docs/HistoricalNotes/2001-06-01-GCCOptimizations2.txt

Date: Fri, 1 Jun 2001 17:08:44 -0500 (CDT)
From: Chris Lattner <sabre@nondot.org>
To: Vikram S. Adve <vadve@cs.uiuc.edu>
Subject: RE: Interesting: GCC passes

> That is very interesting.  I agree that some of these could be done on LLVM
> at link-time, but it is the extra time required that concerns me.  Link-time
> optimization is severely time-constrained.

If we were to reimplement any of these optimizations, I assume that we
could do them a translation unit at a time, just as GCC does now.  This
would lead to a pipeline like this:

Static optimizations, xlation unit at a time:
.c --GCC--> .llvm --llvmopt--> .llvm 

Link time optimizations:
.llvm --llvm-ld--> .llvm --llvm-link-opt--> .llvm 

Of course, many optimizations could be shared between llvmopt and
llvm-link-opt, but the wouldn't need to be shared...  Thus compile time
could be faster, because we are using a "smarter" IR (SSA based).

> BTW, about SGI, "borrowing" SSA-based optimizations from one compiler and
> putting it into another is not necessarily easier than re-doing it.
> Optimization code is usually heavily tied in to the specific IR they use.

Understood.  The only reason that I brought this up is because SGI's IR is
more similar to LLVM than it is different in many respects (SSA based,
relatively low level, etc), and could be easily adapted.  Also their
optimizations are written in C++ and are actually somewhat
structured... of course it would be no walk in the park, but it would be
much less time consuming to adapt, say, SSA-PRE than to rewrite it.

> But your larger point is valid that adding SSA based optimizations is
> feasible and should be fun.  (Again, link time cost is the issue.)

Assuming linktime cost wasn't an issue, the question is: 
Does using GCC's backend buy us anything?

> It also occurs to me that GCC is probably doing quite a bit of back-end
> optimization (step 16 in your list).  Do you have a breakdown of that?

Not really.  The irritating part of GCC is that it mixes it all up and
doesn't have a clean seperation of concerns.  A lot of the "back end
optimization" happens right along with other data optimizations (ie, CSE
of machine specific things).

As far as REAL back end optimizations go, it looks something like this:

1. Instruction combination: try to make CISCy instructions, if available
2. Register movement: try to get registers in the right places for the
architecture to avoid register to register moves.  For example, try to get
the first argument of a function to naturally land in %o0 for sparc.
3. Instruction scheduling: 'nuff said :)
4. Register class preferencing: ??
5. Local register allocation
6. global register allocation
7. Spilling
8. Local regalloc
9. Jump optimization
10. Delay slot scheduling
11. Branch shorting for CISC machines
12. Instruction selection & peephole optimization
13. Debug info output

But none of this would be usable for LLVM anyways, unless we were using
GCC as a static compiler.

-Chris