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diff --git a/docs/HistoricalNotes/2000-12-06-MeetingSummary.txt b/docs/HistoricalNotes/2000-12-06-MeetingSummary.txt new file mode 100644 index 0000000000..b66e18556f --- /dev/null +++ b/docs/HistoricalNotes/2000-12-06-MeetingSummary.txt @@ -0,0 +1,83 @@ +SUMMARY +------- + +We met to discuss the LLVM instruction format and bytecode representation: + +ISSUES RESOLVED +--------------- + +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. + +ARGUMENT AGAINST: + * A two dimensional namespace would be valuable when doing alias + analysis because the extra information can help limit the scope of + analysis. + +ARGUMENT FOR: + * 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. + +REASONING: + * 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 + runtime. + + +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 occurance in the bytecode representation. + +2. Including loop header and body information. This would facilitate + detection of intervals and natural loops. + +UNRESOLVED ISSUES +----------------- + +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 + currently implemented. + +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. + +-Chris + |