Disable the -gmlt optimization implemented in r218129 under Darwin due to issues with dsymutil.

r218129 omits DW_TAG_subprograms which have no inlined subroutines when
emitting -gmlt data. This makes -gmlt very low cost for -O0 builds.

Darwin's dsymutil reasonably considers a CU empty if it has no
subprograms (which occurs with the above optimization in -O0 programs
without any force_inline function calls) and drops the line table, CU,
and everything in this situation, making backtraces impossible.

Until dsymutil is modified to account for this, disable this
optimization on Darwin to preserve the desired functionality.
(see r218545, which should be reverted after this patch, for other
discussion/details)

Footnote:
In the long term, it doesn't look like this scheme (of simplified debug
info to describe inlining to enable backtracing) is tenable, it is far
too size inefficient for optimized code (the DW_TAG_inlined_subprograms,
even once compressed, are nearly twice as large as the line table
itself (also compressed)) and we'll be considering things like Cary's
two level line table proposal to encode all this information directly in
the line table.

llvm-svn: 218702

2 files changed, 3 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 7cd06931249..71d076eb480 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -171,6 +171,7 @@ DwarfDebug::DwarfDebug(AsmPrinter *A, Module *M)
       GlobalRangeCount(0), InfoHolder(A, "info_string", DIEValueAllocator),
       UsedNonDefaultText(false),
       SkeletonHolder(A, "skel_string", DIEValueAllocator),
+      IsDarwin(Triple(A->getTargetTriple()).isOSDarwin()),
       AccelNames(DwarfAccelTable::Atom(dwarf::DW_ATOM_die_offset,
                                        dwarf::DW_FORM_data4)),
       AccelObjC(DwarfAccelTable::Atom(dwarf::DW_ATOM_die_offset,
@@ -190,8 +191,6 @@ DwarfDebug::DwarfDebug(AsmPrinter *A, Module *M)
 
   // Turn on accelerator tables for Darwin by default, pubnames by
   // default for non-Darwin, and handle split dwarf.
-  bool IsDarwin = Triple(A->getTargetTriple()).isOSDarwin();
-
   if (DwarfAccelTables == Default)
     HasDwarfAccelTables = IsDarwin;
   else
@@ -1698,7 +1697,7 @@ void DwarfDebug::endFunction(const MachineFunction *MF) {
   // Under -gmlt, skip building the subprogram if there are no inlined
   // subroutines inside it.
   if (TheCU.getCUNode().getEmissionKind() == DIBuilder::LineTablesOnly &&
-      LScopes.getAbstractScopesList().empty()) {
+      LScopes.getAbstractScopesList().empty() && !IsDarwin) {
     assert(ScopeVariables.empty());
     assert(CurrentFnArguments.empty());
     assert(DbgValues.empty());
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
index f082373cbdb..d0b1ee8a274 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
@@ -319,6 +319,7 @@ class DwarfDebug : public AsmPrinterHandler {
 
   // True iff there are multiple CUs in this module.
   bool SingleCU;
+  bool IsDarwin;
 
   AddressPool AddrPool;