[llvm-objcopy][ELF] Implement --only-keep-debug

--only-keep-debug produces a debug file as the output that only
preserves contents of sections useful for debugging purposes (the
binutils implementation preserves SHT_NOTE and non-SHF_ALLOC sections),
by changing their section types to SHT_NOBITS and rewritting file
offsets.

See https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html

The intended use case is:

```
llvm-objcopy --only-keep-debug a a.dbg
llvm-objcopy --strip-debug a b
llvm-objcopy --add-gnu-debuglink=a.dbg b
```

The current layout algorithm is incapable of deleting contents and
shrinking segments, so it is not suitable for implementing the
functionality.

This patch adds a new algorithm which assigns sh_offset to sections
first, then modifies p_offset/p_filesz of program headers. It bears a
resemblance to lld/ELF/Writer.cpp.

Reviewed By: jhenderson, jakehehrlich

Differential Revision: https://reviews.llvm.org/D67137

1 files changed, 0 insertions, 22 deletions

diff --git a/llvm/test/tools/llvm-objcopy/ELF/basic-only-keep-debug.test b/llvm/test/tools/llvm-objcopy/ELF/basic-only-keep-debug.test
deleted file mode 100644
index 95cd07d0c26..00000000000
--- a/llvm/test/tools/llvm-objcopy/ELF/basic-only-keep-debug.test
+++ /dev/null
@@ -1,22 +0,0 @@
-# NOTE: This test is only intended to be valid as long as --only-keep-debug is
-#       implemented as a NOP. This test should fail when that changes and you
-#       will need to update this test.
-
-# RUN: yaml2obj %s > %t
-# RUN: llvm-objcopy %t %t2
-# RUN: llvm-objcopy --only-keep-debug %t %t3
-# RUN: cmp %t2 %t3
-# RUN: llvm-strip --only-keep-debug --no-strip-all %t -o %t4
-# RUN: cmp %t2 %t4
-
-!ELF
-FileHeader:
-  Class:           ELFCLASS64
-  Data:            ELFDATA2LSB
-  Type:            ET_EXEC
-  Machine:         EM_X86_64
-Sections:
-  - Name:            .text
-    Type:            SHT_PROGBITS
-    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
-    Content:         "DEADBEEF"