Reimplement debug info compression by compressing the whole section, rather than a fragment.

To support compressing the debug_line section that contains multiple
fragments (due, I believe, to variation in choices of line table
encoding depending on the size of instruction ranges in the actual
program code) we needed to support compressing multiple MCFragments in a
single pass.

This patch implements that behavior by mutating the post-relaxed and
relocated section to be the compressed form of its former self,
including renaming the section.

This is a more flexible (and less invasive, to a degree) implementation
that will allow for other features such as "use compression only if it's
smaller than the uncompressed data".

Compressing debug_frame would be a possible further extension to this
work, but I've left it for now. The hurdle there is alignment sections -
which might require going as far as to refactor
MCAssembler.cpp:writeFragment to handle writing to a byte buffer or an
MCObjectWriter (there's already a virtual call there, so it shouldn't
add substantial compile-time cost) which could in turn involve
refactoring MCAsmBackend::writeNopData to use that same abstraction...
which involves touching all the backends. This would remove the limited
handling of fragment writing seen in
ELFObjectWriter.cpp:getUncompressedData which would be nice - but it's
more invasive.

I did discover that I (perhaps obviously) don't need to handle
relocations when I rewrite the fragments - since the relocations have
already been applied and computed (and stored into
ELFObjectWriter::Relocations) by this stage (necessarily, because we
need to have written any immediate values or assembly-time relocations
into the data already before we compress it, which we have). The test
case doesn't necessarily cover that in detail - I can add more test
coverage if that's preferred.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205990 91177308-0d34-0410-b5e6-96231b3b80d8

test/MC/ELF/compression.s

@@ -1,25 +1,35 @@
-// RUN: llvm-mc -filetype=obj -compress-debug-sections -triple x86_64-pc-linux-gnu %s -o - | llvm-objdump -s - | FileCheck %s
-
-// XFAIL: *
+// RUN: llvm-mc -filetype=obj -compress-debug-sections -triple x86_64-pc-linux-gnu %s -o %t
+// RUN: llvm-objdump -s %t | FileCheck %s
+// RUN: llvm-dwarfdump -debug-dump=abbrev %t | FileCheck --check-prefix=ABBREV %s
 
 // REQUIRES: zlib
 
-// CHECK: Contents of section .debug_line:
-// FIXME: Figure out how to handle debug_line that currently uses multiple section fragments
+// CHECK: Contents of section .zdebug_line:
+// Check for the 'ZLIB' file magic at the start of the section only
+// CHECK-NEXT: ZLIB
 // CHECK-NOT: ZLIB
+// CHECK: Contents of
 
 // CHECK: Contents of section .zdebug_abbrev:
-// Check for the 'ZLIB' file magic at the start of the section
 // CHECK-NEXT: ZLIB
 
-// We shouldn't compress the debug_frame section, since it can be relaxed
-// CHECK: Contents of section .debug_frame
+// FIXME: Handle compressing alignment fragments to support compressing debug_frame
+// CHECK: Contents of section .debug_frame:
 // CHECK-NOT: ZLIB
+// CHECK: Contents of
+
+// Decompress one valid dwarf section just to check that this roundtrips
+// ABBREV: Abbrev table for offset: 0x00000000
+// ABBREV: [1] DW_TAG_compile_unit DW_CHILDREN_no
 
 	.section	.debug_line,"",@progbits
 
 	.section	.debug_abbrev,"",@progbits
 	.byte	1                       # Abbreviation Code
+	.byte	17                      # DW_TAG_compile_unit
+	.byte	0                       # DW_CHILDREN_no
+	.byte	0                       # EOM(1)
+	.byte	0                       # EOM(2)
 	.text
 foo:
 	.cfi_startproc