[SLSR] handles off bounds GEPs

[jingyue]

Summary

The old requirement on GEP candidates being in bounds is unnecessary.
For off-bound GEPs, we still have

&B[i * S] = B + (i * S) * e = B + (i * e) * S

[meheff]

LGTM. Thanks!

test/Transforms/StraightLineStrengthReduce/slsr-gep.ll

112

nit: "out of bounds" is probably the term you want to use, not "off bounds".

[jingyue]

fixed grammar

lib/Transforms/Scalar/StraightLineStrengthReduce.cpp

if (LHS != RHS) {
     // Symmetrically, try to split RHS to Base + Index.
     allocateCandidateAndFindBasisForMul(RHS, LHS, I);
   }
 }
 
 void StraightLineStrengthReduce::allocateCandidateAndFindBasisForGEP(
     const SCEV *B, ConstantInt *Idx, Value *S, uint64_t ElementSize,
     Instruction *I) {
-  // I = B + sext(Idx *nsw S) *nsw ElementSize
+  // I = B + sext(Idx *nsw S) * ElementSize
+  //   = B + (sext(Idx) * sext(S)) * ElementSize
   //   = B + (sext(Idx) * ElementSize) * sext(S)
   // Casting to IntegerType is safe because we skipped vector GEPs.
   IntegerType *IntPtrTy = cast<IntegerType>(DL->getIntPtrType(I->getType()));
   ConstantInt *ScaledIdx = ConstantInt::get(
       IntPtrTy, Idx->getSExtValue() * (int64_t)ElementSize, true);
   allocateCandidateAndFindBasis(Candidate::GEP, B, ScaledIdx, S, I);
 }
 
void StraightLineStrengthReduce::factorArrayIndex(Value *ArrayIdx,
   // would have to translate SCEVs back to IR instructions. This translation
   // is difficult when LHS is further evaluated to a composite SCEV.
   //
   // 2. ScalarEvolution is designed to be control-flow oblivious. It tends
   // to strip nsw/nuw flags which are critical for SLSR to trace into
   // sext'ed multiplication.
   if (match(ArrayIdx, m_NSWMul(m_Value(LHS), m_ConstantInt(RHS)))) {
     // SLSR is currently unsafe if i * S may overflow.
-    // GEP = Base + sext(LHS *nsw RHS) *nsw ElementSize
+    // GEP = Base + sext(LHS *nsw RHS) * ElementSize
     allocateCandidateAndFindBasisForGEP(Base, RHS, LHS, ElementSize, GEP);
   }
 }
 
 void StraightLineStrengthReduce::allocateCandidateAndFindBasisForGEP(
     GetElementPtrInst *GEP) {
   // TODO: handle vector GEPs
   if (GEP->getType()->isVectorTy())
void StraightLineStrengthReduce::rewriteCandidateWithBasis(
   Value *Reduced = nullptr; // equivalent to but weaker than C.Ins
   switch (C.CandidateKind) {
   case Candidate::Mul:
     Reduced = Builder.CreateAdd(Basis.Ins, Bump);
     break;
   case Candidate::GEP:
     {
       Type *IntPtrTy = DL->getIntPtrType(C.Ins->getType());
+      bool InBounds = cast<GetElementPtrInst>(C.Ins)->isInBounds();
       if (BumpWithUglyGEP) {
         // C = (char *)Basis + Bump
         unsigned AS = Basis.Ins->getType()->getPointerAddressSpace();
         Type *CharTy = Type::getInt8PtrTy(Basis.Ins->getContext(), AS);
         Reduced = Builder.CreateBitCast(Basis.Ins, CharTy);
-        // We only considered inbounds GEP as candidates.
+        if (InBounds)
           Reduced = Builder.CreateInBoundsGEP(Reduced, Bump);
+        else
+          Reduced = Builder.CreateGEP(Reduced, Bump);
         Reduced = Builder.CreateBitCast(Reduced, C.Ins->getType());
       } else {
         // C = gep Basis, Bump
         // Canonicalize bump to pointer size.
         Bump = Builder.CreateSExtOrTrunc(Bump, IntPtrTy);
+        if (InBounds)
           Reduced = Builder.CreateInBoundsGEP(Basis.Ins, Bump);
+        else
+          Reduced = Builder.CreateGEP(Basis.Ins, Bump);
       }
     }
     break;
   default:
     llvm_unreachable("C.CandidateKind is invalid");
   };
   Reduced->takeName(C.Ins);
   C.Ins->replaceAllUsesWith(Reduced);

test/Transforms/StraightLineStrengthReduce/slsr-gep.ll

 ; CHECK: getelementptr inbounds i8, i8* %{{[0-9]+}}, i64 [[BUMP]]
   %v2 = load i64, i64* %p2
 
   ; return v0 + v1 + v2;
   %1 = add i64 %v0, %v1
   %2 = add i64 %1, %v2
   ret i64 %2
 }
+
+define i32 @slsr_offbound_gep(i32* %input, i32 %s) {
+; CHECK-LABEL: @slsr_offbound_gep(
+  ; v0 = input[0];
+  %p0 = getelementptr i32, i32* %input, i64 0
+  %v0 = load i32, i32* %p0
+
+  ; v1 = input[(long)s];
+  %t = sext i32 %s to i64
+  %p1 = getelementptr i32, i32* %input, i64 %t
+; CHECK: %p1 = getelementptr i32, i32* %input, i64 %t
+  %v1 = load i32, i32* %p1
+
+  ; v2 = input[(long)(s * 2)];
+  %s2 = mul nsw i32 %s, 2
+  %t2 = sext i32 %s2 to i64
+  %p2 = getelementptr i32, i32* %input, i64 %t2
+; CHECK: %p2 = getelementptr i32, i32* %p1, i64 %t
+  %v2 = load i32, i32* %p2
+
+  ; return v0 + v1 + v2;
+  %1 = add i32 %v0, %v1
+  %2 = add i32 %1, %v2
+  ret i32 %2
+}