[DWARFLinker] Preserve DWARF discriminators while linking (#96124)

DWARF-4 introduced DWARF discriminators but both, the classic and the
parallel DWARF linkers discarded them so far.

After applying this patch dsymutil, which uses the DWARF linkers,
correctly preserves discriminator information.

Author: dtellenbach

llvm/lib/DWARFLinker/Classic/DWARFStreamer.cpp

@@ -1060,6 +1060,7 @@ void DwarfStreamer::emitLineTableRows(
   unsigned FileNum = 1;
   unsigned LastLine = 1;
   unsigned Column = 0;
+  unsigned Discriminator = 0;
   unsigned IsStatement = 1;
   unsigned Isa = 0;
   uint64_t Address = -1ULL;
@@ -1098,9 +1099,17 @@ void DwarfStreamer::emitLineTableRows(
       MS->emitULEB128IntValue(Column);
       LineSectionSize += 1 + getULEB128Size(Column);
     }
-
-    // FIXME: We should handle the discriminator here, but dsymutil doesn't
-    // consider it, thus ignore it for now.
+    if (Discriminator != Row.Discriminator &&
+        MS->getContext().getDwarfVersion() >= 4) {
+      Discriminator = Row.Discriminator;
+      unsigned Size = getULEB128Size(Discriminator);
+      MS->emitIntValue(dwarf::DW_LNS_extended_op, 1);
+      MS->emitULEB128IntValue(Size + 1);
+      MS->emitIntValue(dwarf::DW_LNE_set_discriminator, 1);
+      MS->emitULEB128IntValue(Discriminator);
+      LineSectionSize += /* extended op */ 1 + getULEB128Size(Size + 1) +
+                         /* discriminator */ 1 + Size;
+    }
 
     if (Isa != Row.Isa) {
       Isa = Row.Isa;
@@ -1156,7 +1165,7 @@ void DwarfStreamer::emitLineTableRows(
       EncodingBuffer.resize(0);
       Address = -1ULL;
       LastLine = FileNum = IsStatement = 1;
-      RowsSinceLastSequence = Column = Isa = 0;
+      RowsSinceLastSequence = Column = Discriminator = Isa = 0;
     }
   }
 

llvm/lib/DWARFLinker/Parallel/DebugLineSectionEmitter.h

@@ -315,6 +315,7 @@ class DebugLineSectionEmitter {
     unsigned FileNum = 1;
     unsigned LastLine = 1;
     unsigned Column = 0;
+    unsigned Discriminator = 0;
     unsigned IsStatement = 1;
     unsigned Isa = 0;
     uint64_t Address = -1ULL;
@@ -350,9 +351,14 @@ class DebugLineSectionEmitter {
         Section.emitIntVal(dwarf::DW_LNS_set_column, 1);
         encodeULEB128(Column, Section.OS);
       }
-
-      // FIXME: We should handle the discriminator here, but dsymutil doesn't
-      // consider it, thus ignore it for now.
+      if (Discriminator != Row.Discriminator && MC->getDwarfVersion() >= 4) {
+        Discriminator = Row.Discriminator;
+        unsigned Size = getULEB128Size(Discriminator);
+        Section.emitIntVal(dwarf::DW_LNS_extended_op, 1);
+        encodeULEB128(Size + 1, Section.OS);
+        Section.emitIntVal(dwarf::DW_LNE_set_discriminator, 1);
+        encodeULEB128(Discriminator, Section.OS);
+      }
 
       if (Isa != Row.Isa) {
         Isa = Row.Isa;
@@ -397,7 +403,7 @@ class DebugLineSectionEmitter {
         EncodingBuffer.resize(0);
         Address = -1ULL;
         LastLine = FileNum = IsStatement = 1;
-        RowsSinceLastSequence = Column = Isa = 0;
+        RowsSinceLastSequence = Column = Discriminator = Isa = 0;
       }
     }
 

llvm/test/tools/dsymutil/Inputs/discriminator.arm64.dylib

@@ -0,0 +1,23 @@
+; The input at Inputs/discriminator.arm64.dylib[.o] produced by compiling
+;
+;   int foo(int i, int a, int b) {
+;     return i ? a : a * b;
+;   }
+;
+; with -g -fdebug-info-for-profiling -O2.
+
+RUN: dsymutil --flat --linker=classic -oso-prepend-path %p -o - \
+RUN:  --oso-prepend-path %p/Inputs --verify-dwarf=none \
+RUN:  %p/Inputs/discriminator.arm64.dylib | llvm-dwarfdump -debug-line - \
+RUN:  | FileCheck %s
+
+RUN: dsymutil --flat --linker=parallel -oso-prepend-path %p -o - \
+RUN:  --oso-prepend-path %p/Inputs --verify-dwarf=none \
+RUN:  %p/Inputs/discriminator.arm64.dylib | llvm-dwarfdump -debug-line - \
+RUN:  | FileCheck %s
+
+CHECK:          Address            Line   Column File   ISA Discriminator OpIndex Flags
+CHECK-NEXT:     ------------------ ------ ------ ------ --- ------------- ------- -------------
+CHECK-NEXT:     0x0000000000003f98      2     10      0   0             0       0  is_stmt prologue_end
+CHECK-NEXT:     0x0000000000003fa4      2      3      0   0             6       0
+CHECK-NEXT:     0x0000000000003fa8      2      3      0   0             0       0  end_sequence