[RISCV] Separate base from offset in lowerGlobalAddress

Summary:
When lowering global address, lower the base as a TargetGlobal first then
 create an SDNode for the offset separately and chain it to the address calculation

 This optimization will create a DAG where the base address of a global access will
 be reused between different access. The offset can later be folded into the immediate
 part of the memory access instruction.

  With this optimization we generate:

    lui a0, %hi(s)
    addi a0, a0, %lo(s) ; shared base address.

    addi a1, zero, 20 ; 2 instructions per access.
    sw a1, 44(a0)

    addi a1, zero, 10
    sw a1, 8(a0)

    addi a1, zero, 30
    sw a1, 80(a0)

    Instead of:

    lui a0, %hi(s+44) ; 3 instructions per access.
    addi a1, zero, 20
    sw a1, %lo(s+44)(a0)

    lui a0, %hi(s+8)
    addi a1, zero, 10
    sw a1, %lo(s+8)(a0)

    lui a0, %hi(s+80)
    addi a1, zero, 30
    sw a1, %lo(s+80)(a0)

    Which will save one instruction per access.

Reviewers: asb, apazos

Reviewed By: asb

Subscribers: rbar, johnrusso, simoncook, jordy.potman.lists, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, mgrang, apazos, asb, llvm-commits

Differential Revision: https://reviews.llvm.org/D46989

llvm-svn: 332641

1 files changed, 10 insertions, 5 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 52c027ba314..d3d0d0370b9 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -293,17 +293,22 @@ SDValue RISCVTargetLowering::lowerGlobalAddress(SDValue Op,
   GlobalAddressSDNode *N = cast<GlobalAddressSDNode>(Op);
   const GlobalValue *GV = N->getGlobal();
   int64_t Offset = N->getOffset();
+  MVT XLenVT = Subtarget.getXLenVT();
 
   if (isPositionIndependent() || Subtarget.is64Bit())
     report_fatal_error("Unable to lowerGlobalAddress");
-
-  SDValue GAHi =
-    DAG.getTargetGlobalAddress(GV, DL, Ty, Offset, RISCVII::MO_HI);
-  SDValue GALo =
-    DAG.getTargetGlobalAddress(GV, DL, Ty, Offset, RISCVII::MO_LO);
+  // In order to maximise the opportunity for common subexpression elimination,
+  // emit a separate ADD node for the global address offset instead of folding
+  // it in the global address node. Later peephole optimisations may choose to
+  // fold it back in when profitable.
+  SDValue GAHi = DAG.getTargetGlobalAddress(GV, DL, Ty, 0, RISCVII::MO_HI);
+  SDValue GALo = DAG.getTargetGlobalAddress(GV, DL, Ty, 0, RISCVII::MO_LO);
   SDValue MNHi = SDValue(DAG.getMachineNode(RISCV::LUI, DL, Ty, GAHi), 0);
   SDValue MNLo =
     SDValue(DAG.getMachineNode(RISCV::ADDI, DL, Ty, MNHi, GALo), 0);
+  if (Offset != 0)
+    return DAG.getNode(ISD::ADD, DL, Ty, MNLo,
+                       DAG.getConstant(Offset, DL, XLenVT));
   return MNLo;
 }