forked from OSchip/llvm-project
200 lines
9.7 KiB
Plaintext
200 lines
9.7 KiB
Plaintext
Date: Sun, 19 Nov 2000 16:23:57 -0600 (CST)
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From: Chris Lattner <sabre@nondot.org>
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To: Vikram Adve <vadve@cs.uiuc.edu>
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Subject: Re: a few thoughts
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Okay... here are a few of my thoughts on this (it's good to know that we
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think so alike!):
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> 1. We need to be clear on our goals for the VM. Do we want to emphasize
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> portability and safety like the Java VM? Or shall we focus on the
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> architecture interface first (i.e., consider the code generation and
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> processor issues), since the architecture interface question is also
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> important for portable Java-type VMs?
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I forsee the architecture looking kinda like this: (which is completely
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subject to change)
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1. The VM code is NOT guaranteed safe in a java sense. Doing so makes it
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basically impossible to support C like languages. Besides that,
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certifying a register based language as safe at run time would be a
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pretty expensive operation to have to do. Additionally, we would like
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to be able to statically eliminate many bounds checks in Java
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programs... for example.
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2. Instead, we can do the following (eventually):
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* Java bytecode is used as our "safe" representation (to avoid
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reinventing something that we don't add much value to). When the
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user chooses to execute Java bytecodes directly (ie, not
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precompiled) the runtime compiler can do some very simple
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transformations (JIT style) to convert it into valid input for our
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VM. Performance is not wonderful, but it works right.
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* The file is scheduled to be compiled (rigorously) at a later
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time. This could be done by some background process or by a second
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processor in the system during idle time or something...
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* To keep things "safe" ie to enforce a sandbox on Java/foreign code,
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we could sign the generated VM code with a host specific private
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key. Then before the code is executed/loaded, we can check to see if
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the trusted compiler generated the code. This would be much quicker
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than having to validate consistency (especially if bounds checks have
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been removed, for example)
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> This is important because the audiences for these two goals are very
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> different. Architects and many compiler people care much more about
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> the second question. The Java compiler and OS community care much more
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> about the first one.
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3. By focusing on a more low level virtual machine, we have much more room
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for value add. The nice safe "sandbox" VM can be provided as a layer
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on top of it. It also lets us focus on the more interesting compilers
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related projects.
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> 2. Design issues to consider (an initial list that we should continue
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> to modify). Note that I'm not trying to suggest actual solutions here,
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> but just various directions we can pursue:
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Understood. :)
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> a. A single-assignment VM, which we've both already been thinking
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> about.
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Yup, I think that this makes a lot of sense. I am still intrigued,
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however, by the prospect of a minimally allocated VM representation... I
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think that it could have definite advantages for certain applications
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(think very small machines, like PDAs). I don't, however, think that our
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initial implementations should focus on this. :)
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Here are some other auxiliary goals that I think we should consider:
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1. Primary goal: Support a high performance dynamic compilation
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system. This means that we have an "ideal" division of labor between
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the runtime and static compilers. Of course, the other goals of the
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system somewhat reduce the importance of this point (f.e. portability
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reduces performance, but hopefully not much)
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2. Portability to different processors. Since we are most familiar with
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x86 and solaris, I think that these two are excellent candidates when
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we get that far...
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3. Support for all languages & styles of programming (general purpose
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VM). This is the point that disallows java style bytecodes, where all
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array refs are checked for bounds, etc...
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4. Support linking between different language families. For example, call
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C functions directly from Java without using the nasty/slow/gross JNI
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layer. This involves several subpoints:
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A. Support for languages that require garbage collectors and integration
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with languages that don't. As a base point, we could insist on
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always using a conservative GC, but implement free as a noop, f.e.
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> b. A strongly-typed VM. One question is do we need the types to be
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> explicitly declared or should they be inferred by the dynamic
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> compiler?
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B. This is kind of similar to another idea that I have: make OOP
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constructs (virtual function tables, class heirarchies, etc) explicit
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in the VM representation. I believe that the number of additional
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constructs would be fairly low, but would give us lots of important
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information... something else that would/could be important is to
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have exceptions as first class types so that they would be handled in
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a uniform way for the entire VM... so that C functions can call Java
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functions for example...
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> c. How do we get more high-level information into the VM while keeping
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> to a low-level VM design?
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> o Explicit array references as operands? An alternative is
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> to have just an array type, and let the index computations be
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> separate 3-operand instructions.
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C. In the model I was thinking of (subject to change of course), we
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would just have an array type (distinct from the pointer
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types). This would allow us to have arbitrarily complex index
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expressions, while still distinguishing "load" from "Array load",
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for example. Perhaps also, switch jump tables would be first class
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types as well? This would allow better reasoning about the program.
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5. Support dynamic loading of code from various sources. Already
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mentioned above was the example of loading java bytecodes, but we want
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to support dynamic loading of VM code as well. This makes the job of
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the runtime compiler much more interesting: it can do interprocedural
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optimizations that the static compiler can't do, because it doesn't
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have all of the required information (for example, inlining from
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shared libraries, etc...)
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6. Define a set of generally useful annotations to add to the VM
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representation. For example, a function can be analysed to see if it
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has any sideeffects when run... also, the MOD/REF sets could be
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calculated, etc... we would have to determine what is reasonable. This
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would generally be used to make IP optimizations cheaper for the
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runtime compiler...
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> o Explicit instructions to handle aliasing, e.g.s:
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> -- an instruction to say "I speculate that these two values are not
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> aliased, but check at runtime", like speculative execution in
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> EPIC?
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> -- or an instruction to check whether two values are aliased and
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> execute different code depending on the answer, somewhat like
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> predicated code in EPIC
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These are also very good points... if this can be determined at compile
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time. I think that an epic style of representation (not the instruction
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packing, just the information presented) could be a very interesting model
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to use... more later...
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> o (This one is a difficult but powerful idea.)
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> A "thread-id" field on every instruction that allows the static
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> compiler to generate a set of parallel threads, and then have
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> the runtime compiler and hardware do what they please with it.
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> This has very powerful uses, but thread-id on every instruction
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> is expensive in terms of instruction size and code size.
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> We would need to compactly encode it somehow.
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Yes yes yes! :) I think it would be *VERY* useful to include this kind
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of information (which EPIC architectures *implicitly* encode. The trend
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that we are seeing supports this greatly:
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1. Commodity processors are getting massive SIMD support:
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* Intel/Amd MMX/MMX2
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* AMD's 3Dnow!
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* Intel's SSE/SSE2
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* Sun's VIS
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2. SMP is becoming much more common, especially in the server space.
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3. Multiple processors on a die are right around the corner.
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If nothing else, not designing this in would severely limit our future
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expansion of the project...
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> Also, this will require some reading on at least two other
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> projects:
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> -- Multiscalar architecture from Wisconsin
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> -- Simultaneous multithreading architecture from Washington
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>
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> o Or forget all this and stick to a traditional instruction set?
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Heh... :) Well, from a pure research point of view, it is almost more
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attactive to go with the most extreme/different ISA possible. On one axis
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you get safety and conservatism, and on the other you get degree of
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influence that the results have. Of course the problem with pure research
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is that often times there is no concrete product of the research... :)
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> BTW, on an unrelated note, after the meeting yesterday, I did remember
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> that you had suggested doing instruction scheduling on SSA form instead
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> of a dependence DAG earlier in the semester. When we talked about
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> it yesterday, I didn't remember where the idea had come from but I
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> remembered later. Just giving credit where its due...
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:) Thanks.
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> Perhaps you can save the above as a file under RCS so you and I can
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> continue to expand on this.
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I think it makes sense to do so when we get our ideas more formalized and
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bounce it back and forth a couple of times... then I'll do a more formal
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writeup of our goals and ideas. Obviously our first implementation will
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not want to do all of the stuff that I pointed out above... be we will
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want to design the project so that we do not artificially limit ourselves
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at sometime in the future...
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Anyways, let me know what you think about these ideas... and if they sound
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reasonable...
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-Chris
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