[RISCV] Add codegen support for atomic load/stores with RV32A

Fences are inserted according to table A.6 in the current draft of version 2.3
of the RISC-V Instruction Set Manual, which incorporates the memory model
changes and definitions contributed by the RISC-V Memory Consistency Model
task group.

Instruction selection failures will now occur for 8/16/32-bit atomicrmw and 
cmpxchg operations when targeting RV32IA until lowering for these operations 
is added in a follow-on patch.

Differential Revision: https://reviews.llvm.org/D47589

llvm-svn: 334591

4 files changed, 56 insertions, 2 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index b98f0575caa..dda26a73701 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -137,8 +137,10 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
   setOperationAction(ISD::BlockAddress, XLenVT, Custom);
   setOperationAction(ISD::ConstantPool, XLenVT, Custom);
 
-  // Atomic operations aren't suported in the base RV32I ISA.
-  setMaxAtomicSizeInBitsSupported(0);
+  if (Subtarget.hasStdExtA())
+    setMaxAtomicSizeInBitsSupported(Subtarget.getXLen());
+  else
+    setMaxAtomicSizeInBitsSupported(0);
 
   setBooleanContents(ZeroOrOneBooleanContent);
 
@@ -1553,3 +1555,21 @@ RISCVTargetLowering::getRegForInlineAsmConstraint(const TargetRegisterInfo *TRI,
 
   return TargetLowering::getRegForInlineAsmConstraint(TRI, Constraint, VT);
 }
+
+Instruction *RISCVTargetLowering::emitLeadingFence(IRBuilder<> &Builder,
+                                                   Instruction *Inst,
+                                                   AtomicOrdering Ord) const {
+  if (isa<LoadInst>(Inst) && Ord == AtomicOrdering::SequentiallyConsistent)
+    return Builder.CreateFence(Ord);
+  if (isa<StoreInst>(Inst) && isReleaseOrStronger(Ord))
+    return Builder.CreateFence(AtomicOrdering::Release);
+  return nullptr;
+}
+
+Instruction *RISCVTargetLowering::emitTrailingFence(IRBuilder<> &Builder,
+                                                    Instruction *Inst,
+                                                    AtomicOrdering Ord) const {
+  if (isa<LoadInst>(Inst) && isAcquireOrStronger(Ord))
+    return Builder.CreateFence(AtomicOrdering::Acquire);
+  return nullptr;
+}
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.h b/llvm/lib/Target/RISCV/RISCVISelLowering.h
index 52bbfa1e75b..41b9d2789a2 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.h
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.h
@@ -66,6 +66,14 @@ public:
   EVT getSetCCResultType(const DataLayout &DL, LLVMContext &Context,
                          EVT VT) const override;
 
+  bool shouldInsertFencesForAtomic(const Instruction *I) const override {
+    return isa<LoadInst>(I) || isa<StoreInst>(I);
+  }
+  Instruction *emitLeadingFence(IRBuilder<> &Builder, Instruction *Inst,
+                                AtomicOrdering Ord) const override;
+  Instruction *emitTrailingFence(IRBuilder<> &Builder, Instruction *Inst,
+                                 AtomicOrdering Ord) const override;
+
 private:
   void analyzeInputArgs(MachineFunction &MF, CCState &CCInfo,
                         const SmallVectorImpl<ISD::InputArg> &Ins,
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.td b/llvm/lib/Target/RISCV/RISCVInstrInfo.td
index 7dedc0ffcc1..71e058f1958 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.td
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.td
@@ -757,6 +757,12 @@ def : Pat<(atomic_fence (i32 6), (imm)), (FENCE_TSO)>;
 // fence seq_cst -> fence rw, rw
 def : Pat<(atomic_fence (i32 7), (imm)), (FENCE 0b11, 0b11)>;
 
+// Lowering for atomic load and store is defined in RISCVInstrInfoA.td.
+// Although these are lowered to fence+load/store instructions defined in the
+// base RV32I/RV64I ISA, this lowering is only used when the A extension is
+// present. This is necessary as it isn't valid to mix __atomic_* libcalls
+// with inline atomic operations for the same object.
+
 /// Other pseudo-instructions
 
 // Pessimistically assume the stack pointer will be clobbered
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfoA.td b/llvm/lib/Target/RISCV/RISCVInstrInfoA.td
index 33e863ba6a1..37932206043 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfoA.td
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfoA.td
@@ -75,3 +75,23 @@ defm AMOMAX_D   : AMO_rr_aq_rl<0b10100, 0b011, "amomax.d">;
 defm AMOMINU_D  : AMO_rr_aq_rl<0b11000, 0b011, "amominu.d">;
 defm AMOMAXU_D  : AMO_rr_aq_rl<0b11100, 0b011, "amomaxu.d">;
 } // Predicates = [HasStedExtA, IsRV64]
+
+//===----------------------------------------------------------------------===//
+// Pseudo-instructions and codegen patterns
+//===----------------------------------------------------------------------===//
+
+let Predicates = [HasStdExtA] in {
+
+/// Atomic loads and stores
+
+// Fences will be inserted for atomic load/stores according to the logic in
+// RISCVTargetLowering::{emitLeadingFence,emitTrailingFence}.
+
+defm : LdPat<atomic_load_8,  LB>;
+defm : LdPat<atomic_load_16, LH>;
+defm : LdPat<atomic_load_32, LW>;
+
+defm : StPat<atomic_store_8,  SB, GPR>;
+defm : StPat<atomic_store_16,  SH, GPR>;
+defm : StPat<atomic_store_32, SW, GPR>;
+} // Predicates = [HasStdExtF]