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

[Jon Chesterfield via Phabricator via cfe-commits]

JonChesterfield added inline comments.


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Comment at: clang/lib/CodeGen/CGBuiltin.cpp:13651
+      llvm::getConstantStringInfo(Scope, scp);
+      SSID = getLLVMContext().getOrInsertSyncScopeID(scp);
+
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sameerds wrote:
> saiislam wrote:
> > sameerds wrote:
> > > This seems to be creating a new ID for any arbitrary string passed as sync scope. This should be validated against LLVMContext::getSyncScopeNames(). 
> > As the FE is not aware about specific target and implementation of sync scope for that target, getSyncScopeNames() here returns llvm'sdefault sync scopes, which only supports singlethreaded and system as valid scopes. Validity checking of memory scope string is being intentionally left for the later stages which deal with the generated IR.
> That's pretty strange. At this point, Clang should know what the target is, and it should have a chance to update the list of sync scopes somewhere. @arsenm, do you see a way around this?
There is already sufficient IR level checking for the string at the instruction level. Warning in clang as well could be a nicer user experience, but that seems low priority to me.


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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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sameerds wrote:
> JonChesterfield wrote:
> > saiislam wrote:
> > > sameerds wrote:
> > > > 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
> > > As you pointed out, the range of acquire to sequentiallly consistent memory orders for llvm::AtomicOrdering is [4, 7], while for llvm::AtomicOrderingCABI is [2, 5]. Enums of C ABI was taken to ensure easy of use for the users who are familiar with C/C++ standard memory model. It allows them to use macros like __ATOMIC_ACQUIRE etc.
> > > Clang CodeGen of the builtin internally maps C ABI ordering to llvm atomic ordering.
> > What language, implemented in clang, do you have in mind that reusing the existing __ATOMIC_* macros would be incorrect for?
> I think we agreed that this builtin exposes the LLVM fence exactly. That would mean it takes arguments defined by LLVM. If you are implementing something different from that, then it first needs to be specified properly. Perhaps you could say that this is a C ABI compatible builtin, that happens to take target specific scopes? That should cover OpenCL whose scope enum is designed to be compatible with C.
> 
> Whatever it is that you are trying to implement here, it definitely does not expose a raw LLVM fence.
The llvm fence, in text form, uses a symbol for the memory scope. Not an enum.

This symbol is set using these macros for the existing atomic builtins. Using an implementation detail of clang instead is strictly worse, by layering and by precedent.

ABI is not involved here. Nor is OpenCl.


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

https://reviews.llvm.org/D75917