[MC/DWARF] Generate multiple .debug_line entries for adjacent .loc directives

The compiler occasionally generates multiple .loc directives in a row
(at the same instruction address).  These need to be transformed into
multple actual .debug_line table entries, since they are used to signal
certain information to the debugger (e.g. if the opening brace of a
function body is on the same line as the declaration).

The MCAsmStreamer version of EmitDwarfLocDirective handles this
correctly by emitting a .loc directive every time it is called.
However, the MCObjectStream version simply defaults to recording
the information and emitting only a single table entry later,
e.g. when EmitInstruction is called.

This patch introduces a MCAsmStreamer::EmitDwarfLocDirective
version that emits a line table entry for a .loc directive
that may already be pending before recording the new directive.
(This is similar to how this is handled in GNU as.)

With this patch (and the code alignment factor patch) applied,
I'm now getting identical DWARF .debug sections for all test-suite
object files on PowerPC for the internal and the external assembler.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184357 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCObjectStreamer.h

@@ -102,6 +102,10 @@ public:
   virtual void EmitCodeAlignment(unsigned ByteAlignment,
                                  unsigned MaxBytesToEmit = 0);
   virtual bool EmitValueToOffset(const MCExpr *Offset, unsigned char Value);
+  virtual void EmitDwarfLocDirective(unsigned FileNo, unsigned Line,
+                                     unsigned Column, unsigned Flags,
+                                     unsigned Isa, unsigned Discriminator,
+                                     StringRef FileName);
   virtual void EmitDwarfAdvanceLineAddr(int64_t LineDelta,
                                         const MCSymbol *LastLabel,
                                         const MCSymbol *Label,

lib/MC/MCObjectStreamer.cpp

@@ -259,6 +259,19 @@ void MCObjectStreamer::EmitBundleUnlock() {
   llvm_unreachable(BundlingNotImplementedMsg);
 }
 
+void MCObjectStreamer::EmitDwarfLocDirective(unsigned FileNo, unsigned Line,
+                                             unsigned Column, unsigned Flags,
+                                             unsigned Isa,
+                                             unsigned Discriminator,
+                                             StringRef FileName) {
+  // In case we see two .loc directives in a row, make sure the
+  // first one gets a line entry.
+  MCLineEntry::Make(this, getCurrentSection().first);
+
+  this->MCStreamer::EmitDwarfLocDirective(FileNo, Line, Column, Flags,
+                                          Isa, Discriminator, FileName);
+}
+
 void MCObjectStreamer::EmitDwarfAdvanceLineAddr(int64_t LineDelta,
                                                 const MCSymbol *LastLabel,
                                                 const MCSymbol *Label,

test/MC/ELF/debug-line2.s

@@ -0,0 +1,32 @@
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - | llvm-readobj -s -sd | FileCheck %s
+
+// Test that two subsequent .loc directives generate two
+// distinct line table entries.
+
+// CHECK:        Section {
+// CHECK:          Name: .debug_line
+// CHECK-NEXT:     Type: SHT_PROGBITS
+// CHECK-NEXT:     Flags [
+// CHECK-NEXT:     ]
+// CHECK-NEXT:     Address: 0x0
+// CHECK-NEXT:     Offset:
+// CHECK-NEXT:     Size: 56
+// CHECK-NEXT:     Link: 0
+// CHECK-NEXT:     Info: 0
+// CHECK-NEXT:     AddressAlignment: 1
+// CHECK-NEXT:     EntrySize: 0
+// CHECK-NEXT:     SectionData (
+// CHECK-NEXT:       0000: 34000000 02001C00 00000101 FB0E0D00
+// CHECK-NEXT:       0010: 01010101 00000001 00000100 666F6F2E
+// CHECK-NEXT:       0020: 63000000 00000009 02000000 00000000
+// CHECK-NEXT:       0030: 00011302 01000101
+// CHECK-NEXT:     )
+// CHECK-NEXT:   }
+
+	.section	.debug_line,"",@progbits
+	.text
+
+	.file 1 "foo.c"
+	.loc 1 1 0
+	.loc 1 2 0
+	nop