1Date: Sat, 18 Nov 2000 09:19:35 -0600 (CST)
2From: Vikram Adve <vadve@cs.uiuc.edu>
3To: Chris Lattner <lattner@cs.uiuc.edu>
4Subject: a few thoughts
5
6I've been mulling over the virtual machine problem and I had some
7thoughts about some things for us to think about discuss:
8
91. 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
15   This is important because the audiences for these two goals are very
16   different.  Architects and many compiler people care much more about
17   the second question.  The Java compiler and OS community care much more
18   about the first one.
19
20   Also, while the architecture interface question is important for
21   Java-type VMs, the design constraints are very different.
22
23
242. Design issues to consider (an initial list that we should continue
25   to modify).  Note that I'm not trying to suggest actual solutions here,
26   but just various directions we can pursue:
27
28   a. A single-assignment VM, which we've both already been thinking about.
29
30   b. A strongly-typed VM.  One question is do we need the types to be
31      explicitly declared or should they be inferred by the dynamic compiler?
32
33   c. How do we get more high-level information into the VM while keeping
34      to a low-level VM design?
35
36        o  Explicit array references as operands?  An alternative is
37           to have just an array type, and let the index computations be
38           separate 3-operand instructions.
39
40        o  Explicit instructions to handle aliasing, e.g.s:
41           -- an instruction to say "I speculate that these two values are not
42              aliased, but check at runtime", like speculative execution in
43              EPIC?
44           -- or an instruction to check whether two values are aliased and
45              execute different code depending on the answer, somewhat like
46              predicated code in EPIC
47
48        o  (This one is a difficult but powerful idea.)
49           A "thread-id" field on every instruction that allows the static
50           compiler to generate a set of parallel threads, and then have
51           the runtime compiler and hardware do what they please with it.
52           This has very powerful uses, but thread-id on every instruction
53           is expensive in terms of instruction size and code size.
54           We would need to compactly encode it somehow.
55
56           Also, this will require some reading on at least two other
57           projects:
58                -- Multiscalar architecture from Wisconsin
59                -- Simultaneous multithreading architecture from Washington
60
61        o  Or forget all this and stick to a traditional instruction set?
62
63
64BTW, on an unrelated note, after the meeting yesterday, I did remember
65that you had suggested doing instruction scheduling on SSA form instead
66of a dependence DAG earlier in the semester.  When we talked about
67it yesterday, I didn't remember where the idea had come from but I
68remembered later.  Just giving credit where its due...
69
70Perhaps you can save the above as a file under RCS so you and I can
71continue to expand on this.
72
73--Vikram
74
75