Various crash reporting tools have a problem with the dwarf generated for

assembly source when it generates the TAG_subprogram dwarf debug info for
the labels that have nothing between them as in this bit of assembly source:

% cat ZeroLength.s 
_func1:
_func2:
 nop

One solution would be to not emit the subsequent labels with the same address
and use the next label with a different address or the end of the section for
the AT_high_pc value of the TAG_subprogram.

Turns out in llvm-mc it is not possible in all cases to determine of two
symbols have the same value at the point we put out the TAG_subprogram dwarf
debug info.

So we will have llvm-mc instead of putting out TAG_subprogram's put out
DW_TAG_label's.  And the DW_TAG_label does not have a AT_high_pc value which
avoids the problem.

This commit is only the functional change to make the diffs clear as to what is
really being changed.  The next commit will be to clean up the names of such
things like MCGenDwarfSubprogramEntry to something like MCGenDwarfLabelEntry.

rdar://10666925

llvm-svn: 147860

1 files changed, 5 insertions, 17 deletions

diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index 986b8b67df5..83fa0690e95 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -454,15 +454,14 @@ static void EmitGenDwarfAbbrev(MCStreamer *MCOS) {
   EmitAbbrev(MCOS, dwarf::DW_AT_language, dwarf::DW_FORM_data2);
   EmitAbbrev(MCOS, 0, 0);
 
-  // DW_TAG_subprogram DIE abbrev (2).
+  // DW_TAG_label DIE abbrev (2).
   MCOS->EmitULEB128IntValue(2);
-  MCOS->EmitULEB128IntValue(dwarf::DW_TAG_subprogram);
+  MCOS->EmitULEB128IntValue(dwarf::DW_TAG_label);
   MCOS->EmitIntValue(dwarf::DW_CHILDREN_yes, 1);
   EmitAbbrev(MCOS, dwarf::DW_AT_name, dwarf::DW_FORM_string);
   EmitAbbrev(MCOS, dwarf::DW_AT_decl_file, dwarf::DW_FORM_data4);
   EmitAbbrev(MCOS, dwarf::DW_AT_decl_line, dwarf::DW_FORM_data4);
   EmitAbbrev(MCOS, dwarf::DW_AT_low_pc, dwarf::DW_FORM_addr);
-  EmitAbbrev(MCOS, dwarf::DW_AT_high_pc, dwarf::DW_FORM_addr);
   EmitAbbrev(MCOS, dwarf::DW_AT_prototyped, dwarf::DW_FORM_flag);
   EmitAbbrev(MCOS, 0, 0);
 
@@ -545,7 +544,7 @@ static void EmitGenDwarfAranges(MCStreamer *MCOS) {
 
 // When generating dwarf for assembly source files this emits the data for
 // .debug_info section which contains three parts.  The header, the compile_unit
-// DIE and a list of subprogram DIEs.
+// DIE and a list of label DIEs.
 static void EmitGenDwarfInfo(MCStreamer *MCOS) {
   MCContext &context = MCOS->getContext();
 
@@ -630,7 +629,7 @@ static void EmitGenDwarfInfo(MCStreamer *MCOS) {
   // draft has no standard code for assembler.
   MCOS->EmitIntValue(dwarf::DW_LANG_Mips_Assembler, 2);
 
-  // Third part: the list of subprogram DIEs.
+  // Third part: the list of label DIEs.
 
   // Loop on saved info for dwarf subprograms and create the DIEs for them.
   const std::vector<const MCGenDwarfSubprogramEntry *> &Entries =
@@ -640,7 +639,7 @@ static void EmitGenDwarfInfo(MCStreamer *MCOS) {
        ++it) {
     const MCGenDwarfSubprogramEntry *Entry = *it;
 
-    // The DW_TAG_subprogram DIE abbrev (2).
+    // The DW_TAG_label DIE abbrev (2).
     MCOS->EmitULEB128IntValue(2);
 
     // AT_name, of the label without any leading underbar.
@@ -658,17 +657,6 @@ static void EmitGenDwarfInfo(MCStreamer *MCOS) {
                                              MCSymbolRefExpr::VK_None, context);
     MCOS->EmitAbsValue(AT_low_pc, AddrSize);
 
-    // AT_high_pc, end address which is the next label or end of the section.
-    std::vector<const MCGenDwarfSubprogramEntry *>::const_iterator next = it+1;
-    if (next != Entries.end()){
-      const MCGenDwarfSubprogramEntry *NextEntry = *next;
-      const MCExpr *AT_high_pc = MCSymbolRefExpr::Create(NextEntry->getLabel(),
-                                             MCSymbolRefExpr::VK_None, context);
-      MCOS->EmitAbsValue(AT_high_pc, AddrSize);
-    } else {
-      MCOS->EmitAbsValue(End, AddrSize);
-    }
-
     // DW_AT_prototyped, a one byte flag value of 0 saying we have no prototype.
     MCOS->EmitIntValue(0, 1);