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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218702 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 7cd0693124916653078e3594799d1d05a592cd61..71d076eb4801018d10f6cd40c1672cda68b98c87 100644 (file)
--- a/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/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/lib/CodeGen/AsmPrinter/DwarfDebug.h b/lib/CodeGen/AsmPrinter/DwarfDebug.h
index f082373cbdb6ee44f5f83a2b3ccacb108e1a4acb..d0b1ee8a274ee84ddbba06b7ca60c6c8fabbb023 100644 (file)
--- a/lib/CodeGen/AsmPrinter/DwarfDebug.h
+++ b/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;
 

diff --git a/test/DebugInfo/gmlt.ll b/test/DebugInfo/gmlt.ll
index 3912134b2d1094c9d2b2f824d3b9e69c4748a098..a643931927f54e779eaa73c44bd2390ca60c7864 100644 (file)
--- a/test/DebugInfo/gmlt.ll
+++ b/test/DebugInfo/gmlt.ll
@@ -1,5 +1,7 @@
 ; REQUIRES: object-emission
 ; RUN: %llc_dwarf -O0 -filetype=obj < %s | llvm-dwarfdump - | FileCheck %s
+; RUN: %llc_dwarf -O0 -filetype=obj < %s -mtriple x86_64-apple-darwin | llvm-dwarfdump - \
+; RUN:     | FileCheck --check-prefix=CHECK --check-prefix=DARWIN %s
 
 ; Generated from the following source compiled with clang++ -gmlt:
 ; void f1() {}
@@ -17,6 +19,25 @@
 ; CHECK:   DW_AT_ranges [DW_FORM_sec_offset] (0x00000000)
 ; CHECK-NOT: {{DW_TAG|NULL}}
 
+; Omitting the subprograms without inlined subroutines is not possible
+; currently on Darwin as dsymutil will drop the whole CU if it has no subprograms
+; (which happens with this optimization if there are no inlined subroutines).
+
+; DARWIN:  DW_TAG_subprogram
+; DARWIN-NOT: DW_TAG
+; DARWIN:    DW_AT_name {{.*}} "f1"
+; DARWIN-NOT: {{DW_TAG|NULL}}
+; DARWIN:  DW_TAG_subprogram
+; DARWIN-NOT: DW_TAG
+; DARWIN:    DW_AT_name {{.*}} "f2"
+; DARWIN-NOT: {{DW_TAG|NULL}}
+; DARWIN:  DW_TAG_subprogram
+; DARWIN-NOT: DW_TAG
+; Can't check the abstract_origin value across the DARWIN/CHECK checking and
+; ordering, so don't bother - just trust me, it refers to f3 down there.
+; DARWIN:    DW_AT_abstract_origin
+; DARWIN-NOT: {{DW_TAG|NULL}}
+
 
 ; FIXME: Emitting separate abstract definitions is inefficient when we could
 ; just attach the DW_AT_name to the inlined_subroutine directly. Except that