mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-10-30 15:32:52 +01:00
84 lines
3.6 KiB
Plaintext
84 lines
3.6 KiB
Plaintext
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
|
|
|