[RISCV][ISEL] Lowering to load-acquire/store-release for RISCV. (The Zalasr extension)

Currently uses the psABI lowerings for WMO load-acquire/store-release (which are identical to A.7).
These are incompatable with the A.6 lowerings currently used by LLVM.
This should be OK for now since Zalasr is behind the enable experimental extensions flag, but needs to be fixed before it is removed from that.

For TSO, it uses the standard Ztso mappings except for lowering seq_cst loads/store to load-acquire/store-release, I had Andrea review that.

Author: Brendan Sweeney

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -20832,6 +20832,32 @@ unsigned RISCVTargetLowering::getCustomCtpopCost(EVT VT,
   return isCtpopFast(VT) ? 0 : 1;
 }
 
+bool RISCVTargetLowering::shouldInsertFencesForAtomic(
+    const Instruction *I) const {
+  if (Subtarget.hasStdExtZalasr()) {
+    if (Subtarget.hasStdExtZtso()) {
+      // Zalasr + TSO means that atomic_load_acquire and atomic_store_release
+      //  should be lowered to plain load/store. The easiest way to do this is
+      //  to say we should insert fences for them, and the fence insertion code
+      //  will just not insert any fences
+      auto LI = dyn_cast<LoadInst>(I);
+      auto SI = dyn_cast<StoreInst>(I);
+      if ((LI &&
+           (LI->getOrdering() == AtomicOrdering::SequentiallyConsistent)) ||
+          (SI &&
+           (SI->getOrdering() == AtomicOrdering::SequentiallyConsistent))) {
+        // Here, this is a load or store which is seq_cst, and needs a .aq or
+        //  .rl therefore we shouldn't try to insert fences
+        return false;
+      }
+      // Here, we are a TSO inst that isn't a seq_cst load/store
+      return isa<LoadInst>(I) || isa<StoreInst>(I);
+    }
+    return false;
+  }
+  return isa<LoadInst>(I) || isa<StoreInst>(I);
+}
+
 bool RISCVTargetLowering::fallBackToDAGISel(const Instruction &Inst) const {
 
   // GISel support is in progress or complete for these opcodes.

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -650,9 +650,8 @@ class RISCVTargetLowering : public TargetLowering {
 
   bool preferZeroCompareBranch() const override { return true; }
 
-  bool shouldInsertFencesForAtomic(const Instruction *I) const override {
-    return isa<LoadInst>(I) || isa<StoreInst>(I);
-  }
+  bool shouldInsertFencesForAtomic(const Instruction *I) const override;
+
   Instruction *emitLeadingFence(IRBuilderBase &Builder, Instruction *Inst,
                                 AtomicOrdering Ord) const override;
   Instruction *emitTrailingFence(IRBuilderBase &Builder, Instruction *Inst,

llvm/lib/Target/RISCV/RISCVInstrInfoA.td

@@ -105,22 +105,65 @@ defm AMOMAXU_D  : AMO_rr_aq_rl<0b11100, 0b011, "amomaxu.d">,
 // Pseudo-instructions and codegen patterns
 //===----------------------------------------------------------------------===//
 
+let IsAtomic = 1 in {
+// An atomic load operation that does not need either acquire or release
+// semantics.
+class relaxed_load<PatFrags base>
+    : PatFrag<(ops node:$ptr), (base node:$ptr)> {
+  let IsAtomicOrderingAcquireOrStronger = 0;
+}
+
+// A atomic load operation that actually needs acquire semantics.
+class acquiring_load<PatFrags base>
+    : PatFrag<(ops node:$ptr), (base node:$ptr)> {
+  let IsAtomicOrderingAcquire = 1;
+}
+
+// An atomic load operation that needs sequential consistency.
+class seq_cst_load<PatFrags base>
+    : PatFrag<(ops node:$ptr), (base node:$ptr)> {
+  let IsAtomicOrderingSequentiallyConsistent = 1;
+}
+
+// An atomic store operation that does not need either acquire or release
+// semantics.
+class relaxed_store<PatFrag base>
+    : PatFrag<(ops node:$val, node:$ptr), (base node:$val, node:$ptr)> {
+  let IsAtomicOrderingReleaseOrStronger = 0;
+}
+
+// A store operation that actually needs release semantics.
+class releasing_store<PatFrag base>
+    : PatFrag<(ops node:$val, node:$ptr), (base node:$val, node:$ptr)> {
+  let IsAtomicOrderingRelease = 1;
+}
+
+// A store operation that actually needs sequential consistency.
+class seq_cst_store<PatFrag base>
+    : PatFrag<(ops node:$val, node:$ptr), (base node:$val, node:$ptr)> {
+  let IsAtomicOrderingSequentiallyConsistent = 1;
+}
+} // IsAtomic = 1
+
 // Atomic load/store are available under both +a and +force-atomics.
 // Fences will be inserted for atomic load/stores according to the logic in
 // RISCVTargetLowering::{emitLeadingFence,emitTrailingFence}.
+// The normal loads/store are relaxed (unordered) loads that don't have any
+// ordering. This is necessary because AtomicExpandPass has added fences to
+// atomic load/stores and changed them to unordered ones.
 let Predicates = [HasAtomicLdSt] in {
-  def : LdPat<atomic_load_8,  LB>;
-  def : LdPat<atomic_load_16, LH>;
-  def : LdPat<atomic_load_32, LW>;
+  def : LdPat<relaxed_load<atomic_load_8>,  LB>;
+  def : LdPat<relaxed_load<atomic_load_16>, LH>;
+  def : LdPat<relaxed_load<atomic_load_32>, LW>;
 
-  def : StPat<atomic_store_8,  SB, GPR, XLenVT>;
-  def : StPat<atomic_store_16, SH, GPR, XLenVT>;
-  def : StPat<atomic_store_32, SW, GPR, XLenVT>;
+  def : StPat<relaxed_store<atomic_store_8>,  SB, GPR, XLenVT>;
+  def : StPat<relaxed_store<atomic_store_16>, SH, GPR, XLenVT>;
+  def : StPat<relaxed_store<atomic_store_32>, SW, GPR, XLenVT>;
 }
 
 let Predicates = [HasAtomicLdSt, IsRV64] in {
-  def : LdPat<atomic_load_64, LD, i64>;
-  def : StPat<atomic_store_64, SD, GPR, i64>;
+  def : LdPat<relaxed_load<atomic_load_64>, LD, i64>;
+  def : StPat<relaxed_store<atomic_store_64>, SD, GPR, i64>;
 }
 
 /// AMOs
@@ -423,12 +466,12 @@ let Predicates = [HasStdExtA, IsRV64] in
 defm : PseudoCmpXchgPat<"atomic_cmp_swap_32", PseudoCmpXchg32, i32>;
 
 let Predicates = [HasAtomicLdSt] in {
-  def : LdPat<atomic_load_8,  LB, i32>;
-  def : LdPat<atomic_load_16, LH, i32>;
-  def : LdPat<atomic_load_32, LW, i32>;
+  def : LdPat<relaxed_load<atomic_load_8>,  LB, i32>;
+  def : LdPat<relaxed_load<atomic_load_16>, LH, i32>;
+  def : LdPat<relaxed_load<atomic_load_32>, LW, i32>;
 
-  def : StPat<atomic_store_8,  SB, GPR, i32>;
-  def : StPat<atomic_store_16, SH, GPR, i32>;
-  def : StPat<atomic_store_32, SW, GPR, i32>;
+  def : StPat<relaxed_store<atomic_store_8>,  SB, GPR, i32>;
+  def : StPat<relaxed_store<atomic_store_16>, SH, GPR, i32>;
+  def : StPat<relaxed_store<atomic_store_32>, SW, GPR, i32>;
 }
 

llvm/lib/Target/RISCV/RISCVInstrInfoZalasr.td

@@ -57,3 +57,50 @@ let Predicates = [HasStdExtZalasr, IsRV64] in {
 defm LD : LAQ_r_aq_rl<0b011, "ld">;
 defm SD : SRL_r_aq_rl<0b011, "sd">;
 } // Predicates = [HasStdExtZalasr, IsRV64]
+
+//===----------------------------------------------------------------------===//
+// Pseudo-instructions and codegen patterns
+//===----------------------------------------------------------------------===//
+
+class PatLAQ<SDPatternOperator OpNode, RVInst Inst, ValueType vt = XLenVT>
+    : Pat<(vt (OpNode (vt GPRMemZeroOffset:$rs1))), (Inst GPRMemZeroOffset:$rs1)>;
+
+// n.b. this switches order of arguments
+//  to deal with the fact that SRL has addr, data
+//  while atomic_store has data, addr
+class PatSRL<SDPatternOperator OpNode, RVInst Inst, ValueType vt = XLenVT>
+    : Pat<(OpNode (vt GPR:$rs2), (vt GPRMemZeroOffset:$rs1)),
+          (Inst GPRMemZeroOffset:$rs1, GPR:$rs2)>; 
+                              
+
+let Predicates = [HasStdExtZalasr] in {
+  // the sequentially consistent loads use
+  //  .aq instead of .aqrl to match the psABI/A.7
+  def : PatLAQ<acquiring_load<atomic_load_8>, LB_AQ>;
+  def : PatLAQ<seq_cst_load<atomic_load_8>, LB_AQ>; 
+
+  def : PatLAQ<acquiring_load<atomic_load_16>, LH_AQ>;
+  def : PatLAQ<seq_cst_load<atomic_load_16>, LH_AQ>;
+
+  def : PatLAQ<acquiring_load<atomic_load_32>, LW_AQ>;
+  def : PatLAQ<seq_cst_load<atomic_load_32>, LW_AQ>;
+
+  // the sequentially consistent stores use
+  //  .rl instead of .aqrl to match the psABI/A.7
+  def : PatSRL<releasing_store<atomic_store_8>, SB_RL>;
+  def : PatSRL<seq_cst_store<atomic_store_8>, SB_RL>; 
+
+  def : PatSRL<releasing_store<atomic_store_16>, SH_RL>;
+  def : PatSRL<seq_cst_store<atomic_store_16>, SH_RL>;
+
+  def : PatSRL<releasing_store<atomic_store_32>, SW_RL>;
+  def : PatSRL<seq_cst_store<atomic_store_32>, SW_RL>;
+} // Predicates = [HasStdExtZalasr]
+
+let Predicates = [HasStdExtZalasr, IsRV64] in {
+  def : PatLAQ<acquiring_load<atomic_load_64>, LD_AQ>;
+  def : PatLAQ<seq_cst_load<atomic_load_64>, LD_AQ>;
+
+  def : PatSRL<releasing_store<atomic_store_64>, SD_RL>;
+  def : PatSRL<seq_cst_store<atomic_store_64>, SD_RL>;
+} // Predicates = [HasStdExtZalasr, IsRV64]