[ELF] Change tombstone values to (.debug_ranges/.debug_loc) 1 and (other .debug_*) 0

tl;dr See D81784 for the 'tombstone value' concept. This patch changes our behavior to be almost the same as GNU ld (except that we also use 1 for .debug_loc):

* .debug_ranges & .debug_loc: 1 (LLD<11: 0+addend; GNU ld uses 1 for .debug_ranges)
* .debug_*: 0 (LLD<11: 0+addend; GNU ld uses 0; future LLD: -1)

We make the tweaks because:

1) The new tombstone is novel and needs more time to be adopted by consumers before it's the default.
2) The old (gold) strategy had problems with zero-length functions - so rather than going back that, we're going to the GNU ld strategy which doesn't have that problem.
3) One slight tweak to (2) is to apply the .debug_ranges workaround to .debug_loc for the same reasons it applies to debug_ranges - to avoid terminating lists early.

-----

http://lists.llvm.org/pipermail/llvm-dev/2020-July/143482.html

The tombstone value -1 in .debug_line caused problems to lldb (fixed by D83957;
will be included in 11.0.0) and breakpad (fixed by
https://crrev.com/c/2321300). It may potentially affects other DWARF consumers.

For .debug_ranges & .debug_loc: 1, an argument preferring 1 (GNU ld for .debug_ranges) over -2 is that:
```
{-1, -2}    <<< base address selection entry
{0, length} <<< address range
```
may create a situation where low_pc is greater than high_pc. So we use
1, the GNU ld behavior for .debug_ranges

For other .debug_* sections, there haven't been many reports. One issue is that
bloaty (src/dwarf.cc) can incorrectly count address ranges in .debug_ranges . To
reduce similar disruption, this patch changes the tombstone values to be similar to GNU ld.

This does mean another behavior change to the default trunk behavior. Sorry
about it. The default trunk behavior will be similar to release/11.x while we work on a transition plan for LLD users.

Reviewed By: dblaikie, echristo

Differential Revision: https://reviews.llvm.org/D84825

Author: MaskRay

lld/ELF/InputSection.cpp

@@ -945,14 +945,17 @@ void InputSection::relocateNonAlloc(uint8_t *buf, ArrayRef<RelTy> rels) {
       // the folded-in function, so exclude .debug_line.
       //
       // For pre-DWARF-v5 .debug_loc and .debug_ranges, -1 is a reserved value
-      // (base address selection entry), so -2 is used.
+      // (base address selection entry), use 1 (which is used by GNU ld for
+      // .debug_ranges).
+      //
+      // TODO To reduce disruption, we use 0 instead of -1 as the tombstone
+      // value. Enable -1 in a future release.
       auto *ds = dyn_cast<Defined>(&sym);
       if (!sym.getOutputSection() ||
           (ds && ds->section->repl != ds->section && !isDebugLine)) {
         // If -z dead-reloc-in-nonalloc= is specified, respect it.
-        const uint64_t value =
-            tombstone ? SignExtend64<bits>(*tombstone)
-                      : (isDebugLocOrRanges ? UINT64_MAX - 1 : UINT64_MAX);
+        const uint64_t value = tombstone ? SignExtend64<bits>(*tombstone)
+                                         : (isDebugLocOrRanges ? 1 : 0);
         target->relocateNoSym(bufLoc, type, value);
         continue;
       }

lld/test/ELF/dead-reloc-in-nonalloc.s

@@ -11,7 +11,7 @@
 # RUN:   -z dead-reloc-in-nonalloc=.not_debug=0xbbbbbbbb %t.o -o - | cmp %t -
 
 # COMMON:      Contents of section .debug_addr:
-# COMMON-NEXT:  0000 [[ADDR:[0-9a-f]+]] 00000000 ffffffff ffffffff
+# COMMON-NEXT:  0000 [[ADDR:[0-9a-f]+]] 00000000 00000000 00000000
 
 # AA:          Contents of section .debug_info:
 # AA-NEXT:      0000 [[ADDR]] 00000000 aaaaaaaa 00000000

lld/test/ELF/debug-dead-reloc-32.s

@@ -8,11 +8,11 @@
 # RUN: llvm-objdump -s %t | FileCheck %s
 
 # CHECK:      Contents of section .debug_loc:
-# CHECK-NEXT:  0000 feffffff
+# CHECK-NEXT:  0000 01000000
 # CHECK-NEXT: Contents of section .debug_ranges:
-# CHECK-NEXT:  0000 feffffff
+# CHECK-NEXT:  0000 01000000
 # CHECK-NEXT: Contents of section .debug_addr:
-# CHECK-NEXT:  0000 ffffffff
+# CHECK-NEXT:  0000 00000000
 
 .section .text.1,"axe"
   .byte 0
@@ -24,6 +24,6 @@
 .section .debug_ranges
   .long .text.1+16
 
-## Resolved to UINT32_C(-1), with the addend ignored.
+## Resolved to UINT32_C(0), with the addend ignored.
 .section .debug_addr
   .long .text.1+8

lld/test/ELF/debug-dead-reloc-icf.s

@@ -9,7 +9,7 @@
 # RUN: llvm-objdump -s %t | FileCheck %s
 
 # CHECK:      Contents of section .debug_info:
-# CHECK-NEXT:  0000 {{[0-9a-f]+}}000 00000000 ffffffff ffffffff
+# CHECK-NEXT:  0000 {{[0-9a-f]+}}000 00000000 00000000 00000000
 # CHECK:      Contents of section .debug_line:
 # CHECK-NEXT:  0000 [[ADDR:[0-9a-f]+]] 00000000
 # CHECK-SAME:                                   [[ADDR]] 00000000

lld/test/ELF/debug-dead-reloc-tls-arm.s

@@ -7,7 +7,7 @@
 # RUN: llvm-objdump -s %t | FileCheck %s
 
 # CHECK:      Contents of section .debug_info:
-# CHECK-NEXT:  0000 ffffffff
+# CHECK-NEXT:  0000 00000000
 
 .globl _start
 _start:

lld/test/ELF/debug-dead-reloc-tls.s

@@ -7,8 +7,8 @@
 # RUN: llvm-objdump -s %t | FileCheck %s
 
 # CHECK:      Contents of section .debug_info:
-# CHECK-NEXT:  0000 ffffffff ffffffff ffffffff ffffffff
-# CHECK-NEXT:  0010 ffffffff ffffffff
+# CHECK-NEXT:  0000 00000000 00000000 00000000 00000000
+# CHECK-NEXT:  0010 00000000 ffffffff
 
 .globl _start
 _start:

lld/test/ELF/debug-dead-reloc.s

@@ -9,15 +9,15 @@
 # RUN: llvm-objdump -s %t | FileCheck %s
 
 # CHECK:      Contents of section .debug_loc:
-# CHECK-NEXT:  0000 feffffff ffffffff feffffff ffffffff
+# CHECK-NEXT:  0000 01000000 00000000 01000000 00000000
 # CHECK-NEXT: Contents of section .debug_ranges:
-# CHECK-NEXT:  0000 feffffff ffffffff feffffff ffffffff
+# CHECK-NEXT:  0000 01000000 00000000 01000000 00000000
 # CHECK-NEXT: Contents of section .debug_addr:
 # CHECK-NEXT:  0000 {{.*}}000 00000000 {{.*}}000 00000000
-# CHECK-NEXT:  0010 ffffffff  ffffffff {{.*}}000 00000000
+# CHECK-NEXT:  0010 00000000  00000000 {{.*}}000 00000000
 # CHECK-NEXT: Contents of section .debug_foo:
-# CHECK-NEXT:  0000 ffffffff ffffffff 08000000 00000000
-# CHECK-NEXT:  0010 ffffffff ffffffff 08000000 00000000
+# CHECK-NEXT:  0000 00000000 00000000 08000000 00000000
+# CHECK-NEXT:  0010 00000000 00000000 08000000 00000000
 
 ## -z dead-reloc-in-nonalloc= can override the tombstone value.
 # RUN: ld.lld --gc-sections -z dead-reloc-in-nonalloc=.debug_loc=42 %t.o %t1.o %t1.o -o %t42
@@ -35,7 +35,7 @@
 group:
   .byte 0
 
-## Resolved to UINT64_C(-2), with the addend ignored.
+## Resolved to UINT64_C(1), with the addend ignored.
 ## UINT64_C(-1) is a reserved value (base address selection entry) which can't be used.
 .section .debug_loc
   .quad .text.1+8
@@ -44,7 +44,7 @@ group:
 
 .section .debug_addr
 ## .text.3 is a local symbol. The symbol defined in a non-prevailing group is
-## discarded. Resolved to UINT64_C(-1).
+## discarded. Resolved to UINT64_C(0).
   .quad .text.3+24
 ## group is a non-local symbol. The relocation from the second %t1.o gets
 ## resolved to the prevailing copy.