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LTO: introduce object file-based on-disk module format.

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This format is simply a regular object file with the bitcode stored in a
section named ".llvmbc", plus any number of other (non-allocated) sections.

One immediate use case for this is to accommodate compilation processes
which expect the object file to contain metadata in non-allocated sections,
such as the ".go_export" section used by some Go compilers [1], although I
imagine that in the future we could consider compiling parts of the module
(such as large non-inlinable functions) directly into the object file to
improve LTO efficiency.

[1] http://golang.org/doc/install/gccgo#Imports

Differential Revision: http://reviews.llvm.org/D4371

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218078 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Peter Collingbourne
2014-09-18 21:28:49 +00:00
parent 65edced76b
commit 394be6c159
15 changed files with 178 additions and 25 deletions
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docs/BitCodeFormat.rst
+16 -2
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@@ -28,8 +28,9 @@ Unlike XML, the bitstream format is a binary encoding, and unlike XML it
provides a mechanism for the file to self-describe "abbreviations", which are
effectively size optimizations for the content.
LLVM IR files may be optionally embedded into a `wrapper`_ structure that makes
it easy to embed extra data along with LLVM IR files.
LLVM IR files may be optionally embedded into a `wrapper`_ structure, or in a
`native object file`_. Both of these mechanisms make it easy to embed extra
data along with LLVM IR files.
This document first describes the LLVM bitstream format, describes the wrapper
format, then describes the record structure used by LLVM IR files.
@@ -460,6 +461,19 @@ to the start of the bitcode stream in the file, and the Size field is the size
in bytes of the stream. CPUType is a target-specific value that can be used to
encode the CPU of the target.
.. _native object file:
Native Object File Wrapper Format
=================================
Bitcode files for LLVM IR may also be wrapped in a native object file
(i.e. ELF, COFF, Mach-O). The bitcode must be stored in a section of the
object file named ``.llvmbc``. This wrapper format is useful for accommodating
LTO in compilation pipelines where intermediate objects must be native object
files which contain metadata in other sections.
Not all tools support this format.
.. _encoding of LLVM IR:
LLVM IR Encoding