2000-12-06-MeetingSummary.txt [plain text]
We met to discuss the LLVM instruction format and bytecode representation:
1. We decided that we shall use a flat namespace to represent our
variables in SSA form, as opposed to having a two dimensional namespace
of the original variable and the SSA instance subscript.
* A two dimensional namespace would be valuable when doing alias
analysis because the extra information can help limit the scope of
* Including this information would require that all users of the LLVM
bytecode would have to parse and handle it. This would slow down the
common case and inflate the instruction representation with another
infinite variable space.
* It was decided that because original variable sources could be
reconstructed from SSA form in linear time, that it would be an
unjustified expense for the common case to include the extra
information for one optimization. Alias analysis itself is typically
greater than linear in asymptotic complexity, so this extra analaysis
would not affect the runtime of the optimization in a significant
way. Additionally, this would be an unlikely optimization to do at
IDEAS TO CONSIDER
1. Including dominator information in the LLVM bytecode
representation. This is one example of an analysis result that may be
packaged with the bytecodes themselves. As a conceptual implementation
idea, we could include an immediate dominator number for each basic block
in the LLVM bytecode program. Basic blocks could be numbered according
to the order of occurrence in the bytecode representation.
2. Including loop header and body information. This would facilitate
detection of intervals and natural loops.
1. Will oSUIF provide enough of an infrastructure to support the research
that we will be doing? We know that it has less than stellar
performance, but hope that this will be of little importance for our
static compiler. This could affect us if we decided to do some IP
research. Also we do not yet understand the level of exception support
2. Should we consider the requirements of a direct hardware implementation
of the LLVM when we design it? If so, several design issues should
have their priorities shifted. The other option is to focus on a
software layer interpreting the LLVM in all cases.
3. Should we use some form of packetized format to improve forward
compatibility? For example, we could design the system to encode a
packet type and length field before analysis information, to allow a
runtime to skip information that it didn't understand in a bytecode
stream. The obvious benefit would be for compatibility, the drawback
is that it would tend to splinter that 'standard' LLVM definition.
4. Should we use fixed length instructions or variable length
instructions? Fetching variable length instructions is expensive (for
either hardware or software based LLVM runtimes), but we have several
'infinite' spaces that instructions operate in (SSA register numbers,
type spaces, or packet length [if packets were implemented]). Several
options were mentioned including:
A. Using 16 or 32 bit numbers, which would be 'big enough'
B. A scheme similar to how UTF-8 works, to encode infinite numbers
while keeping small number small.
C. Use something similar to Huffman encoding, so that the most common
numbers are the smallest.