forked from OSchip/llvm-project
56 lines
2.4 KiB
Plaintext
56 lines
2.4 KiB
Plaintext
Date: Sun, 12 May 2002 17:12:53 -0500 (CDT)
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From: Chris Lattner <sabre@nondot.org>
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To: "Vikram S. Adve" <vadve@cs.uiuc.edu>
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Subject: LLVM change
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There is a fairly fundemental change that I would like to make to the LLVM
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infrastructure, but I'd like to know if you see any drawbacks that I
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don't...
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Basically right now at the basic block level, each basic block contains an
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instruction list (returned by getInstList()) that is a ValueHolder of
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instructions. To iterate over instructions, we must actually iterate over
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the instlist, and access the instructions through the instlist.
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To add or remove an instruction from a basic block, we need to get an
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iterator to an instruction, which, given just an Instruction*, requires a
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linear search of the basic block the instruction is contained in... just
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to insert an instruction before another instruction, or to delete an
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instruction! This complicates algorithms that should be very simple (like
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simple constant propogation), because they aren't actually sparse anymore,
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they have to traverse basic blocks to remove constant propogated
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instructions.
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Additionally, adding or removing instructions to a basic block
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_invalidates all iterators_ pointing into that block, which is really
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irritating.
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To fix these problems (and others), I would like to make the ordering of
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the instructions be represented with a doubly linked list in the
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instructions themselves, instead of an external data structure. This is
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how many other representations do it, and frankly I can't remember why I
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originally implemented it the way I did.
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Long term, all of the code that depends on the nasty features in the
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instruction list (which can be found by grep'ing for getInstList()) will
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be changed to do nice local transformations. In the short term, I'll
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change the representation, but preserve the interface (including
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getInstList()) so that all of the code doesn't have to change.
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Iteration over the instructions in a basic block remains the simple:
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) ...
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But we will also support:
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for (Instruction *I = BB->front(); I; I = I->getNext()) ...
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After converting instructions over, I'll convert basic blocks and
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functions to have a similar interface.
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The only negative aspect of this change that I see is that it increases
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the amount of memory consumed by one pointer per instruction. Given the
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benefits, I think this is a very reasonable tradeoff.
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What do you think?
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-Chris
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