[llvm-commits] Proposal/patch: Enable bitcode streaming

[Chris Lattner]

Hi Derek,

Have you measured the performance impact of this patch on the non-streaming case?  How much slower does "llvm-dis -disable-output foo.bc" go (with a release build)?

-Chris

On Nov 9, 2011, at 4:57 PM, Derek Schuff wrote:

> Hello all,
> The following is a proposal (and a prototype patch) to enable bitcode streaming. The overall goal is to be able to overlap bitcode reading/download with compilation, a functionality useful obviously for pnacl and renderscript but also potentially for any situation where the interface between the frontend and backend is something other than a file. 
> 
> In the current state of the world, at a high level, there are 2 things keeping this from happening. The first is that BitcodeReader construction takes a MemoryBuffer which it expects to be filled with bitcode, and inside BitcodeReader, the BitstreamCursor (which is the primary interface to the bitcode itself) gets pointers to the bitcode in memory, and does all of its magic with pointer arithmetic. The second issue is that in BitcodeReader::ParseModule (which is run when right after the Module and BitcodeReader objects are created), the reader makes a pass over the entire bitcode file. This step does everything except read the function bodies, but it records the bit locations of each function for future materialization.
> 
> High-level change description:
> This patch creates a class called BitcodeStream, which is a very simple interface with one method (GetBytes), which fetches the requested number of bytes from the stream, and writes them into the requested destination. This method may block the calling thread if there are not yet enough bytes available in the stream buffer (similarly to a stdin or socket read).
> 
> The first issue above is addressed by introducing the BitstreamVector, an abstraction that wraps the bitcode in memory. Instead of using pointers, the BitstreamCursor uses indices and gets bitcode bytes by indexing (i.e. operator[] ) the BitstreamVector. When streaming is not used, the BitstreamVector itself keeps pointers to the start and end of the backing MemoryBuffer and the indexing operator is just a pointer dereference. For streaming use, the BitstreamVector has a BitcodeStream object. If a byte is requested that has not yet been fetched, it calls GetBytes to get more, until it has enough to return the requested byte.
> This model of allowing any byte to be requested and blocking the caller has the advantage that there is no structural/architectural change required at this lowest level, nor at the high level (A FunctionPassManager is used to iterate over all the functions and compile each one).
> 
> The second issue is solved by 2 simple changes. The first is in ParseModule. Instead of a single pass over all the bitcode, ParseModule becomes resumable.  ParseModule will do its normal handling for top-level records, type table blocks, metadata, etc, but if streaming is in use, it will save its state and return as soon as a function subblock is encountered (rather than saving its location and skipping over it). Each subsequent time it is called, it bookmarks and skips one function block. Later, when a function needs to be materialized, if the function body has been seen already, then materialization is the same as before. Otherwise, Materialize will keep calling ParseModule (each time bookmarking and skipping one function body) until the requested function is found. The one other change required to make this work simply is that the bitcode writer writes function bodies as the last subblock (currently the attachment metadata and value symbol table are written after the function bodies).
> 
> The prototype patch is attached and can also be viewed online at http://codereview.chromium.org/8393017/ . Feedback is welcome, as well as guidance from the relevant code owners/reviewers regarding what the next step needs to be toward committing this.
> 
> Thanks,
> -Derek
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