[llvm-branch-commits] [llvm] [CGP] Simple loop strength reduction for vector values (PR #226197)

[Graham Hunter via llvm-branch-commits]

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@@ -8944,6 +8944,146 @@ static bool optimizeBranch(CondBrInst *Branch, const TargetLowering &TLI,
   return false;
 }
 
+// Performs very basic loop strength reduction to vector values with a known
+// evolution from one iteration to the next that are used as the value operand
+// for a store.
+// A simple example to illustrate:
+//
+//  %vec.ind = phi <vscale x 2 x i64> [ %start, %entry ],
+//                                    [ %vec.ind.next, %vector.body ]
+//  %data.gep = getelementptr inbounds nuw [72 x i8], ptr %datap,
+//                            <vscale x 2 x i64> %vec.ind
+//  %addr.gep = getelementptr inbounds nuw [8 x i8], ptr %addrp, i64 %index
+//  store <vscale x 2 x ptr> %data.gep, ptr %addr.gep, align 8
+//  %vec.ind.next = add nuw nsw <vscale x 2 x i64> %vec.ind, %elt.cnt.splat
+//
+// This will end up with a multiply by a vscale-scaled term in the loop, as
+// well as adding to the base. Changing it to add a splat based on vscale *
+// min.elt.cnt * sizeof(ptrdiff) and remove the gep removes the multiply.
+//
+// TODO: Support more cases, such as the address instead of value operand for
+//       strided memory operations.
+static bool strengthReduceVectorPhiUsers(PHINode *Phi, LoopInfo *LI) {
+  // We're only interested in header phis in innermost loops.
+  // We want a loop with an identifiable preheader and single latch.
+  Loop *L = LI->getLoopFor(Phi->getParent());
+  if (!L || !L->isInnermost())
+    return false;
+
+  BasicBlock *Header = L->getHeader();
+  BasicBlock *PreHeader = L->getLoopPreheader();
+  BasicBlock *Latch = L->getLoopLatch();
+  if (!PreHeader || !Latch || Phi->getParent() != Header)
+    return false;
+
+  // Check the progression of the phi.
+  Value *Start = Phi->getIncomingValueForBlock(PreHeader);
+  Value *Step = Phi->getIncomingValueForBlock(Latch);
+
+  // TODO: Handle offsets from stepvector.
+  if (!match(Start, m_Intrinsic<Intrinsic::stepvector>()))
+    return false;
+
+  Value *LoopStride = nullptr;
+  if (!match(Step, m_c_Add(m_Specific(Phi), m_Value(LoopStride))))
+    return false;
+
+  const APInt *ShiftAmt = nullptr;
+  if (!match(LoopStride, m_Splat(m_Shl(m_VScale(), m_APInt(ShiftAmt)))))
+    return false;
+
+  // Record users of interest.
+  struct OffsetGEP {
+    GetElementPtrInst *GEP;
+    Value *Offset;
+  };
+  SmallVector<OffsetGEP, 4> Candidates;
+  Value *CommonBase = nullptr;
+  unsigned CommonSize = 0;
+  DataLayout DL = Phi->getFunction()->getDataLayout();
+  for (User *U : Phi->users()) {
+    Instruction *I = cast<Instruction>(U);
+    // Skip over the step, handled above.
+    if (Step == I)
+      continue;
+
+    // If we have an offset from the Phi values, record that and then look
+    // for a GEP.
+    Value *Offset = nullptr;
+    if (match(I, m_OneUse(m_c_Add(m_Specific(Phi), m_Value(Offset)))))
+      I = cast<Instruction>(I->getSingleUndroppableUse()->getUser());
+
+    // We're only interested in single index GEPs used only as the value
+    // operand in a store for now.
+    auto *GEP = dyn_cast<GetElementPtrInst>(I);
+    if (!GEP || GEP->getNumOperands() != 2)
+      return false;
+
+    Use *GEPUse = GEP->getSingleUndroppableUse();
+    if (!GEPUse || !isa<StoreInst>(GEPUse->getUser()) ||
+        GEPUse->getOperandNo() != 0)
+      return false;
+
+    // Reject anything with a loop-varying base or if we have different bases
+    // for different GEPs.
+    // TODO: Support multiple bases.
+    Value *Base = I->getOperand(0);
+    if (!L->isLoopInvariant(Base) || (CommonBase && CommonBase != Base))
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huntergr-arm wrote:

Yeah, I didn't add negative tests yet. I wasn't sure if there would be pushback to adding this to CGP, but if it's fine to do it here I can add more test cases.

https://github.com/llvm/llvm-project/pull/226197