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.

llvm-svn: 205990

1 files changed, 15 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCContext.cpp b/llvm/lib/MC/MCContext.cpp
index 9dfd6367ed7..8c2b5095304 100644
--- a/llvm/lib/MC/MCContext.cpp
+++ b/llvm/lib/MC/MCContext.cpp
@@ -251,6 +251,21 @@ getELFSection(StringRef Section, unsigned Type, unsigned Flags,
   return getELFSection(Section, Type, Flags, Kind, 0, "");
 }
 
+void MCContext::renameELFSection(const MCSectionELF *Section, StringRef Name) {
+  if (ELFUniquingMap == 0)
+    ELFUniquingMap = new ELFUniqueMapTy();
+  ELFUniqueMapTy &Map = *(ELFUniqueMapTy*)ELFUniquingMap;
+
+  StringRef GroupName;
+  if (const MCSymbol *Group = Section->getGroup())
+    GroupName = Group->getName();
+
+  Map.erase(SectionGroupPair(Section->getSectionName(), GroupName));
+  auto I = Map.insert(std::make_pair(SectionGroupPair(Name, GroupName),
+                                     Section)).first;
+  const_cast<MCSectionELF*>(Section)->setSectionName(I->first.first);
+}
+
 const MCSectionELF *MCContext::
 getELFSection(StringRef Section, unsigned Type, unsigned Flags,
               SectionKind Kind, unsigned EntrySize, StringRef Group) {