[PATCH] D129694: [OPENMP] Make declare target static global externally visible

[Sunil Shrestha via Phabricator via cfe-commits]

ssquare08 added inline comments.


================
Comment at: clang/lib/CodeGen/TargetInfo.cpp:9439
     const Decl *D, llvm::GlobalValue *GV, CodeGen::CodeGenModule &M) const {
+  // Make sure any variable with OpenMP declare target is visible to runtime
+  // except for those with hidden visibility
----------------
jhuber6 wrote:
> jhuber6 wrote:
> > ssquare08 wrote:
> > > jhuber6 wrote:
> > > > Just spitballing, is it possible to do this when we make the global instead?
> > > This is something I was wondering as well.  In CodeGenModule::GetOrCreateLLVMGlobal, when it creates a new global variable, it always uses the llvm::GlobalValue::ExternalLinkage. Seems like this changes somewhere later to internal for static globals. Do you know where that would be?
> > I'm not exactly sure, I remember deleting some code in D117806 that did something like that, albeit incorrectly. But I'm not sure if you'd have the necessary information to check whether or not there are updates attached to it. We don't want to externalize things if we don't need to, otherwise we'd get a lot of our device runtime variables with external visibility that now can't be optimized out.
> Were you able to find a place for this when we generate the variable? You should be able to do something similar to the patch above if it's a declare target static to force it to have external visibility, but as mentioned before I would prefer we only do this if necessary which might take some extra analysis.
If you are asking about the GV, it is created in 'CodeGenModule::GetOrCreateLLVMGlobal' with external linkage always.
```
  auto *GV = new llvm::GlobalVariable(
      getModule(), Ty, false, llvm::GlobalValue::ExternalLinkage, nullptr,
      MangledName, nullptr, llvm::GlobalVariable::NotThreadLocal,
      getContext().getTargetAddressSpace(DAddrSpace));
```
The linkage, however, changes in 'CodeGenModule::EmitGlobalVarDefinition' based on the information VarDecl

```
  llvm::GlobalValue::LinkageTypes Linkage =
      getLLVMLinkageVarDefinition(D, GV->isConstant());
```

Maybe you are suggesting changing the linkage information in 'VarDecl' itself?


================
Comment at: clang/test/OpenMP/declare_target_visibility_codegen.cpp:11-12
 // HOST: @z = global i32 0
-// HOST-NOT: @.omp_offloading.entry.c
-// HOST-NOT: @.omp_offloading.entry.x
+// HOST: @.omp_offloading.entry.c__static__{{[0-9a-z]+_[0-9a-z]+}}
+// HOST: @.omp_offloading.entry.x__static__{{[0-9a-z]+_[0-9a-z]+}}
 // HOST-NOT: @.omp_offloading.entry.y
----------------
jhuber6 wrote:
> ssquare08 wrote:
> > ssquare08 wrote:
> > > jhuber6 wrote:
> > > > If there are no updates between the host and device we can keep these static without emitting an offloading entry.
> > > That 's a good point. I'll fix that.
> > I thought about this more and I think the behavior for these  declare target static globals should be the same as the other declare target. Checking for update is not enough because users could also map these variables. For update, it could be mapped with a pointer or the users could pass address of these variables to an external function. Please let me know what you think of these cases below:
> > ```
> > #pragma omp declare target
> > static int x[10];
> > #pragma omp end declare target
> > 
> > //case 1
> > #pragma omp target update to(x)
> > 
> > //case 2
> > int* y = &x[2];
> > #pragma omp target update to(y[0])
> > 
> > //case 3
> > #pragma omp target map(always to:x)
> > {
> >  x[0]= 111;
> > }
> > 
> > //case 4
> > #pragma omp target
> > { 
> >   foo(&x[3]);
> > }
> > ```
> We should still be able to do this if there are either no updates at all in the module, or if the declare type is `nohost`. Doing anything more complicated would require some optimizations between the host and device we can't do yet. I'm making this point because making these statics external is a performance regression so we should only do it when needed. To that end we may even want a flag that entirely disables this feature.
I'll add a check to see if there are any updates in the module.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D129694/new/

https://reviews.llvm.org/D129694