[clang] [llvm] [BPF] Fix linking issues in static map initializers (PR #91310)

[Nick Zavaritsky via cfe-commits]

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@@ -1950,8 +1950,22 @@ ConstantLValueEmitter::tryEmitBase(const APValue::LValueBase &base) {
     if (D->hasAttr<WeakRefAttr>())
       return CGM.GetWeakRefReference(D).getPointer();
 
-    if (auto FD = dyn_cast<FunctionDecl>(D))
-      return CGM.GetAddrOfFunction(FD);
+    if (auto FD = dyn_cast<FunctionDecl>(D)) {
+      auto *C = CGM.GetAddrOfFunction(FD);
+
+      // we don't normally emit debug info for extern fns referenced via
+      // variable initialisers; BPF needs it since it generates BTF from
+      // debug info and bpftool demands BTF for every symbol linked
+      if (CGM.getTarget().getTriple().isBPF() && FD->getStorageClass() == SC_Extern) {
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mejedi wrote:

@efriedma-quic Thank you for insightful comments! As a newbie, I need more guidance.

It is pretty much clear that `allowDebugInfoForExternalRef` is a preferred way to check whether we should emit additional debug info. It is currently only overriden by BPF target and is being used for exact same purpose we have here.

I need help getting up to speed irt. `CompleteExternalDeclaration` proposal. Having grepped through the code, I can see that BPF faced the same challenge with `extern` variables. At the moment, there's a vector of `VariableDecl`s populated in `SemaDecl.cpp` and later used to attach debug info to variables.

A different mechanism is applied to discovering `extern` functions in need for debug info (prior to my patch, `extern` functions referenced through code were handled). This is accomplished by code in `CGExpr.cpp`. I am proposing a similar extension in `CGExprConstant.cpp`.

Would you like to have functions on `ExternalDeclarations` list instead?

What would be a good spot to get ahold of `FunctionDecl` to put it on `ExternalDeclarations` list?

https://github.com/llvm/llvm-project/pull/91310