| Date: Fri, 1 Jun 2001 16:38:17 -0500 (CDT) | |
| From: Chris Lattner <sabre@nondot.org> | |
| To: Vikram S. Adve <vadve@cs.uiuc.edu> | |
| Subject: Interesting: GCC passes | |
| Take a look at this document (which describes the order of optimizations | |
| that GCC performs): | |
| http://gcc.gnu.org/onlinedocs/gcc_17.html | |
| The rundown is that after RTL generation, the following happens: | |
| 1 . [t] jump optimization (jumps to jumps, etc) | |
| 2 . [t] Delete unreachable code | |
| 3 . Compute live ranges for CSE | |
| 4 . [t] Jump threading (jumps to jumps with identical or inverse conditions) | |
| 5 . [t] CSE | |
| 6 . *** Conversion to SSA | |
| 7 . [t] SSA Based DCE | |
| 8 . *** Conversion to LLVM | |
| 9 . UnSSA | |
| 10. GCSE | |
| 11. LICM | |
| 12. Strength Reduction | |
| 13. Loop unrolling | |
| 14. [t] CSE | |
| 15. [t] DCE | |
| 16. Instruction combination, register movement, scheduling... etc. | |
| I've marked optimizations with a [t] to indicate things that I believe to | |
| be relatively trivial to implement in LLVM itself. The time consuming | |
| things to reimplement would be SSA based PRE, Strength reduction & loop | |
| unrolling... these would be the major things we would miss out on if we | |
| did LLVM creation from tree code [inlining and other high level | |
| optimizations are done on the tree representation]. | |
| Given the lack of "strong" optimizations that would take a long time to | |
| reimplement, I am leaning a bit more towards creating LLVM from the tree | |
| code. Especially given that SGI has GPL'd their compiler, including many | |
| SSA based optimizations that could be adapted (besides the fact that their | |
| code looks MUCH nicer than GCC :) | |
| Even if we choose to do LLVM code emission from RTL, we will almost | |
| certainly want to move LLVM emission from step 8 down until at least CSE | |
| has been rerun... which causes me to wonder if the SSA generation code | |
| will still work (due to global variable dependencies and stuff). I assume | |
| that it can be made to work, but might be a little more involved than we | |
| would like. | |
| I'm continuing to look at the Tree -> RTL code. It is pretty gross | |
| because they do some of the translation a statement at a time, and some | |
| of it a function at a time... I'm not quite clear why and how the | |
| distinction is drawn, but it does not appear that there is a wonderful | |
| place to attach extra info. | |
| Anyways, I'm proceeding with the RTL -> LLVM conversion phase for now. We | |
| can talk about this more on Monday. | |
| Wouldn't it be nice if there were a obvious decision to be made? :) | |
| -Chris | |