[CGDebugInfo] Emit subprograms for decls when AT_tail_call is understood

Currently, clang emits subprograms for declared functions when the
target debugger or DWARF standard is known to support entry values
(DW_OP_entry_value & the GNU equivalent).

Treat DW_AT_tail_call the same way to allow debuggers to follow cross-TU
tail calls.

Pre-patch debug session with a cross-TU tail call:

```
  * frame #0: 0x0000000100000fa4 main`target at b.c:4:3 [opt]
    frame #1: 0x0000000100000f99 main`main at a.c:8:10 [opt]
```

Post-patch (note that the tail-calling frame, "helper", is visible):

```
  * frame #0: 0x0000000100000fa4 main`target at b.c:4:3 [opt]
    frame #1: 0x0000000100000f80 main`helper [opt] [artificial]
    frame #2: 0x0000000100000f99 main`main at a.c:8:10 [opt]
```

rdar://46577651

Differential Revision: https://reviews.llvm.org/D69743

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -3748,9 +3748,7 @@ void CGDebugInfo::EmitFunctionDecl(GlobalDecl GD, SourceLocation Loc,
 void CGDebugInfo::EmitFuncDeclForCallSite(llvm::CallBase *CallOrInvoke,
                                           QualType CalleeType,
                                           const FunctionDecl *CalleeDecl) {
-  auto &CGOpts = CGM.getCodeGenOpts();
-  if (!CGOpts.EnableDebugEntryValues || !CGM.getLangOpts().Optimize ||
-      !CallOrInvoke)
+  if (!CallOrInvoke || getCallSiteRelatedAttrs() == llvm::DINode::FlagZero)
     return;
 
   auto *Func = CallOrInvoke->getCalledFunction();
@@ -4824,10 +4822,10 @@ llvm::DINode::DIFlags CGDebugInfo::getCallSiteRelatedAttrs() const {
   bool SupportsDWARFv4Ext =
       CGM.getCodeGenOpts().DwarfVersion == 4 &&
       (CGM.getCodeGenOpts().getDebuggerTuning() == llvm::DebuggerKind::LLDB ||
-       (CGM.getCodeGenOpts().EnableDebugEntryValues &&
-       CGM.getCodeGenOpts().getDebuggerTuning() == llvm::DebuggerKind::GDB));
+       CGM.getCodeGenOpts().getDebuggerTuning() == llvm::DebuggerKind::GDB);
 
-  if (!SupportsDWARFv4Ext && CGM.getCodeGenOpts().DwarfVersion < 5)
+  if (!SupportsDWARFv4Ext && CGM.getCodeGenOpts().DwarfVersion < 5 &&
+      !CGM.getCodeGenOpts().EnableDebugEntryValues)
     return llvm::DINode::FlagZero;
 
   return llvm::DINode::FlagAllCallsDescribed;

clang/test/CodeGen/debug-info-extern-call.c

@@ -1,8 +1,21 @@
-// RUN: %clang -Xclang -femit-debug-entry-values -g -O2 -target x86_64-none-linux-gnu -S -emit-llvm %s -o - | FileCheck %s -check-prefix=CHECK-EXT
-// CHECK-EXT: !DISubprogram(name: "fn1"
+// When entry values are emitted, expect a subprogram for extern decls so that
+// the dwarf generator can describe call site parameters at extern call sites.
+//
+// RUN: %clang -Xclang -femit-debug-entry-values -g -O2 -target x86_64-none-linux-gnu -S -emit-llvm %s -o - | FileCheck %s -check-prefix=ENTRY-VAL
+// ENTRY-VAL: !DISubprogram(name: "fn1"
 
-// RUN: %clang -g -O2 -target x86_64-none-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
-// CHECK-NOT: !DISubprogram(name: "fn1"
+// Similarly, when the debugger tuning is gdb, expect a subprogram for extern
+// decls so that the dwarf generator can describe information needed for tail
+// call frame reconstrution.
+//
+// RUN: %clang -g -O2 -target x86_64-none-linux-gnu -ggdb -S -emit-llvm %s -o - | FileCheck %s -check-prefix=GDB
+// GDB: !DISubprogram(name: "fn1"
+//
+// Do not emit a subprogram for extern decls when entry values are disabled and
+// the tuning is not set to gdb.
+//
+// RUN: %clang -g -O2 -target x86_64-none-linux-gnu -gsce -S -emit-llvm %s -o - | FileCheck %s -check-prefix=SCE
+// SCE-NOT: !DISubprogram(name: "fn1"
 
 extern int fn1(int a, int b);
 

clang/test/CodeGenCXX/dbg-info-all-calls-described.cpp

@@ -56,6 +56,7 @@
 
 // NO-ATTR-NOT: FlagAllCallsDescribed
 
+// HAS-ATTR-DAG: DISubprogram(name: "declaration1", {{.*}}, flags: DIFlagPrototyped
 // HAS-ATTR-DAG: DISubprogram(name: "declaration2", {{.*}}, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition
 // HAS-ATTR-DAG: DISubprogram(name: "struct1", {{.*}}, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized)
 // HAS-ATTR-DAG: DISubprogram(name: "struct1", {{.*}}, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition

llvm/test/DebugInfo/X86/dwarf-callsite-related-attrs.ll

@@ -25,6 +25,14 @@
 
 @sink = global i32 0, align 4, !dbg !0
 
+define void @__has_no_subprogram() {
+entry:
+  %0 = load volatile i32, i32* @sink, align 4
+  %inc = add nsw i32 %0, 1
+  store volatile i32 %inc, i32* @sink, align 4
+  ret void
+}
+
 ; ASM: DW_TAG_subprogram
 ; ASM:   DW_AT_call_all_calls
 ; OBJ: [[bat_sp:.*]]: DW_TAG_subprogram
@@ -70,6 +78,7 @@ entry:
 ; OBJ:     DW_AT_call_tail_call
 define void @_Z3foov() !dbg !25 {
 entry:
+  tail call void @__has_no_subprogram()
   tail call void @_Z3barv(), !dbg !26
   tail call void @_Z3batv(), !dbg !27
   tail call void @_Z3barv(), !dbg !26