[llvm] Redesign Straight-Line Strength Reduction (SLSR) (PR #162930)

[Artem Belevich via llvm-commits]

================
@@ -561,118 +1098,161 @@ void StraightLineStrengthReduce::allocateCandidatesAndFindBasisForGEP(
             DL->getIndexSizeInBits(GEP->getAddressSpace())) {
       // Skip factoring if TruncatedArrayIdx is wider than the pointer size,
       // because TruncatedArrayIdx is implicitly truncated to the pointer size.
-      factorArrayIndex(TruncatedArrayIdx, BaseExpr, ElementSize, GEP);
+      allocateCandidatesAndFindBasis(Candidate::GEP, BaseExpr, ElementSizeIdx,
+                                     TruncatedArrayIdx, GEP);
     }
 
     IndexExprs[I - 1] = OrigIndexExpr;
   }
 }
 
-// A helper function that unifies the bitwidth of A and B.
-static void unifyBitWidth(APInt &A, APInt &B) {
-  if (A.getBitWidth() < B.getBitWidth())
-    A = A.sext(B.getBitWidth());
-  else if (A.getBitWidth() > B.getBitWidth())
-    B = B.sext(A.getBitWidth());
-}
-
 Value *StraightLineStrengthReduce::emitBump(const Candidate &Basis,
                                             const Candidate &C,
                                             IRBuilder<> &Builder,
                                             const DataLayout *DL) {
-  APInt Idx = C.Index->getValue(), BasisIdx = Basis.Index->getValue();
-  unifyBitWidth(Idx, BasisIdx);
-  APInt IndexOffset = Idx - BasisIdx;
-
-  // Compute Bump = C - Basis = (i' - i) * S.
-  // Common case 1: if (i' - i) is 1, Bump = S.
-  if (IndexOffset == 1)
-    return C.Stride;
-  // Common case 2: if (i' - i) is -1, Bump = -S.
-  if (IndexOffset.isAllOnes())
-    return Builder.CreateNeg(C.Stride);
-
-  // Otherwise, Bump = (i' - i) * sext/trunc(S). Note that (i' - i) and S may
-  // have different bit widths.
-  IntegerType *DeltaType =
-      IntegerType::get(Basis.Ins->getContext(), IndexOffset.getBitWidth());
-  Value *ExtendedStride = Builder.CreateSExtOrTrunc(C.Stride, DeltaType);
-  if (IndexOffset.isPowerOf2()) {
-    // If (i' - i) is a power of 2, Bump = sext/trunc(S) << log(i' - i).
-    ConstantInt *Exponent = ConstantInt::get(DeltaType, IndexOffset.logBase2());
-    return Builder.CreateShl(ExtendedStride, Exponent);
+  auto CreateMul = [&](Value *LHS, Value *RHS) {
+    if (ConstantInt *CR = dyn_cast<ConstantInt>(RHS)) {
+      const APInt &ConstRHS = CR->getValue();
+      IntegerType *DeltaType =
+          IntegerType::get(C.Ins->getContext(), ConstRHS.getBitWidth());
+      if (ConstRHS.isPowerOf2()) {
+        ConstantInt *Exponent =
+            ConstantInt::get(DeltaType, ConstRHS.logBase2());
+        return Builder.CreateShl(LHS, Exponent);
+      }
+      if (ConstRHS.isNegatedPowerOf2()) {
+        ConstantInt *Exponent =
+            ConstantInt::get(DeltaType, (-ConstRHS).logBase2());
+        return Builder.CreateNeg(Builder.CreateShl(LHS, Exponent));
+      }
+    }
+
+    return Builder.CreateMul(LHS, RHS);
+  };
+
+  Value *Delta = C.Delta;
+  // If Delta is 0, C is a fully redundant of C.Basis,
+  // just replace C.Ins with Basis.Ins
+  if (ConstantInt *CI = dyn_cast<ConstantInt>(Delta)) {
+    APInt DeltaValue = CI->getValue();
+    if (DeltaValue.isZero())
+      return nullptr;
   }
----------------
Artem-B wrote:

Nit: just
```
if (ConstantInt *CI = dyn_cast<ConstantInt>(Delta);  CI && CI->getValue().isZero())
      return nullptr;
```

https://github.com/llvm/llvm-project/pull/162930