1 Date: Sun, 12 May 2002 17:12:53 -0500 (CDT) 2 From: Chris Lattner <sabre (a] nondot.org> 3 To: "Vikram S. Adve" <vadve (a] cs.uiuc.edu> 4 Subject: LLVM change 5 6 There is a fairly fundemental change that I would like to make to the LLVM 7 infrastructure, but I'd like to know if you see any drawbacks that I 8 don't... 9 10 Basically right now at the basic block level, each basic block contains an 11 instruction list (returned by getInstList()) that is a ValueHolder of 12 instructions. To iterate over instructions, we must actually iterate over 13 the instlist, and access the instructions through the instlist. 14 15 To add or remove an instruction from a basic block, we need to get an 16 iterator to an instruction, which, given just an Instruction*, requires a 17 linear search of the basic block the instruction is contained in... just 18 to insert an instruction before another instruction, or to delete an 19 instruction! This complicates algorithms that should be very simple (like 20 simple constant propagation), because they aren't actually sparse anymore, 21 they have to traverse basic blocks to remove constant propogated 22 instructions. 23 24 Additionally, adding or removing instructions to a basic block 25 _invalidates all iterators_ pointing into that block, which is really 26 irritating. 27 28 To fix these problems (and others), I would like to make the ordering of 29 the instructions be represented with a doubly linked list in the 30 instructions themselves, instead of an external data structure. This is 31 how many other representations do it, and frankly I can't remember why I 32 originally implemented it the way I did. 33 34 Long term, all of the code that depends on the nasty features in the 35 instruction list (which can be found by grep'ing for getInstList()) will 36 be changed to do nice local transformations. In the short term, I'll 37 change the representation, but preserve the interface (including 38 getInstList()) so that all of the code doesn't have to change. 39 40 Iteration over the instructions in a basic block remains the simple: 41 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) ... 42 43 But we will also support: 44 for (Instruction *I = BB->front(); I; I = I->getNext()) ... 45 46 After converting instructions over, I'll convert basic blocks and 47 functions to have a similar interface. 48 49 The only negative aspect of this change that I see is that it increases 50 the amount of memory consumed by one pointer per instruction. Given the 51 benefits, I think this is a very reasonable tradeoff. 52 53 What do you think? 54 55 -Chris 56
