| 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 |
| |
