[llvm] [RISCV][GISel] Select G_FENCE. (PR #73184)

[Michael Maitland via llvm-commits]

================
@@ -1099,6 +1110,63 @@ bool RISCVInstructionSelector::selectFPCompare(MachineInstr &MI,
   return true;
 }
 
+void RISCVInstructionSelector::emitFence(AtomicOrdering FenceOrdering,
+                                         SyncScope::ID FenceSSID,
+                                         MachineIRBuilder &MIB) const {
+  if (STI.hasStdExtZtso()) {
+    // The only fence that needs an instruction is a sequentially-consistent
+    // cross-thread fence.
+    if (FenceOrdering == AtomicOrdering::SequentiallyConsistent &&
+        FenceSSID == SyncScope::System) {
+      // fence rw, rw
+      MIB.buildInstr(RISCV::FENCE, {}, {})
+          .addImm(RISCVFenceField::R | RISCVFenceField::W)
+          .addImm(RISCVFenceField::R | RISCVFenceField::W);
+      return;
+    }
+
+    // MEMBARRIER is a compiler barrier; it codegens to a no-op.
+    MIB.buildInstr(TargetOpcode::MEMBARRIER, {}, {});
+    return;
+  }
+
+  // singlethread fences only synchronize with signal handlers on the same
+  // thread and thus only need to preserve instruction order, not actually
+  // enforce memory ordering.
+  if (FenceSSID == SyncScope::SingleThread) {
+    MIB.buildInstr(TargetOpcode::MEMBARRIER, {}, {});
+    return;
+  }
+
+  // Refer to Table A.6 in the version 2.3 draft of the RISC-V Instruction Set
----------------
michaelmaitland wrote:

nit: Is 2.3 version a thing? I can only find [version 20191213](https://drive.google.com/file/d/1s0lZxUZaa7eV_O0_WsZzaurFLLww7ou5/view) as the only versioned pdf after 2.2. My google search was `riscv instruction set volume i "this is version 2.3"`.

https://github.com/llvm/llvm-project/pull/73184