These are in-progress notes for the upcoming LLVM 3.6 release. You may prefer the LLVM 3.5 Release Notes.
This document contains the release notes for the LLVM Compiler Infrastructure, release 3.6. Here we describe the status of LLVM, including major improvements from the previous release, improvements in various subprojects of LLVM, and some of the current users of the code. All LLVM releases may be downloaded from the LLVM releases web site.
For more information about LLVM, including information about the latest release, please check out the main LLVM web site. If you have questions or comments, the LLVM Developer’s Mailing List is a good place to send them.
Note that if you are reading this file from a Subversion checkout or the main LLVM web page, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.
The semantics of the prefix attribute have been changed. Users that want the previous prefix semantics should instead use prologue. To motivate this change, let’s examine the primary usecases that these attributes aim to serve,
- Code sanitization metadata (e.g. Clang’s undefined behavior sanitizer)
- Function hot-patching: Enable the user to insert nop operations at the beginning of the function which can later be safely replaced with a call to some instrumentation facility.
- Language runtime metadata: Allow a compiler to insert data for use by the runtime during execution. GHC is one example of a compiler that needs this functionality for its tables-next-to-code functionality.
Previously prefix served cases (1) and (2) quite well by allowing the user to introduce arbitrary data at the entrypoint but before the function body. Case (3), however, was poorly handled by this approach as it required that prefix data was valid executable code.
In this release the concept of prefix data has been redefined to be data which occurs immediately before the function entrypoint (i.e. the symbol address). Since prefix data now occurs before the function entrypoint, there is no need for the data to be valid code.
The previous notion of prefix data now goes under the name “prologue data” to emphasize its duality with the function epilogue.
The intention here is to handle cases (1) and (2) with prologue data and case (3) with prefix data. See the language reference for further details on the semantics of these attributes.
This refactoring arose out of discussions with Reid Kleckner in response to a proposal to introduce the notion of symbol offsets to enable handling of case (3).
An exciting aspect of LLVM is that it is used as an enabling technology for a lot of other language and tools projects. This section lists some of the projects that have already been updated to work with LLVM 3.6.
A wide variety of additional information is available on the LLVM web page, in particular in the documentation section. The web page also contains versions of the API documentation which is up-to-date with the Subversion version of the source code. You can access versions of these documents specific to this release by going into the llvm/docs/ directory in the LLVM tree.
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