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64 lines
2.4 KiB
64 lines
2.4 KiB
4 months ago
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Date: Fri, 1 Jun 2001 16:38:17 -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: Interesting: GCC passes
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Take a look at this document (which describes the order of optimizations
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that GCC performs):
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http://gcc.gnu.org/onlinedocs/gcc_17.html
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The rundown is that after RTL generation, the following happens:
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1 . [t] jump optimization (jumps to jumps, etc)
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2 . [t] Delete unreachable code
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3 . Compute live ranges for CSE
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4 . [t] Jump threading (jumps to jumps with identical or inverse conditions)
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5 . [t] CSE
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6 . *** Conversion to SSA
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7 . [t] SSA Based DCE
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8 . *** Conversion to LLVM
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9 . UnSSA
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10. GCSE
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11. LICM
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12. Strength Reduction
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13. Loop unrolling
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14. [t] CSE
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15. [t] DCE
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16. Instruction combination, register movement, scheduling... etc.
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I've marked optimizations with a [t] to indicate things that I believe to
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be relatively trivial to implement in LLVM itself. The time consuming
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things to reimplement would be SSA based PRE, Strength reduction & loop
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unrolling... these would be the major things we would miss out on if we
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did LLVM creation from tree code [inlining and other high level
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optimizations are done on the tree representation].
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Given the lack of "strong" optimizations that would take a long time to
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reimplement, I am leaning a bit more towards creating LLVM from the tree
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code. Especially given that SGI has GPL'd their compiler, including many
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SSA based optimizations that could be adapted (besides the fact that their
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code looks MUCH nicer than GCC :)
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Even if we choose to do LLVM code emission from RTL, we will almost
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certainly want to move LLVM emission from step 8 down until at least CSE
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has been rerun... which causes me to wonder if the SSA generation code
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will still work (due to global variable dependencies and stuff). I assume
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that it can be made to work, but might be a little more involved than we
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would like.
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I'm continuing to look at the Tree -> RTL code. It is pretty gross
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because they do some of the translation a statement at a time, and some
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of it a function at a time... I'm not quite clear why and how the
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distinction is drawn, but it does not appear that there is a wonderful
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place to attach extra info.
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Anyways, I'm proceeding with the RTL -> LLVM conversion phase for now. We
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can talk about this more on Monday.
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Wouldn't it be nice if there were a obvious decision to be made? :)
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-Chris
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