Disable indvar widening if arithmetics on the wider type are more expensive

Summary:
Reapply r221772. The old patch breaks the bot because the @indvar_32_bit test
was run whether NVPTX was enabled or not.

IndVarSimplify should not widen an indvar if arithmetics on the wider
indvar are more expensive than those on the narrower indvar. For
instance, although NVPTX64 treats i64 as a legal type, an ADD on i64 is
twice as expensive as that on i32, because the hardware needs to
simulate a 64-bit integer using two 32-bit integers.

Split from D6188, and based on D6195 which adds NVPTXTargetTransformInfo.

Fixes PR21148.

Test Plan:
Added @indvar_32_bit that verifies we do not widen an indvar if the arithmetics
on the wider type are more expensive. This test is run only when NVPTX is
enabled.

Reviewers: jholewinski, eliben, meheff, atrick

Reviewed By: atrick

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D6196



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221799 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -31,6 +31,7 @@
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/LoopPass.h"
 #include "llvm/Analysis/ScalarEvolutionExpander.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
@@ -69,11 +70,12 @@ static cl::opt<bool> ReduceLiveIVs("liv-reduce", cl::Hidden,
 
 namespace {
   class IndVarSimplify : public LoopPass {
-    LoopInfo        *LI;
-    ScalarEvolution *SE;
-    DominatorTree   *DT;
-    const DataLayout *DL;
-    TargetLibraryInfo *TLI;
+    LoopInfo                  *LI;
+    ScalarEvolution           *SE;
+    DominatorTree             *DT;
+    const DataLayout          *DL;
+    TargetLibraryInfo         *TLI;
+    const TargetTransformInfo *TTI;
 
     SmallVector<WeakVH, 16> DeadInsts;
     bool Changed;
@@ -661,7 +663,7 @@ namespace {
 /// extended by this sign or zero extend operation. This is used to determine
 /// the final width of the IV before actually widening it.
 static void visitIVCast(CastInst *Cast, WideIVInfo &WI, ScalarEvolution *SE,
-                        const DataLayout *DL) {
+                        const DataLayout *DL, const TargetTransformInfo *TTI) {
   bool IsSigned = Cast->getOpcode() == Instruction::SExt;
   if (!IsSigned && Cast->getOpcode() != Instruction::ZExt)
     return;
@@ -671,6 +673,19 @@ static void visitIVCast(CastInst *Cast, WideIVInfo &WI, ScalarEvolution *SE,
   if (DL && !DL->isLegalInteger(Width))
     return;
 
+  // Cast is either an sext or zext up to this point.
+  // We should not widen an indvar if arithmetics on the wider indvar are more
+  // expensive than those on the narrower indvar. We check only the cost of ADD
+  // because at least an ADD is required to increment the induction variable. We
+  // could compute more comprehensively the cost of all instructions on the
+  // induction variable when necessary.
+  if (TTI &&
+      TTI->getArithmeticInstrCost(Instruction::Add, Ty) >
+          TTI->getArithmeticInstrCost(Instruction::Add,
+                                      Cast->getOperand(0)->getType())) {
+    return;
+  }
+
   if (!WI.WidestNativeType) {
     WI.WidestNativeType = SE->getEffectiveSCEVType(Ty);
     WI.IsSigned = IsSigned;
@@ -1187,14 +1202,16 @@ namespace {
   class IndVarSimplifyVisitor : public IVVisitor {
     ScalarEvolution *SE;
     const DataLayout *DL;
+    const TargetTransformInfo *TTI;
     PHINode *IVPhi;
 
   public:
     WideIVInfo WI;
 
     IndVarSimplifyVisitor(PHINode *IV, ScalarEvolution *SCEV,
-                          const DataLayout *DL, const DominatorTree *DTree):
-      SE(SCEV), DL(DL), IVPhi(IV) {
+                          const DataLayout *DL, const TargetTransformInfo *TTI,
+                          const DominatorTree *DTree)
+        : SE(SCEV), DL(DL), TTI(TTI), IVPhi(IV) {
       DT = DTree;
       WI.NarrowIV = IVPhi;
       if (ReduceLiveIVs)
@@ -1202,7 +1219,9 @@ namespace {
     }
 
     // Implement the interface used by simplifyUsersOfIV.
-    void visitCast(CastInst *Cast) override { visitIVCast(Cast, WI, SE, DL); }
+    void visitCast(CastInst *Cast) override {
+      visitIVCast(Cast, WI, SE, DL, TTI);
+    }
   };
 }
 
@@ -1236,7 +1255,7 @@ void IndVarSimplify::SimplifyAndExtend(Loop *L,
       PHINode *CurrIV = LoopPhis.pop_back_val();
 
       // Information about sign/zero extensions of CurrIV.
-      IndVarSimplifyVisitor Visitor(CurrIV, SE, DL, DT);
+      IndVarSimplifyVisitor Visitor(CurrIV, SE, DL, TTI, DT);
 
       Changed |= simplifyUsersOfIV(CurrIV, SE, &LPM, DeadInsts, &Visitor);
 
@@ -1895,6 +1914,7 @@ bool IndVarSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
   DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
   DL = DLP ? &DLP->getDataLayout() : nullptr;
   TLI = getAnalysisIfAvailable<TargetLibraryInfo>();
+  TTI = getAnalysisIfAvailable<TargetTransformInfo>();
 
   DeadInsts.clear();
   Changed = false;