/3rd_party/llvm/docs/HistoricalNotes/2000-11-18-EarlyDesignIdeasResp.txt

  1Date: Sun, 19 Nov 2000 16:23:57 -0600 (CST)
  2From: Chris Lattner <sabre@nondot.org>
  3To: Vikram Adve <vadve@cs.uiuc.edu>
  4Subject: Re: a few thoughts
  5
  6Okay... here are a few of my thoughts on this (it's good to know that we
  7think so alike!):
  8
  9> 1. We need to be clear on our goals for the VM.  Do we want to emphasize
 10>    portability and safety like the Java VM?  Or shall we focus on the
 11>    architecture interface first (i.e., consider the code generation and
 12>    processor issues), since the architecture interface question is also
 13>    important for portable Java-type VMs?
 14
 15I forsee the architecture looking kinda like this: (which is completely
 16subject to change)
 17
 181. The VM code is NOT guaranteed safe in a java sense.  Doing so makes it
 19   basically impossible to support C like languages.  Besides that,
 20   certifying a register based language as safe at run time would be a
 21   pretty expensive operation to have to do.  Additionally, we would like
 22   to be able to statically eliminate many bounds checks in Java
 23   programs... for example.
 24
 25 2. Instead, we can do the following (eventually): 
 26   * Java bytecode is used as our "safe" representation (to avoid
 27     reinventing something that we don't add much value to).  When the
 28     user chooses to execute Java bytecodes directly (ie, not
 29     precompiled) the runtime compiler can do some very simple
 30     transformations (JIT style) to convert it into valid input for our
 31     VM.  Performance is not wonderful, but it works right.
 32   * The file is scheduled to be compiled (rigorously) at a later
 33     time.  This could be done by some background process or by a second
 34     processor in the system during idle time or something...
 35   * To keep things "safe" ie to enforce a sandbox on Java/foreign code,
 36     we could sign the generated VM code with a host specific private
 37     key.  Then before the code is executed/loaded, we can check to see if
 38     the trusted compiler generated the code.  This would be much quicker
 39     than having to validate consistency (especially if bounds checks have
 40     been removed, for example)
 41
 42>    This is important because the audiences for these two goals are very
 43>    different.  Architects and many compiler people care much more about
 44>    the second question.  The Java compiler and OS community care much more
 45>    about the first one.
 46
 473. By focusing on a more low level virtual machine, we have much more room
 48   for value add.  The nice safe "sandbox" VM can be provided as a layer
 49   on top of it.  It also lets us focus on the more interesting compilers
 50   related projects.
 51
 52> 2. Design issues to consider (an initial list that we should continue
 53>    to modify).  Note that I'm not trying to suggest actual solutions here,
 54>    but just various directions we can pursue:
 55
 56Understood.  :)
 57
 58>    a. A single-assignment VM, which we've both already been thinking
 59>       about.
 60
 61Yup, I think that this makes a lot of sense.  I am still intrigued,
 62however, by the prospect of a minimally allocated VM representation... I
 63think that it could have definite advantages for certain applications
 64(think very small machines, like PDAs).  I don't, however, think that our
 65initial implementations should focus on this.  :)
 66
 67Here are some other auxiliary goals that I think we should consider:
 68
 691. Primary goal: Support a high performance dynamic compilation
 70   system.  This means that we have an "ideal" division of labor between
 71   the runtime and static compilers.  Of course, the other goals of the
 72   system somewhat reduce the importance of this point (f.e. portability
 73   reduces performance, but hopefully not much)
 742. Portability to different processors.  Since we are most familiar with
 75   x86 and solaris, I think that these two are excellent candidates when
 76   we get that far...
 773. Support for all languages & styles of programming (general purpose
 78   VM).  This is the point that disallows java style bytecodes, where all
 79   array refs are checked for bounds, etc...
 804. Support linking between different language families.  For example, call
 81   C functions directly from Java without using the nasty/slow/gross JNI
 82   layer.  This involves several subpoints:
 83  A. Support for languages that require garbage collectors and integration
 84     with languages that don't.  As a base point, we could insist on
 85     always using a conservative GC, but implement free as a noop, f.e.
 86
 87>    b. A strongly-typed VM.  One question is do we need the types to be
 88>       explicitly declared or should they be inferred by the dynamic
 89>       compiler?
 90
 91  B. This is kind of similar to another idea that I have: make OOP
 92     constructs (virtual function tables, class heirarchies, etc) explicit
 93     in the VM representation.  I believe that the number of additional
 94     constructs would be fairly low, but would give us lots of important
 95     information... something else that would/could be important is to
 96     have exceptions as first class types so that they would be handled in
 97     a uniform way for the entire VM... so that C functions can call Java
 98     functions for example...
 99
100>    c. How do we get more high-level information into the VM while keeping
101>       to a low-level VM design?
102>       o  Explicit array references as operands?  An alternative is
103>          to have just an array type, and let the index computations be
104>          separate 3-operand instructions.
105
106   C. In the model I was thinking of (subject to change of course), we
107      would just have an array type (distinct from the pointer
108      types).  This would allow us to have arbitrarily complex index
109      expressions, while still distinguishing "load" from "Array load",
110      for example.  Perhaps also, switch jump tables would be first class
111      types as well?  This would allow better reasoning about the program.
112
1135. Support dynamic loading of code from various sources.  Already
114   mentioned above was the example of loading java bytecodes, but we want
115   to support dynamic loading of VM code as well.  This makes the job of
116   the runtime compiler much more interesting:  it can do interprocedural
117   optimizations that the static compiler can't do, because it doesn't
118   have all of the required information (for example, inlining from
119   shared libraries, etc...)
120
1216. Define a set of generally useful annotations to add to the VM
122   representation.  For example, a function can be analysed to see if it
123   has any sideeffects when run... also, the MOD/REF sets could be
124   calculated, etc... we would have to determine what is reasonable.  This
125   would generally be used to make IP optimizations cheaper for the
126   runtime compiler...
127
128>       o  Explicit instructions to handle aliasing, e.g.s:
129>            -- an instruction to say "I speculate that these two values are not
130>               aliased, but check at runtime", like speculative execution in
131>             EPIC?
132>          -- or an instruction to check whether two values are aliased and
133>             execute different code depending on the answer, somewhat like
134>             predicated code in EPIC
135
136These are also very good points... if this can be determined at compile
137time.  I think that an epic style of representation (not the instruction
138packing, just the information presented) could be a very interesting model
139to use... more later...
140
141>         o  (This one is a difficult but powerful idea.)
142>          A "thread-id" field on every instruction that allows the static
143>          compiler to generate a set of parallel threads, and then have
144>          the runtime compiler and hardware do what they please with it.
145>          This has very powerful uses, but thread-id on every instruction
146>          is expensive in terms of instruction size and code size.
147>          We would need to compactly encode it somehow.
148
149Yes yes yes!  :)  I think it would be *VERY* useful to include this kind
150of information (which EPIC architectures *implicitly* encode.  The trend
151that we are seeing supports this greatly:
152
1531. Commodity processors are getting massive SIMD support:
154   * Intel/Amd MMX/MMX2
155   * AMD's 3Dnow!
156   * Intel's SSE/SSE2
157   * Sun's VIS
1582. SMP is becoming much more common, especially in the server space.
1593. Multiple processors on a die are right around the corner.
160
161If nothing else, not designing this in would severely limit our future
162expansion of the project...
163
164>          Also, this will require some reading on at least two other
165>          projects:
166>               -- Multiscalar architecture from Wisconsin
167>               -- Simultaneous multithreading architecture from Washington
168>
169>       o  Or forget all this and stick to a traditional instruction set?
170
171Heh... :)  Well, from a pure research point of view, it is almost more
172attactive to go with the most extreme/different ISA possible.  On one axis
173you get safety and conservatism, and on the other you get degree of
174influence that the results have.  Of course the problem with pure research
175is that often times there is no concrete product of the research... :)
176
177> BTW, on an unrelated note, after the meeting yesterday, I did remember
178> that you had suggested doing instruction scheduling on SSA form instead
179> of a dependence DAG earlier in the semester.  When we talked about
180> it yesterday, I didn't remember where the idea had come from but I
181> remembered later.  Just giving credit where its due...
182
183:) Thanks.  
184
185> Perhaps you can save the above as a file under RCS so you and I can
186> continue to expand on this.
187
188I think it makes sense to do so when we get our ideas more formalized and
189bounce it back and forth a couple of times... then I'll do a more formal
190writeup of our goals and ideas.  Obviously our first implementation will
191not want to do all of the stuff that I pointed out above... be we will
192want to design the project so that we do not artificially limit ourselves
193at sometime in the future...
194
195Anyways, let me know what you think about these ideas... and if they sound
196reasonable...
197
198-Chris
199