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