PPCDAGToDAGISel::PostprocessISelDAG()
This patch implements the PPCDAGToDAGISel::PostprocessISelDAG virtual
method to perform post-selection peephole optimizations on the DAG
representation.
One optimization is implemented here: folds to clean up complex
addressing expressions for thread-local storage and medium code
model. It will also be useful for large code model sequences when
those are added later. I originally thought about doing this on the
MI representation prior to register assignment, but it's difficult to
do effective global dead code elimination at that point. DCE is
trivial on the DAG representation.
A typical example of a candidate code sequence in assembly:
addis 3, 2, globalvar@toc@ha
addi 3, 3, globalvar@toc@l
lwz 5, 0(3)
When the final instruction is a load or store with an immediate offset
of zero, the offset from the add-immediate can replace the zero,
provided the relocation information is carried along:
addis 3, 2, globalvar@toc@ha
lwz 5, globalvar@toc@l(3)
Since the addi can in general have multiple uses, we need to only
delete the instruction when the last use is removed.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175697 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/PowerPC/PPCISelDAGToDAG.cpp b/lib/Target/PowerPC/PPCISelDAGToDAG.cpp
index 1453506..8663dd4 100644
--- a/lib/Target/PowerPC/PPCISelDAGToDAG.cpp
+++ b/lib/Target/PowerPC/PPCISelDAGToDAG.cpp
@@ -67,6 +67,8 @@
return true;
}
+ virtual void PostprocessISelDAG();
+
/// getI32Imm - Return a target constant with the specified value, of type
/// i32.
inline SDValue getI32Imm(unsigned Imm) {
@@ -1398,6 +1400,159 @@
return SelectCode(N);
}
+/// PostProcessISelDAG - Perform some late peephole optimizations
+/// on the DAG representation.
+void PPCDAGToDAGISel::PostprocessISelDAG() {
+
+ // Skip peepholes at -O0.
+ if (TM.getOptLevel() == CodeGenOpt::None)
+ return;
+
+ // These optimizations are currently supported only for 64-bit SVR4.
+ if (PPCSubTarget.isDarwin() || !PPCSubTarget.isPPC64())
+ return;
+
+ SelectionDAG::allnodes_iterator Position(CurDAG->getRoot().getNode());
+ ++Position;
+
+ while (Position != CurDAG->allnodes_begin()) {
+ SDNode *N = --Position;
+ // Skip dead nodes and any non-machine opcodes.
+ if (N->use_empty() || !N->isMachineOpcode())
+ continue;
+
+ unsigned FirstOp;
+ unsigned StorageOpcode = N->getMachineOpcode();
+
+ switch (StorageOpcode) {
+ default: continue;
+
+ case PPC::LBZ:
+ case PPC::LBZ8:
+ case PPC::LD:
+ case PPC::LFD:
+ case PPC::LFS:
+ case PPC::LHA:
+ case PPC::LHA8:
+ case PPC::LHZ:
+ case PPC::LHZ8:
+ case PPC::LWA:
+ case PPC::LWZ:
+ case PPC::LWZ8:
+ FirstOp = 0;
+ break;
+
+ case PPC::STB:
+ case PPC::STB8:
+ case PPC::STD:
+ case PPC::STFD:
+ case PPC::STFS:
+ case PPC::STH:
+ case PPC::STH8:
+ case PPC::STW:
+ case PPC::STW8:
+ FirstOp = 1;
+ break;
+ }
+
+ // If this is a load or store with a zero offset, we may be able to
+ // fold an add-immediate into the memory operation.
+ if (!isa<ConstantSDNode>(N->getOperand(FirstOp)) ||
+ N->getConstantOperandVal(FirstOp) != 0)
+ continue;
+
+ SDValue Base = N->getOperand(FirstOp + 1);
+ if (!Base.isMachineOpcode())
+ continue;
+
+ unsigned Flags = 0;
+ bool ReplaceFlags = true;
+
+ // When the feeding operation is an add-immediate of some sort,
+ // determine whether we need to add relocation information to the
+ // target flags on the immediate operand when we fold it into the
+ // load instruction.
+ //
+ // For something like ADDItocL, the relocation information is
+ // inferred from the opcode; when we process it in the AsmPrinter,
+ // we add the necessary relocation there. A load, though, can receive
+ // relocation from various flavors of ADDIxxx, so we need to carry
+ // the relocation information in the target flags.
+ switch (Base.getMachineOpcode()) {
+ default: continue;
+
+ case PPC::ADDI8:
+ case PPC::ADDI8L:
+ case PPC::ADDIL:
+ // In some cases (such as TLS) the relocation information
+ // is already in place on the operand, so copying the operand
+ // is sufficient.
+ ReplaceFlags = false;
+ // For these cases, the immediate may not be divisible by 4, in
+ // which case the fold is illegal for DS-form instructions. (The
+ // other cases provide aligned addresses and are always safe.)
+ if ((StorageOpcode == PPC::LWA ||
+ StorageOpcode == PPC::LD ||
+ StorageOpcode == PPC::STD) &&
+ (!isa<ConstantSDNode>(Base.getOperand(1)) ||
+ Base.getConstantOperandVal(1) % 4 != 0))
+ continue;
+ break;
+ case PPC::ADDIdtprelL:
+ Flags = PPCII::MO_DTPREL16_LO;
+ break;
+ case PPC::ADDItlsldL:
+ Flags = PPCII::MO_TLSLD16_LO;
+ break;
+ case PPC::ADDItocL:
+ Flags = PPCII::MO_TOC16_LO;
+ break;
+ }
+
+ // We found an opportunity. Reverse the operands from the add
+ // immediate and substitute them into the load or store. If
+ // needed, update the target flags for the immediate operand to
+ // reflect the necessary relocation information.
+ DEBUG(dbgs() << "Folding add-immediate into mem-op:\nBase: ");
+ DEBUG(Base->dump(CurDAG));
+ DEBUG(dbgs() << "\nN: ");
+ DEBUG(N->dump(CurDAG));
+ DEBUG(dbgs() << "\n");
+
+ SDValue ImmOpnd = Base.getOperand(1);
+
+ // If the relocation information isn't already present on the
+ // immediate operand, add it now.
+ if (ReplaceFlags) {
+ GlobalAddressSDNode *GA = dyn_cast<GlobalAddressSDNode>(ImmOpnd);
+
+ if (GA) {
+ DebugLoc dl = GA->getDebugLoc();
+ const GlobalValue *GV = GA->getGlobal();
+ ImmOpnd = CurDAG->getTargetGlobalAddress(GV, dl, MVT::i64, 0, Flags);
+ } else {
+ ConstantPoolSDNode *CP = dyn_cast<ConstantPoolSDNode>(ImmOpnd);
+ if (CP) {
+ const Constant *C = CP->getConstVal();
+ ImmOpnd = CurDAG->getTargetConstantPool(C, MVT::i64,
+ CP->getAlignment(),
+ 0, Flags);
+ }
+ }
+ }
+
+ if (FirstOp == 1) // Store
+ (void)CurDAG->UpdateNodeOperands(N, N->getOperand(0), ImmOpnd,
+ Base.getOperand(0), N->getOperand(3));
+ else // Load
+ (void)CurDAG->UpdateNodeOperands(N, ImmOpnd, Base.getOperand(0),
+ N->getOperand(2));
+
+ // The add-immediate may now be dead, in which case remove it.
+ if (Base.getNode()->use_empty())
+ CurDAG->RemoveDeadNode(Base.getNode());
+ }
+}
/// createPPCISelDag - This pass converts a legalized DAG into a