[PATCH] D128741: [RISCV] Fold (add X, [-4096, -2049]) or (add X, [2048,4096]) into load/store address during isel.

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.
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Previously we iseled this to a pair of ADDIs and relied on a post
isel peephole to fold one of the ADDIs into the load/store. Now
we split the immediate in two parts the same way isel does and fold
one of the pieces. If the add has a non-memory use it will emit
two isels and larger one will CSE with the ADDI we created for the
the memory use.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D128741

Files:
  llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp


Index: llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
+++ llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
@@ -1879,6 +1879,26 @@
     }
   }
 
+  // Handle ADD with large immediates.
+  if (Addr.getOpcode() == ISD::ADD && isa<ConstantSDNode>(Addr.getOperand(1))) {
+    int64_t CVal = cast<ConstantSDNode>(Addr.getOperand(1))->getSExtValue();
+    assert(!isInt<12>(CVal) && "simm12 not already handled?");
+
+    if (isInt<12>(CVal / 2) && isInt<12>(CVal - CVal / 2)) {
+      // We can use an ADDI for part of the offset and fold the rest into the
+      // load/store. This mirrors the AddiPair PatFrag in RISCVInstrInfo.td.
+      int64_t Adj = CVal < 0 ? -2048 : 2047;
+      SDLoc DL(Addr);
+      MVT VT = Addr.getSimpleValueType();
+      Base = SDValue(
+          CurDAG->getMachineNode(RISCV::ADDI, DL, VT, Addr.getOperand(0),
+                                 CurDAG->getTargetConstant(Adj, DL, VT)),
+          0);
+      Offset = CurDAG->getTargetConstant(CVal - Adj, DL, VT);
+      return true;
+    }
+  }
+
   Base = Addr;
   Offset = CurDAG->getTargetConstant(0, SDLoc(Addr), Subtarget->getXLenVT());
   return true;


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