[CGDebugInfo] Emit subprograms for decls when AT_tail_call is understood

[vsk]

Summary

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

[djtodoro]

@vsk Thanks for this!
(lgtm)

[dblaikie]

What's the extra DWARF that's emitted from the backend? Is there a corresponding LLVM code change/test, or is the functionality there but dormant in this particular case? (& already running/tested in other cases, such as when fn1 is defined in the same CU)

[vsk]

In D69743#1732967, @dblaikie wrote:

What's the extra DWARF that's emitted from the backend? Is there a corresponding LLVM code change/test, or is the functionality there but dormant in this particular case? (& already running/tested in other cases, such as when fn1 is defined in the same CU)

The extra DWARF would look like:

DW_TAG_subprogram
                DW_AT_name	("fn1")
                DW_AT_declaration	(true)

This clang change has no corresponding llvm logic change. The functionality to emit a subprogram for a declaration is already present & tested in llvm, as is the logic to use such a subprogram to create a TAG_call_site (llvm/test/DebugInfo/X86/dwarf-callsite-related-attrs.ll is a test, and the logic is in DwarfDebug::constructCallSiteEntryDIEs). That test should cover the case where a function has no subprogram, though.

[vsk]

• Add coverage for a callee without a subprogram in llvm/test/DebugInfo/X86/dwarf-callsite-related-attrs.ll.

[dblaikie]

In D69743#1733101, @vsk wrote:

In D69743#1732967, @dblaikie wrote:

What's the extra DWARF that's emitted from the backend? Is there a corresponding LLVM code change/test, or is the functionality there but dormant in this particular case? (& already running/tested in other cases, such as when fn1 is defined in the same CU)

The extra DWARF would look like:

DW_TAG_subprogram
                DW_AT_name	("fn1")
                DW_AT_declaration	(true)

This clang change has no corresponding llvm logic change. The functionality to emit a subprogram for a declaration is already present & tested in llvm, as is the logic to use such a subprogram to create a TAG_call_site (llvm/test/DebugInfo/X86/dwarf-callsite-related-attrs.ll is a test, and the logic is in DwarfDebug::constructCallSiteEntryDIEs). That test should cover the case where a function has no subprogram, though.

OK, thanks for the context!

clang/lib/CodeGen/CGDebugInfo.cpp

if (IsDeclForCallSite)
     Fn->setSubprogram(SP);
 
   DBuilder.retainType(SP);
 }
 
 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();
   if (!Func)
     return;
 
   // If there is no DISubprogram attached to the function being called,
   // create the one describing the function in order to have complete
if (!CGM.getLangOpts().Optimize || DebugKind == codegenoptions::NoDebugInfo ||
     return llvm::DINode::FlagZero;
 
   // Call site-related attributes are available in DWARF v5. Some debuggers,
   // while not fully DWARF v5-compliant, may accept these attributes as if they
   // were part of DWARF v4.
   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

-// 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);
 
 int fn2 () {
   int x = 4, y = 5;
   int res = fn1(x, y);
 
   return res;
 }
 

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

 // Unsupported: DWARF4
 // RUN: %clang_cc1 -emit-llvm -triple %itanium_abi_triple %s -o - \
 // RUN:   -O1 -disable-llvm-passes \
 // RUN:   -debug-info-kind=standalone -dwarf-version=4 \
 // RUN: | FileCheck %s -check-prefix=NO-ATTR
 
 // 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
 // HAS-ATTR-DAG: DISubprogram(name: "method1", {{.*}}, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition
 // HAS-ATTR-DAG: DISubprogram(name: "force_irgen", {{.*}}, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition
 
 // LINE-TABLES-ONLY: DISubprogram(name: "force_irgen", {{.*}}, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition
 

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

 ; RUN: llvm-dwarfdump -statistics %t.o | FileCheck %s -check-prefix=STATS
 ; RUN: llvm-as < %s | llvm-dis | llvm-as | llvm-dis -o /dev/null
 
 ; VERIFY: No errors.
 ; STATS: "call site DIEs":6
 
 @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
 ; OBJ:   DW_AT_call_all_calls (true)
 ; OBJ:   DW_AT_name ("bat")
 define void @_Z3batv() !dbg !13 {
 entry:
   %0 = load volatile i32, i32* @sink, align 4, !dbg !16, !tbaa !17

 ; OBJ:   DW_TAG_call_site
 ; OBJ:     DW_AT_call_origin ([[bar_sp]])
 ; OBJ:     DW_AT_call_return_pc
 ; OBJ:   DW_TAG_call_site
 ; OBJ:     DW_AT_call_origin ([[bat_sp]])
 ; 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
   tail call void @_Z3batv(), !dbg !27
   ret void, !dbg !28
 }
 
 ; ASM: DW_TAG_subprogram