[PATCH] D75917: Expose llvm fence instruction as clang intrinsic

[Sameer Sahasrabuddhe via Phabricator via cfe-commits]

sameerds requested changes to this revision.
sameerds added inline comments.
This revision now requires changes to proceed.


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Comment at: clang/include/clang/Basic/Builtins.def:1583
+// Second argument : target specific sync scope string
+BUILTIN(__builtin_memory_fence, "vUicC*", "n")
+
----------------
This should be moved to be near line 786,  where atomic builtins are listed under the comment "// GCC does not support these, they are a Clang extension."


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Comment at: clang/lib/CodeGen/CGBuiltin.cpp:13630
+
+      // Map C11/C++11 memory ordering to LLVM memory ordering
+      switch (static_cast<llvm::AtomicOrderingCABI>(ord)) {
----------------
There should no mention of any high-level language here. The correct enum to validate against is llvm::AtomicOrdering from llvm/Support/AtomicOrdering.h, and not the C ABI or any other language ABI.


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Comment at: clang/lib/CodeGen/CGBuiltin.cpp:13651
+      llvm::getConstantStringInfo(Scope, scp);
+      SSID = getLLVMContext().getOrInsertSyncScopeID(scp);
+
----------------
This seems to be creating a new ID for any arbitrary string passed as sync scope. This should be validated against LLVMContext::getSyncScopeNames(). 


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Comment at: clang/test/CodeGenHIP/builtin_memory_fence.cpp:5
+
+void test_memory_fence_success() {
+// CHECK-LABEL: test_memory_fence_success
----------------
There should be a line that tries to do:
  __builtin_memory_fence(__ATOMIC_SEQ_CST, "foobar");


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Comment at: clang/test/CodeGenHIP/builtin_memory_fence.cpp:9
+  // CHECK: fence syncscope("workgroup") seq_cst
+  __builtin_memory_fence(__ATOMIC_SEQ_CST,  "workgroup");
+  
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Orderings like `__ATOMIC_SEQ_CST` are defined for C/C++ memory models. They should not be used with the new builtin because this new builtin does not follow any specific language model. For user convenience, the right thing to do is to introduce new tokens in the Clang preprocessor, similar to the `__ATOMIC_*` tokens. The convenient shortcut is to just tell the user to supply numerical values by looking at the LLVM source code.

>From llvm/Support/AtomicOrdering.h, note how the numerical value for `__ATOMIC_SEQ_CST` is 5, but the numerical value for the LLVM SequentiallyConsistent ordering is 7. The numerical value 5 refers to the LLVM ordering "release". So, if the implementation were correct, this line should result in the following unexpected LLVM IR:
  fence syncscope("workgroup") release


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D75917/new/

https://reviews.llvm.org/D75917