Pull in r218241 from upstream llvm trunk (by Oliver Stannard):

  Downgrade DWARF2 section limit error to a warning

  We currently emit an error when trying to assemble a file with more
  than one section using DWARF2 debug info. This should be a warning
  instead, as the resulting file will still be usable, but with a
  degraded debug illusion.

This helps to compile sys/amd64/amd64/xen-locore.S, which mixes multiple
sections into one file, and thus would emit an error about DWARF2 not
supporting that.

Introduced here: http://svnweb.freebsd.org/changeset/base/274968

Index: lib/MC/MCDwarf.cpp
===================================================================
--- lib/MC/MCDwarf.cpp
+++ lib/MC/MCDwarf.cpp
@@ -519,7 +519,8 @@ static void EmitGenDwarfAbbrev(MCStreamer *MCOS) {
   MCOS->EmitULEB128IntValue(dwarf::DW_TAG_compile_unit);
   MCOS->EmitIntValue(dwarf::DW_CHILDREN_yes, 1);
   EmitAbbrev(MCOS, dwarf::DW_AT_stmt_list, dwarf::DW_FORM_data4);
-  if (MCOS->getContext().getGenDwarfSectionSyms().size() > 1) {
+  if (MCOS->getContext().getGenDwarfSectionSyms().size() > 1 &&
+      MCOS->getContext().getDwarfVersion() >= 3) {
     EmitAbbrev(MCOS, dwarf::DW_AT_ranges, dwarf::DW_FORM_data4);
   } else {
     EmitAbbrev(MCOS, dwarf::DW_AT_low_pc, dwarf::DW_FORM_addr);
@@ -855,10 +856,11 @@ void MCGenDwarfInfo::Emit(MCStreamer *MCOS) {
   if (MCOS->getContext().getGenDwarfSectionSyms().empty())
     return;
 
-  // We only need to use the .debug_ranges section if we have multiple
-  // code sections.
+  // We only use the .debug_ranges section if we have multiple code sections,
+  // and we are emitting a DWARF version which supports it.
   const bool UseRangesSection =
-      MCOS->getContext().getGenDwarfSectionSyms().size() > 1;
+      MCOS->getContext().getGenDwarfSectionSyms().size() > 1 &&
+      MCOS->getContext().getDwarfVersion() >= 3;
   CreateDwarfSectionSymbols |= UseRangesSection;
 
   MCOS->SwitchSection(context.getObjectFileInfo()->getDwarfInfoSection());
Index: lib/MC/MCParser/ELFAsmParser.cpp
===================================================================
--- lib/MC/MCParser/ELFAsmParser.cpp
+++ lib/MC/MCParser/ELFAsmParser.cpp
@@ -555,7 +555,7 @@ EndStmt:
         std::make_pair(ELFSection, std::make_pair(nullptr, nullptr)));
     if (InsertResult.second) {
       if (getContext().getDwarfVersion() <= 2)
-        Error(loc, "DWARF2 only supports one section per compilation unit");
+        Warning(loc, "DWARF2 only supports one section per compilation unit");
 
       MCSymbol *SectionStartSymbol = getContext().CreateTempSymbol();
       getStreamer().EmitLabel(SectionStartSymbol);
Index: test/MC/ARM/dwarf-asm-multiple-sections-dwarf-2.s
===================================================================
--- test/MC/ARM/dwarf-asm-multiple-sections-dwarf-2.s
+++ test/MC/ARM/dwarf-asm-multiple-sections-dwarf-2.s
@@ -0,0 +1,66 @@
+// RUN: llvm-mc < %s -triple=armv7-linux-gnueabi -filetype=obj -o %t -g -fdebug-compilation-dir=/tmp -dwarf-version 2 2>&1 | FileCheck -check-prefix MESSAGES %s
+// RUN: llvm-dwarfdump %t | FileCheck -check-prefix DWARF %s
+// RUN: llvm-objdump -r %t | FileCheck -check-prefix RELOC %s
+
+  .section .text, "ax"
+a:
+  mov r0, r0
+
+  .section foo, "ax"
+b:
+  mov r1, r1
+
+// MESSAGES: warning: DWARF2 only supports one section per compilation unit
+
+// DWARF: .debug_abbrev contents:
+// DWARF: Abbrev table for offset: 0x00000000
+// DWARF: [1] DW_TAG_compile_unit DW_CHILDREN_yes
+// DWARF:         DW_AT_stmt_list DW_FORM_data4
+// DWARF:         DW_AT_low_pc    DW_FORM_addr
+// DWARF:         DW_AT_high_pc   DW_FORM_addr
+// DWARF:         DW_AT_name      DW_FORM_string
+// DWARF:         DW_AT_comp_dir  DW_FORM_string
+// DWARF:         DW_AT_producer  DW_FORM_string
+// DWARF:         DW_AT_language  DW_FORM_data2
+
+// DWARF: .debug_info contents:
+// DWARF: 0x{{[0-9a-f]+}}: DW_TAG_compile_unit [1]
+// CHECK-NOT-DWARF: DW_TAG_
+// DWARF:               DW_AT_low_pc [DW_FORM_addr]       (0x0000000000000000)
+// DWARF:               DW_AT_high_pc [DW_FORM_addr]      (0x0000000000000004)
+
+// DWARF: 0x{{[0-9a-f]+}}:   DW_TAG_label [2] *
+// DWARF-NEXT: DW_AT_name [DW_FORM_string]     ("a")
+
+
+// DWARF: .debug_aranges contents:
+// DWARF-NEXT: Address Range Header: length = 0x00000024, version = 0x0002, cu_offset = 0x00000000, addr_size = 0x04, seg_size = 0x00
+// DWARF-NEXT: [0x00000000 - 0x00000004)
+// DWARF-NEXT: [0x00000000 - 0x00000004)
+
+// DWARF: .debug_line contents:
+// DWARF:      0x0000000000000000      7      0      1   0   0 is_stmt
+// DWARF-NEXT: 0x0000000000000004      7      0      1   0   0 is_stmt end_sequence
+// DWARF:      0x0000000000000000     11      0      1   0   0 is_stmt
+// DWARF-NEXT: 0x0000000000000004     11      0      1   0   0 is_stmt end_sequence
+
+
+// DWARF: .debug_ranges contents:
+// DWARF-NOT: {{0-9a-f}}
+// DWARF: .debug_pubnames contents:
+
+
+// RELOC: RELOCATION RECORDS FOR [.rel.debug_info]:
+// RELOC-NEXT: 00000006 R_ARM_ABS32 .debug_abbrev
+// RELOC-NEXT: 0000000c R_ARM_ABS32 .debug_line
+// RELOC-NEXT: R_ARM_ABS32 .text
+// RELOC-NEXT: R_ARM_ABS32 .text
+// RELOC-NEXT: R_ARM_ABS32 .text
+// RELOC-NEXT: R_ARM_ABS32 foo
+
+// RELOC-NOT: RELOCATION RECORDS FOR [.rel.debug_ranges]:
+
+// RELOC: RELOCATION RECORDS FOR [.rel.debug_aranges]:
+// RELOC-NEXT: 00000006 R_ARM_ABS32 .debug_info
+// RELOC-NEXT: 00000010 R_ARM_ABS32 .text
+// RELOC-NEXT: 00000018 R_ARM_ABS32 foo
Index: test/MC/ARM/dwarf-asm-multiple-sections.s
===================================================================
--- test/MC/ARM/dwarf-asm-multiple-sections.s
+++ test/MC/ARM/dwarf-asm-multiple-sections.s
@@ -1,7 +1,7 @@
 // RUN: llvm-mc < %s -triple=armv7-linux-gnueabi -filetype=obj -o %t -g -fdebug-compilation-dir=/tmp
 // RUN: llvm-dwarfdump %t | FileCheck -check-prefix DWARF %s
 // RUN: llvm-objdump -r %t | FileCheck -check-prefix RELOC %s
-// RUN: not llvm-mc < %s -triple=armv7-linux-gnueabi -filetype=obj -o %t -g -dwarf-version 2 2>&1 | FileCheck -check-prefix VERSION %s
+// RUN: llvm-mc < %s -triple=armv7-linux-gnueabi -filetype=obj -o %t -g -dwarf-version 2 2>&1 | FileCheck -check-prefix VERSION %s
 // RUN: not llvm-mc < %s -triple=armv7-linux-gnueabi -filetype=obj -o %t -g -dwarf-version 1 2>&1 | FileCheck -check-prefix DWARF1 %s
 // RUN: not llvm-mc < %s -triple=armv7-linux-gnueabi -filetype=obj -o %t -g -dwarf-version 5 2>&1 | FileCheck -check-prefix DWARF5 %s
   .section .text, "ax"
@@ -73,7 +73,7 @@ b:
 // RELOC-NEXT: 00000018 R_ARM_ABS32 foo
 
 
-// VERSION: {{.*}} error: DWARF2 only supports one section per compilation unit
+// VERSION: {{.*}} warning: DWARF2 only supports one section per compilation unit
 
 // DWARF1: Dwarf version 1 is not supported.
 // DWARF5: Dwarf version 5 is not supported.