mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-10-30 07:22:55 +01:00
64 lines
2.4 KiB
Plaintext
64 lines
2.4 KiB
Plaintext
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
|
|
|