[PATCH] [X86] replace (atomic fetch_add of 0) by (mfence; mov)

[David Majnemer]

================
Comment at: lib/Target/X86/X86ISelDAGToDAG.cpp:1777-1792
@@ +1776,18 @@
+  unsigned Opc;
+  switch(NVT.SimpleTy) {
+    case MVT::i8:
+      Opc = X86::ACQUIRE_MOV8rm;
+      break;
+    case MVT::i16:
+      Opc = X86::ACQUIRE_MOV16rm;
+      break;
+    case MVT::i32:
+      Opc = X86::ACQUIRE_MOV32rm;
+      break;
+    case MVT::i64:
+      Opc = X86::ACQUIRE_MOV64rm;
+      break;
+    default:
+      llvm_unreachable("Unexpected size for LXADD 0");
+  }
+
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http://reviews.llvm.org/D5091