[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
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 52c027b..d3d0d03 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -293,17 +293,22 @@
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;
}