[llvm] [AMDGPU][SDAG] Add target-specific ISD::PTRADD combines (PR #143673)

[Fabian Ritter via llvm-commits]

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@@ -15089,42 +15094,116 @@ SDValue SITargetLowering::performPtrAddCombine(SDNode *N,
                                                DAGCombinerInfo &DCI) const {
   SelectionDAG &DAG = DCI.DAG;
   SDLoc DL(N);
+  EVT VT = N->getValueType(0);
   SDValue N0 = N->getOperand(0);
   SDValue N1 = N->getOperand(1);
 
-  if (N1.getOpcode() == ISD::ADD) {
-    // (ptradd x, (add y, z)) -> (ptradd (ptradd x, y), z) if z is a constant,
-    //    y is not, and (add y, z) is used only once.
-    // (ptradd x, (add y, z)) -> (ptradd (ptradd x, z), y) if y is a constant,
-    //    z is not, and (add y, z) is used only once.
-    // The goal is to move constant offsets to the outermost ptradd, to create
-    // more opportunities to fold offsets into memory instructions.
-    // Together with the generic combines in DAGCombiner.cpp, this also
-    // implements (ptradd (ptradd x, y), z) -> (ptradd (ptradd x, z), y)).
-    //
-    // This transform is here instead of in the general DAGCombiner as it can
-    // turn in-bounds pointer arithmetic out-of-bounds, which is problematic for
-    // AArch64's CPA.
-    SDValue X = N0;
-    SDValue Y = N1.getOperand(0);
-    SDValue Z = N1.getOperand(1);
-    if (N1.hasOneUse()) {
-      bool YIsConstant = DAG.isConstantIntBuildVectorOrConstantInt(Y);
-      bool ZIsConstant = DAG.isConstantIntBuildVectorOrConstantInt(Z);
-      if (ZIsConstant != YIsConstant) {
-        // If both additions in the original were NUW, the new ones are as well.
-        SDNodeFlags Flags =
-            (N->getFlags() & N1->getFlags()) & SDNodeFlags::NoUnsignedWrap;
-        if (YIsConstant)
-          std::swap(Y, Z);
+  // The following folds transform PTRADDs into regular arithmetic in cases
+  // where the PTRADD wouldn't be folded as an immediate offset into memory
+  // instructions anyway. They are target-specific in that other targets might
+  // prefer to not lose information about the pointer arithmetic.
+
+  // Fold (ptradd x, shl(0 - v, k)) -> sub(x, shl(v, k)).
+  // Adapted from DAGCombiner::visitADDLikeCommutative.
+  SDValue V, K;
+  if (sd_match(N1, m_Shl(m_Neg(m_Value(V)), m_Value(K)))) {
+    SDValue Inner = DAG.getNode(ISD::SHL, DL, VT, V, K);
----------------
ritter-x2a wrote:

I think the only case that works is `(ptradd, x, (shl nuw nsw, 0 - v, k)) -> sub(x, (shl nsw, v, k))` (see [alive2](https://alive2.llvm.org/ce/z/g3dT4D), with `add` instead of `gep`). I've added that, but I don't know if we exploit `shl nsw` anywhere, which would be necessary for a test where the flag preservation changes the output.

https://github.com/llvm/llvm-project/pull/143673