[WebAssembly] Add atomic.fence instruction

Summary:
This adds `atomic.fence` instruction:
https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md#fence-operator

And we now emit the new `atomic.fence` instruction for multithread
fences, rather than the prevous `atomic.rmw` hack.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, jfb, tlively, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66794

llvm-svn: 370272

2 files changed, 36 insertions, 85 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
index 4de002ce487..b0c03c13fe6 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
@@ -88,88 +88,36 @@ void WebAssemblyDAGToDAGISel::Select(SDNode *Node) {
 
     uint64_t SyncScopeID =
         cast<ConstantSDNode>(Node->getOperand(2).getNode())->getZExtValue();
+    MachineSDNode *Fence = nullptr;
     switch (SyncScopeID) {
-    case SyncScope::SingleThread: {
+    case SyncScope::SingleThread:
       // We lower a single-thread fence to a pseudo compiler barrier instruction
       // preventing instruction reordering. This will not be emitted in final
       // binary.
-      MachineSDNode *Fence =
-          CurDAG->getMachineNode(WebAssembly::COMPILER_FENCE,
-                                 DL,                 // debug loc
-                                 MVT::Other,         // outchain type
-                                 Node->getOperand(0) // inchain
-          );
-      ReplaceNode(Node, Fence);
-      CurDAG->RemoveDeadNode(Node);
-      return;
-    }
-
-    case SyncScope::System: {
-      // For non-emscripten systems, we have not decided on what we should
-      // traslate fences to yet.
-      if (!Subtarget->getTargetTriple().isOSEmscripten())
-        report_fatal_error(
-            "ATOMIC_FENCE is not yet supported in non-emscripten OSes");
-
-      // Wasm does not have a fence instruction, but because all atomic
-      // instructions in wasm are sequentially consistent, we translate a
-      // fence to an idempotent atomic RMW instruction to a linear memory
-      // address. All atomic instructions in wasm are sequentially consistent,
-      // but this is to ensure a fence also prevents reordering of non-atomic
-      // instructions in the VM. Even though LLVM IR's fence instruction does
-      // not say anything about its relationship with non-atomic instructions,
-      // we think this is more user-friendly.
-      //
-      // While any address can work, here we use a value stored in
-      // __stack_pointer wasm global because there's high chance that area is
-      // in cache.
-      //
-      // So the selected instructions will be in the form of:
-      //   %addr = get_global $__stack_pointer
-      //   %0 = i32.const 0
-      //   i32.atomic.rmw.or %addr, %0
-      SDValue StackPtrSym = CurDAG->getTargetExternalSymbol(
-          "__stack_pointer", TLI->getPointerTy(CurDAG->getDataLayout()));
-      MachineSDNode *GetGlobal =
-          CurDAG->getMachineNode(WebAssembly::GLOBAL_GET_I32, // opcode
-                                 DL,                          // debug loc
-                                 MVT::i32,                    // result type
-                                 StackPtrSym // __stack_pointer symbol
-          );
-
-      SDValue Zero = CurDAG->getTargetConstant(0, DL, MVT::i32);
-      auto *MMO = MF.getMachineMemOperand(
-          MachinePointerInfo::getUnknownStack(MF),
-          // FIXME Volatile isn't really correct, but currently all LLVM
-          // atomic instructions are treated as volatiles in the backend, so
-          // we should be consistent.
-          MachineMemOperand::MOVolatile | MachineMemOperand::MOLoad |
-              MachineMemOperand::MOStore,
-          4, 4, AAMDNodes(), nullptr, SyncScope::System,
-          AtomicOrdering::SequentiallyConsistent);
-      MachineSDNode *Const0 =
-          CurDAG->getMachineNode(WebAssembly::CONST_I32, DL, MVT::i32, Zero);
-      MachineSDNode *AtomicRMW = CurDAG->getMachineNode(
-          WebAssembly::ATOMIC_RMW_OR_I32, // opcode
-          DL,                             // debug loc
-          MVT::i32,                       // result type
-          MVT::Other,                     // outchain type
-          {
-              Zero,                  // alignment
-              Zero,                  // offset
-              SDValue(GetGlobal, 0), // __stack_pointer
-              SDValue(Const0, 0),    // OR with 0 to make it idempotent
-              Node->getOperand(0)    // inchain
-          });
-
-      CurDAG->setNodeMemRefs(AtomicRMW, {MMO});
-      ReplaceUses(SDValue(Node, 0), SDValue(AtomicRMW, 1));
-      CurDAG->RemoveDeadNode(Node);
-      return;
-    }
+      Fence = CurDAG->getMachineNode(WebAssembly::COMPILER_FENCE,
+                                     DL,                 // debug loc
+                                     MVT::Other,         // outchain type
+                                     Node->getOperand(0) // inchain
+      );
+      break;
+    case SyncScope::System:
+      // Currently wasm only supports sequentially consistent atomics, so we
+      // always set the order to 0 (sequentially consistent).
+      Fence = CurDAG->getMachineNode(
+          WebAssembly::ATOMIC_FENCE,
+          DL,                                         // debug loc
+          MVT::Other,                                 // outchain type
+          CurDAG->getTargetConstant(0, DL, MVT::i32), // order
+          Node->getOperand(0)                         // inchain
+      );
+      break;
     default:
       llvm_unreachable("Unknown scope!");
     }
+
+    ReplaceNode(Node, Fence);
+    CurDAG->RemoveDeadNode(Node);
+    return;
   }
 
   case ISD::GlobalTLSAddress: {
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyInstrAtomics.td b/llvm/lib/Target/WebAssembly/WebAssemblyInstrAtomics.td
index e85aa57efc4..fecb895e0d7 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyInstrAtomics.td
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyInstrAtomics.td
@@ -127,6 +127,19 @@ def : WaitPatGlobalAddrOffOnly<i64, int_wasm_atomic_wait_i64, ATOMIC_WAIT_I64>;
 } // Predicates = [HasAtomics]
 
 //===----------------------------------------------------------------------===//
+// Atomic fences
+//===----------------------------------------------------------------------===//
+
+// A compiler fence instruction that prevents reordering of instructions.
+let Defs = [ARGUMENTS] in {
+let isPseudo = 1, hasSideEffects = 1 in
+defm COMPILER_FENCE : ATOMIC_NRI<(outs), (ins), [], "compiler_fence">;
+let hasSideEffects = 1 in
+defm ATOMIC_FENCE : ATOMIC_NRI<(outs), (ins i8imm:$flags), [], "atomic.fence",
+                               0x03>;
+} // Defs = [ARGUMENTS]
+
+//===----------------------------------------------------------------------===//
 // Atomic loads
 //===----------------------------------------------------------------------===//
 
@@ -887,13 +900,3 @@ defm : TerRMWTruncExtPattern<
   ATOMIC_RMW8_U_CMPXCHG_I32, ATOMIC_RMW16_U_CMPXCHG_I32,
   ATOMIC_RMW8_U_CMPXCHG_I64, ATOMIC_RMW16_U_CMPXCHG_I64,
   ATOMIC_RMW32_U_CMPXCHG_I64>;
-
-//===----------------------------------------------------------------------===//
-// Atomic fences
-//===----------------------------------------------------------------------===//
-
-// A compiler fence instruction that prevents reordering of instructions.
-let Defs = [ARGUMENTS] in {
-let isPseudo = 1, hasSideEffects = 1 in
-defm COMPILER_FENCE : ATOMIC_NRI<(outs), (ins), [], "compiler_fence">;
-} // Defs = [ARGUMENTS]