[clang][DebugInfo] Clear retained nodes list of a vararg trunk's DISubprogram

This fixes an issue reported in
#166855 (comment),
that had been revealed after #166855 was merged.

`CodeGenFunction::GenerateVarArgsThunk` creates thunks for vararg
functions by cloning and modifying the function. It is different
from `CodeGenFunction::generateThunk`, which is used for Itanium ABI.

According to https://reviews.llvm.org/D39396,
`CodeGenFunction::GenerateVarArgsThunk` may be called before metadata
nodes are resolved. So, it tries to avoid remapping DISubprogram and all
metadata nodes it references inside `CloneFunction()` by manually
cloning DISubprogram.

When optimization level is not OptNone, all local variables are saved in
DISubprogram's retainedNodes field. When `CodeGenFunction::GenerateVarArgsThunk`
clones such DISubprogram without remapping, it produces a subprogram
with incorrectly-scoped retained nodes. It triggers Verifier checks
added in #166855.

To solve that, retained nodes list of a cloned DISubprogram is cleared.

Author: dzhidzhoev

clang/lib/CodeGen/CGVTables.cpp

@@ -125,6 +125,9 @@ static void resolveTopLevelMetadata(llvm::Function *Fn,
   if (!DIS)
     return;
   auto *NewDIS = llvm::MDNode::replaceWithDistinct(DIS->clone());
+  // As DISubprogram remapping is avoided, clear retained nodes list of
+  // cloned DISubprogram from retained nodes local to original DISubprogram.
+  NewDIS->replaceRetainedNodes(llvm::MDTuple::get(Fn->getContext(), {}));
   VMap.MD()[DIS].reset(NewDIS);
 
   // Find all llvm.dbg.declare intrinsics and resolve the DILocalVariable nodes

clang/test/CodeGenCXX/tmp-md-nodes1.cpp

@@ -2,6 +2,14 @@
 // RUN: %clang_cc1 -O0 -triple %itanium_abi_triple -debug-info-kind=limited -emit-llvm %s -o - | \
 // RUN: FileCheck %s
 
+// Trigger GenerateVarArgsThunk.
+// RUN: %clang_cc1 -O0 -triple riscv64-linux-gnu -debug-info-kind=limited -emit-llvm %s -o - | \
+// RUN: FileCheck %s
+
+// Check that retainedNodes are properly maintained at function cloning.
+// RUN: %clang_cc1 -O1 -triple riscv64-linux-gnu -debug-info-kind=limited -emit-llvm %s -o - | \
+// RUN: FileCheck %s --check-prefixes=CHECK,CHECK-DI
+
 // This test simply checks that the varargs thunk is created. The failing test
 // case asserts.
 
@@ -16,3 +24,11 @@ struct CharlieImpl : Charlie, Alpha {
 } delta;
 
 // CHECK: define {{.*}} void @_ZThn{{[48]}}_N11CharlieImpl5bravoEz(
+
+// CHECK-DI: distinct !DISubprogram({{.*}}, linkageName: "_ZN11CharlieImpl5bravoEz", {{.*}}, retainedNodes: [[RN1:![0-9]+]]
+// A non-empty retainedNodes list of original DISubprogram.
+// CHECK-DI: [[RN1]] = !{!{{.*}}}
+
+// CHECK-DI: distinct !DISubprogram({{.*}}, linkageName: "_ZN11CharlieImpl5bravoEz", {{.*}}, retainedNodes: [[EMPTY:![0-9]+]]
+// An empty retainedNodes list of cloned DISubprogram.
+// CHECK-DI: [[EMPTY]] = !{}

clang/test/CodeGenCXX/tmp-md-nodes2.cpp

@@ -2,6 +2,14 @@
 // RUN: %clang_cc1 -O0 -triple %itanium_abi_triple -debug-info-kind=limited -emit-llvm %s -o - | \
 // RUN: FileCheck %s
 
+// Trigger GenerateVarArgsThunk.
+// RUN: %clang_cc1 -O0 -triple riscv64-linux-gnu -debug-info-kind=limited -emit-llvm %s -o - | \
+// RUN: FileCheck %s
+
+// Check that retainedNodes are properly maintained at function cloning.
+// RUN: %clang_cc1 -O1 -triple riscv64-linux-gnu -debug-info-kind=limited -emit-llvm %s -o - | \
+// RUN: FileCheck %s --check-prefixes=CHECK,CHECK-DI
+
 // This test simply checks that the varargs thunk is created. The failing test
 // case asserts.
 
@@ -31,3 +39,11 @@ BOOL CBdVfsImpl::ReqCacheHint( CMsgAgent* p_ma, CACHE_HINT hint, ... ) {
 }
 
 // CHECK: define {{.*}} @_ZThn{{[48]}}_N10CBdVfsImpl12ReqCacheHintEP9CMsgAgentN3CFs10CACHE_HINTEz(
+
+// An empty retainedNodes list of cloned DISubprogram.
+// CHECK-DI: [[EMPTY:![0-9]+]] = !{}
+// CHECK-DI: distinct !DISubprogram({{.*}}, linkageName: "_ZN10CBdVfsImpl12ReqCacheHintEP9CMsgAgentN3CFs10CACHE_HINTEz", {{.*}}, retainedNodes: [[RN1:![0-9]+]]
+// A non-empty retainedNodes list of original DISubprogram.
+// CHECK-DI: [[RN1]] = !{!{{.*}}}
+
+// CHECK-DI: distinct !DISubprogram({{.*}}, linkageName: "_ZN10CBdVfsImpl12ReqCacheHintEP9CMsgAgentN3CFs10CACHE_HINTEz", {{.*}}, retainedNodes: [[EMPTY]]