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

[Fei Peng via llvm-commits]

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@@ -561,118 +1067,164 @@ 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;
+  auto CreateMul = [&](Value *LHS, Value *RHS) {
+    if (isa<ConstantInt>(RHS)) {
+      APInt ConstRHS = cast<ConstantInt>(RHS)->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);
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fiigii wrote:

Hmm, for this shl, sometimes we may preserve nsw/nuw. I can give it a try in future PRs.

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