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1 Date: Sun, 19 Nov 2000 16:23:57 -0600 (CST)
2 From: Chris Lattner <sabre@nondot.org>
3 To: Vikram Adve <vadve@cs.uiuc.edu>
4 Subject: Re: a few thoughts
6 Okay... here are a few of my thoughts on this (it's good to know that we
7 think so alike!):
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?
15 I forsee the architecture looking kinda like this: (which is completely
16 subject to change)
18 1. 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.
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)
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.
47 3. 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.
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:
56 Understood. :)
58 > a. A single-assignment VM, which we've both already been thinking
59 > about.
61 Yup, I think that this makes a lot of sense. I am still intrigued,
62 however, by the prospect of a minimally allocated VM representation... I
63 think that it could have definite advantages for certain applications
64 (think very small machines, like PDAs). I don't, however, think that our
65 initial implementations should focus on this. :)
67 Here are some other auxiliary goals that I think we should consider:
69 1. 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)
74 2. 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...
77 3. 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...
80 4. 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.
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?
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...
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.
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.
113 5. 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...)
121 6. 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...
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
136 These are also very good points... if this can be determined at compile
137 time. I think that an epic style of representation (not the instruction
138 packing, just the information presented) could be a very interesting model
139 to use... more later...
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.
149 Yes yes yes! :) I think it would be *VERY* useful to include this kind
150 of information (which EPIC architectures *implicitly* encode. The trend
151 that we are seeing supports this greatly:
153 1. 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
158 2. SMP is becoming much more common, especially in the server space.
159 3. Multiple processors on a die are right around the corner.
161 If nothing else, not designing this in would severely limit our future
162 expansion of the project...
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?
171 Heh... :) Well, from a pure research point of view, it is almost more
172 attactive to go with the most extreme/different ISA possible. On one axis
173 you get safety and conservatism, and on the other you get degree of
174 influence that the results have. Of course the problem with pure research
175 is that often times there is no concrete product of the research... :)
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...
183 :) Thanks.
185 > Perhaps you can save the above as a file under RCS so you and I can
186 > continue to expand on this.
188 I think it makes sense to do so when we get our ideas more formalized and
189 bounce it back and forth a couple of times... then I'll do a more formal
190 writeup of our goals and ideas. Obviously our first implementation will
191 not want to do all of the stuff that I pointed out above... be we will
192 want to design the project so that we do not artificially limit ourselves
193 at sometime in the future...
195 Anyways, let me know what you think about these ideas... and if they sound
196 reasonable...
198 -Chris