1 Date: Tue, 13 Feb 2001 18:25:42 -0600 2 From: Vikram S. Adve <vadve (a] cs.uiuc.edu> 3 To: Chris Lattner <sabre (a] nondot.org> 4 Subject: RE: LLVM Concerns... 5 6 > 1. Reference types 7 > Right now, I've spec'd out the language to have a pointer type, which 8 > works fine for lots of stuff... except that Java really has 9 > references: constrained pointers that cannot be manipulated: added and 10 > subtracted, moved, etc... Do we want to have a type like this? It 11 > could be very nice for analysis (pointer always points to the start of 12 > an object, etc...) and more closely matches Java semantics. The 13 > pointer type would be kept for C++ like semantics. Through analysis, 14 > C++ pointers could be promoted to references in the LLVM 15 > representation. 16 17 18 You're right, having references would be useful. Even for C++ the *static* 19 compiler could generate references instead of pointers with fairly 20 straightforward analysis. Let's include a reference type for now. But I'm 21 also really concerned that LLVM is becoming big and complex and (perhaps) 22 too high-level. After we get some initial performance results, we may have 23 a clearer idea of what our goals should be and we should revisit this 24 question then. 25 26 > 2. Our "implicit" memory references in assembly language: 27 > After thinking about it, this model has two problems: 28 > A. If you do pointer analysis and realize that two stores are 29 > independent and can share the same memory source object, 30 31 not sure what you meant by "share the same memory source object" 32 33 > there is 34 > no way to represent this in either the bytecode or assembly. 35 > B. When parsing assembly/bytecode, we effectively have to do a full 36 > SSA generation/PHI node insertion pass to build the dependencies 37 > when we don't want the "pinned" representation. This is not 38 > cool. 39 40 I understand the concern. But again, let's focus on the performance first 41 and then look at the language design issues. E.g., it would be good to know 42 how big the bytecode files are before expanding them further. I am pretty 43 keen to explore the implications of LLVM for mobile devices. Both bytecode 44 size and power consumption are important to consider there. 45 46 --Vikram 47 48
