Treat __sync_synchronize and asm("":::"memory") as stronger fences.

lib/CodeGen/CGBuiltin.cpp

Index: lib/CodeGen/CGBuiltin.cpp
diff --git a/lib/CodeGen/CGBuiltin.cpp b/lib/CodeGen/CGBuiltin.cpp
index a655966d6e32e34423c096f59efd96da9b942cf8..b2decffefafe37d465c870bc75015f00c98cf8a3 100644
--- a/lib/CodeGen/CGBuiltin.cpp
+++ b/lib/CodeGen/CGBuiltin.cpp
@@ -20,6 +20,7 @@
 #include "clang/Basic/TargetBuiltins.h"
 #include "clang/Basic/TargetInfo.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/InlineAsm.h" // @LOCALMOD
 #include "llvm/IR/Intrinsics.h"
 
 using namespace clang;
@@ -1034,7 +1035,27 @@ RValue CodeGenFunction::EmitBuiltinExpr(const FunctionDecl *FD,
     // any way to safely use it... but in practice, it mostly works
     // to use it with non-atomic loads and stores to get acquire/release
     // semantics.
+    // @LOCALMOD-START
+    // For PNaCl, surround __sync_synchronize with compiler fences. This
+    // should enforce ordering of more than just atomic memory accesses,
+    // though it won't guarantee that all accesses (e.g. those to
+    // non-escaping objects) will not be reordered.
+    llvm::FunctionType *FTy = llvm::FunctionType::get(VoidTy, false);
+    std::string AsmString;  // Empty.
+    std::string Constraints("~{memory}");
+    bool HasSideEffect = true;
+    if (getTarget().getTriple().getArch() == llvm::Triple::le32)

[eliben, 2013/08/06 16:26:56]

It would be cleaner to have a single LOCAMOD if-branch, leaving the current code
unmodified in the "else". That's easier to read (clearer what the LOCALMOD
applies to and what was there before), and easier to merge later.

[JF, 2013/08/07 17:58:28]

Done, and as suggested offline the code now uses TargetInfo/getTargetHooks.

+      Builder.CreateCall(
+          llvm::InlineAsm::get(FTy, AsmString, Constraints, HasSideEffect))->
+          addAttribute(llvm::AttributeSet::FunctionIndex,
+                       llvm::Attribute::NoUnwind);
     Builder.CreateFence(llvm::SequentiallyConsistent);
+    if (getTarget().getTriple().getArch() == llvm::Triple::le32)
+      Builder.CreateCall(
+          llvm::InlineAsm::get(FTy, AsmString, Constraints, HasSideEffect))->
+          addAttribute(llvm::AttributeSet::FunctionIndex,
+                       llvm::Attribute::NoUnwind);
+    // @LOCALMOD-END
     return RValue::get(0);
   }