(Function &F, InstCombineWorklist &Worklist,
 
     InstCombiner IC(Worklist, &Builder, F.optForMinSize(), ExpensiveCombines,
                     AA, AC, TLI, DT, DL, LI);
+    IC.MaxArraySizeForCombine = MaxArraySize;
     Changed |= IC.run();
 
     if (!Changed)