[all-commits] [llvm/llvm-project] e6ad78: [ELF] Don't resolve a relocation in .debug_line re...

[Fangrui Song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: e6ad78fe050392e6807593793f59e65b0e6dc433
      https://github.com/llvm/llvm-project/commit/e6ad78fe050392e6807593793f59e65b0e6dc433
  Author: Fangrui Song <maskray at google.com>
  Date:   2020-07-01 (Wed, 01 Jul 2020)

  Changed paths:
    M lld/ELF/InputSection.cpp
    M lld/test/ELF/debug-dead-reloc-icf.s

  Log Message:
  -----------
  [ELF] Don't resolve a relocation in .debug_line referencing an ICF folded symbol to the tombstone value

After D81784, we resolve a relocation in .debug_* referencing an ICF folded
section symbol to a tombstone value.

Doing this for .debug_line has a problem (https://reviews.llvm.org/D81784#2116925 ):
.debug_line may describe folded lines as having addresses UINT64_MAX or
some wraparound small addresses.

```
int foo(int x) {
  return x; // line 2
}

int bar(int x) {
  return x; // line 6
}
```

```
Address            Line   Column File   ISA Discriminator Flags
------------------ ------ ------ ------ --- ------------- -------------
0x00000000002016c0      1      0      1   0             0  is_stmt
0x00000000002016c7      2      9      1   0             0  is_stmt
prologue_end
0x00000000002016ca      2      2      1   0             0
0x00000000002016cc      2      2      1   0             0  end_sequence
// UINT64_MAX and wraparound small addresses
0xffffffffffffffff      5      0      1   0             0  is_stmt
0x0000000000000006      6      9      1   0             0  is_stmt
prologue_end
0x0000000000000009      6      2      1   0             0
0x000000000000000b      6      2      1   0             0  end_sequence
0x00000000002016d0      9      0      1   0             0  is_stmt
0x00000000002016df     10      6      1   0             0  is_stmt prologue_end
0x00000000002016e6     11     11      1   0             0  is_stmt
...
```

These entries can confuse debuggers:

gdb before 2020-07-01 (binutils-gdb a8caed5d7faa639a1e6769eba551d15d8ddd9510 "Recognize -1 as a tombstone value in .debug_line")
(can't continue due to a breakpoint in an invalid region of memory):
```
Warning:
Cannot insert breakpoint 1.
Cannot access memory at address 0x6
```
lldb (breakpoint has no effect):
```
(lldb) b 6
Breakpoint 1: no locations (pending).
WARNING:  Unable to resolve breakpoint to any actual locations.
```

This patch special cases .debug_line to not use the tombstone value,
restoring the previous behavior: .debug_line will have entries with the
same addresses (ICF) but different line numbers. A breakpoint on line 2
or 6 will trigger on both functions.

Reviewed By: dblaikie, jhenderson

Differential Revision: https://reviews.llvm.org/D82828