1 Date: Sat, 18 Nov 2000 09:19:35 -0600 (CST) 2 From: Vikram Adve <vadve (a] cs.uiuc.edu> 3 To: Chris Lattner <lattner (a] cs.uiuc.edu> 4 Subject: a few thoughts 5 6 I've been mulling over the virtual machine problem and I had some 7 thoughts about some things for us to think about discuss: 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 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 24 2. 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 64 BTW, on an unrelated note, after the meeting yesterday, I did remember 65 that you had suggested doing instruction scheduling on SSA form instead 66 of a dependence DAG earlier in the semester. When we talked about 67 it yesterday, I didn't remember where the idea had come from but I 68 remembered later. Just giving credit where its due... 69 70 Perhaps you can save the above as a file under RCS so you and I can 71 continue to expand on this. 72 73 --Vikram 74 75
