[BPF] preserve btf_decl_tag for parameters of extern functions

Generate DILocalVariable entries for parameters of extern functions,
the "annotations" field of DILocalVariable is used to link
"btf_decl_tag" annotation with the parameter.

Do this only for BPF backend as there are no other users for this
information. Final DWARF is valid as "Appendix A" is very much lax in
what is allowed as attributes for "DW_TAG_formal_parameter":

    DWARF does not in general require that a given debugging information
    entry contain a particular attribute or set of attributes. Instead,
    a DWARF producer is free to generate any, all, or none of the
    attributes ... other attributes ... may also appear in a given
    debugging information entry.

DWARF Debugging Information Format Version 5,
Appendix A: Attributes by Tag Value (Informative)
Page 251, Line 3.

Differential Revision: https://reviews.llvm.org/D140970

Author: eddyz87

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -4206,10 +4206,28 @@ void CGDebugInfo::EmitFunctionDecl(GlobalDecl GD, SourceLocation Loc,
     SPFlags |= llvm::DISubprogram::SPFlagOptimized;
 
   llvm::DINodeArray Annotations = CollectBTFDeclTagAnnotations(D);
-  llvm::DISubprogram *SP = DBuilder.createFunction(
-      FDContext, Name, LinkageName, Unit, LineNo,
-      getOrCreateFunctionType(D, FnType, Unit), ScopeLine, Flags, SPFlags,
-      TParamsArray.get(), getFunctionDeclaration(D), nullptr, Annotations);
+  llvm::DISubroutineType *STy = getOrCreateFunctionType(D, FnType, Unit);
+  llvm::DISubprogram *SP =
+      DBuilder.createFunction(FDContext, Name, LinkageName, Unit, LineNo, STy,
+                              ScopeLine, Flags, SPFlags, TParamsArray.get(),
+                              getFunctionDeclaration(D), nullptr, Annotations);
+
+  // Preserve btf_decl_tag attributes for parameters of extern functions
+  // for BPF target. The parameters created in this loop are attached as
+  // DISubprogram's retainedNodes in the subsequent finalizeSubprogram call.
+  if (IsDeclForCallSite && CGM.getTarget().getTriple().isBPF()) {
+    if (auto *FD = dyn_cast<FunctionDecl>(D)) {
+      llvm::DITypeRefArray ParamTypes = STy->getTypeArray();
+      unsigned ArgNo = 1;
+      for (ParmVarDecl *PD : FD->parameters()) {
+        llvm::DINodeArray ParamAnnotations = CollectBTFDeclTagAnnotations(PD);
+        DBuilder.createParameterVariable(
+            SP, PD->getName(), ArgNo, Unit, LineNo, ParamTypes[ArgNo], true,
+            llvm::DINode::FlagZero, ParamAnnotations);
+        ++ArgNo;
+      }
+    }
+  }
 
   if (IsDeclForCallSite)
     Fn->setSubprogram(SP);

clang/test/CodeGen/bpf-decl-tag-extern-func-args.c

@@ -0,0 +1,38 @@
+// REQUIRES: bpf-registered-target
+// RUN: %clang -target bpf -S -emit-llvm -g -O2 -o - %s  | FileCheck %s
+
+#define __decl_tag(x) __attribute__((btf_decl_tag(x)))
+
+extern void foo(int aa __decl_tag("aa_tag"), long bb __decl_tag("bb_tag"));
+extern void bar(char cc __decl_tag("cc_tag"));
+extern void buz(int __decl_tag("buz_arg_tag"));
+
+void root(void) {
+  foo(0, 1);
+  bar(0);
+  buz(0);
+}
+// CHECK: [[foo:![0-9]+]] = !DISubprogram(name: "foo", {{.*}}, retainedNodes: [[foo_nodes:![0-9]+]])
+// CHECK: [[foo_nodes]] = !{[[aa:![0-9]+]], [[bb:![0-9]+]]}
+
+// CHECK: [[aa]] = !DILocalVariable(name: "aa", arg: 1, scope: [[foo]], {{.*}}, annotations: [[aa_annot:![0-9]+]])
+// CHECK: [[aa_annot]] = !{[[aa_tag:![0-9]+]]}
+// CHECK: [[aa_tag]] = !{!"btf_decl_tag", !"aa_tag"}
+
+// CHECK: [[bb]] = !DILocalVariable(name: "bb", arg: 2, scope: [[foo]], {{.*}}, annotations: [[bb_annot:![0-9]+]])
+// CHECK: [[bb_annot]] = !{[[bb_tag:![0-9]+]]}
+// CHECK: [[bb_tag]] = !{!"btf_decl_tag", !"bb_tag"}
+
+// CHECK: [[bar:![0-9]+]] = !DISubprogram(name: "bar", {{.*}}, retainedNodes: [[bar_nodes:![0-9]+]])
+// CHECK: [[bar_nodes]] = !{[[cc:![0-9]+]]}
+
+// CHECK: [[cc]] = !DILocalVariable(name: "cc", arg: 1, scope: [[bar]], {{.*}}, annotations: [[cc_annot:![0-9]+]])
+// CHECK: [[cc_annot]] = !{[[cc_tag:![0-9]+]]}
+// CHECK: [[cc_tag]] = !{!"btf_decl_tag", !"cc_tag"}
+
+// CHECK: [[buz:![0-9]+]] = !DISubprogram(name: "buz", {{.*}}, retainedNodes: [[buz_nodes:![0-9]+]])
+// CHECK: [[buz_nodes]] = !{[[buz_arg:![0-9]+]]}
+
+// CHECK: [[buz_arg]] = !DILocalVariable(arg: 1, scope: [[buz]], {{.*}}, annotations: [[buz_arg_annot:![0-9]+]])
+// CHECK: [[buz_arg_annot]] = !{[[buz_arg_tag:![0-9]+]]}
+// CHECK: [[buz_arg_tag]] = !{!"btf_decl_tag", !"buz_arg_tag"}