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

2 files changed, 4 insertions, 0 deletions

diff --git a/include/llvm/MC/MCContext.h b/include/llvm/MC/MCContext.h
index 5ae27a9104..ea0217693d 100644
--- a/include/llvm/MC/MCContext.h
+++ b/include/llvm/MC/MCContext.h
@@ -259,6 +259,8 @@ namespace llvm {
                                       unsigned Flags, SectionKind Kind,
                                       unsigned EntrySize, StringRef Group);
 
+    void renameELFSection(const MCSectionELF *Section, StringRef Name);
+
     const MCSectionELF *CreateELFGroupSection();
 
     const MCSectionCOFF *getCOFFSection(StringRef Section,
diff --git a/include/llvm/MC/MCSectionELF.h b/include/llvm/MC/MCSectionELF.h
index 89c02cce1d..c161c8bf4b 100644
--- a/include/llvm/MC/MCSectionELF.h
+++ b/include/llvm/MC/MCSectionELF.h
@@ -53,6 +53,8 @@ private:
     : MCSection(SV_ELF, K), SectionName(Section), Type(type), Flags(flags),
       EntrySize(entrySize), Group(group) {}
   ~MCSectionELF();
+
+  void setSectionName(StringRef Name) { SectionName = Name; }
 public:
 
   /// ShouldOmitSectionDirective - Decides whether a '.section' directive