The Future of Compiler Optimization
blog.regehr.org
blog.regehr.org
Now while, LLVM may not do this, it's far from the only compiler on the block; especially in the non-open-source world.
The IR argument is a non-argument. Certain optimizations work better with certain IRs than others, that's just the way it is. It's unlikely that there will be one to rule them all. More likely, we'll convert between them to use the optimal one.
Superoptimizers are an interesting case, it's kind of like having a database of optimality. While it may not give much speedup compared to time invested, if you're desperate for that last little bit of performance, it's a great tool. Plus it should make itself faster over time. Could even locate it in the cloud, such that a compiler send its hottest methods over to a remote machine, and it'll use its massive database of optimality to optimize it as best as possible.
Verifiable transformations are great in theory, but it's true, it's extremely hard to implement currently.
The author definitely has a point about AOT/JIT compilers being the future of optimization. Static compilers have been done to death, while dynamic compilers are still (comparatively) new and fresh.
Non-open-source compilers don't get much press around here. Could you point out the most interesting ones?
In general, the Microsoft and Intel C++ compilers have pretty incredible performance on the x86 platform. The last time I talked to the Intel folks, they didn't really consider GCC to be competitive. It's possible things have changed in the last five years on that front; apologies if I'm misrepresenting the state of GCC optimization quality.
The PGI Fortran folks are pretty incredible in the parallel space.
On embedded platforms, most serious companies seem to buy the Green Hills C++ compiler.
One of the program-management types from the Visual C++ team could probably do this much better justice than my quickly-fading memories. But, I think the HN crowd would be quite shocked by the market share that commercial compilers have, even on *NIX platforms.
gcc is "free" but is it free enough to double the bill for hardware? Benchmark and find out...
I measure something as simple as a loop in which some short floating point calculations is made (actually only two additions per loop and one FP comparison) and my measurements on the latest Intel i5 give (in seconds):
0.31 VS2010 -O2 SSE2
0.35 VC 6 -O2
0.44 cygwin gcc-4.3.2 -O2
0.95 cygwin gcc-3.4.4 -O2
I haven't tested the later 4.x gcc but the results are clear -- even VC 6, 12 years old, is better than a quite recent gcc, at least for FPU calculations.
(It actually is the default for linux and darwin now, because DWARF unwind tables obsolete frame pointers for most uses.)
gcc supports profile-guided optimization just fine, llvm has some code for it but I'm not sure if it's hooked up. Neither of them use iterative techniques for optimization - they're already too slow as it is for most people, anyway.
There was some other stuff, but I cannot remember (I am actually glad to have been able to come up with IPS.)
Instruction scheduling of any kind doesn't really help on x86 anyway, and register pressure is usually surprisingly good already (since temporary values are moved close to their uses when combining instructions). I think the most important thing missing thing is rematerialization - recalculating values instead of saving them on stack would save a lot of memory loads.
The difficult (compute intensive) part is training the database, and you only have to do that once for each optimization...
I am a little confused by why he stated that the results of the super optimizer were not very impressive. Presumably, man-years have been invested in defining optimizations for compilers like GCC and the intel C compiler, by human experts...
To have a machine that even gets close to those results is impressive to me. If the technique were used for an actual compiler, it would mean that no one would have to write peephole optimizations, instead, the compiler would just get better and better as it ages.
Combined with aggressive in-lining (or maybe dynamic recompilation? e.x. lisp style function compilation rather than C), it seems to me that this sort of technology could be incredibly powerful. Am I wrong?
Just tell me what optimization could learn from verification? Everything verification does is too slow to do within a compiler. Sure, you can produce better code, if you are allowed to use NP algorithms, but in reality i have never seen any algorithm more complex than O(n^2) in a compiler. Usually O(nlogn) is the most one can afford.
It would also be rather useful, although probably quite a bit of work, to have a "verify the optimizations you did" mode. While most errors aren't compiler errors, gcc has been known to optimize bugs into programs...
Another candidate for the most advanced compiler is Stalin.
* LLVM is probably the Open-Source compiler which fits your intention most.
* GCC has man-years of efforts for little bit/byte tweaks, which no other compiler does. This is not advanced, but tedious.
* libFirm is the only compiler which does not deconstruct SSA form. This is advanced in a mostly academic sense.
* GHC is probably the best functional (CPS) compiler.
* Sun JVM is probably the best Hotspot (online) compiler.
* I'm not sure which compiler is "most advanced" for dynamically typed code. Some Smalltalk or Javascript compiler? PyPy?
When do we expect Hitler, Churchill and Roosevelt?
libFirm is the only compiler which does not deconstruct SSA form
I thought that continuation-passing-style compilers didn't use SSA, and that most Scheme implementations were CPS. No?
In SSA form most operations (add,sub,...) are side-effect free and the rest (load,store,call,...) can be understood as using something like a "Memory Monad".
* http://ctuning.org/wiki/index.php/CTools:MilepostGCC
Smalltalk JITs are pretty darn good, e.g., the Strongtalk project is known to be pretty fast. As of 2006, Sun released the Strongtalk source code. Its publication record is relatively weak (aside of type system related publications), but it contains a wealth of relevant optimizations and I am sure the interested reader/programmer will find something valuable in there. (http://www.strongtalk.org)
Eliot Miranda has been implementing Smalltalk VMs for quite a while, and I think his recent addition ("Cog") to the Squeak implementation is probably the most recent addition to JIT compilers for Smalltalk VMs. Given his in-depth experience and expertise (particularly with inline caching), this could probably serve as a blueprint for other (Smalltalk) JITs.
V8 for Javascript is supposedly very fast (interesting side information: Robert Griesemer is working on V8, but worked on the Strongtalk interpreter before), but I don't know about the involved benchmarks, and how they stack up against each other--particularly since the TraceMonkey trace-based JITs came along.
Mike Pall's LuaJIT is a very interesting project (only one-man JIT project I know of), too.
PS: I am sorry for the overly long post...
His argumentation is flawed though. He says "I could always get a good optimization result by running all of my optimization passes until a fixpoint is reached," but unfortunately there is no such fixpoint. Many optimizations reverse each other (e.g. loop fusion vs loop spitting) or just arbitrarily choose some normalization (e.g. 2*x vs x+x vs x<<1).
You can build a superoptimizer, which constructs all variations (e.g. equality saturation http://portal.acm.org/citation.cfm?doid=1480881.1480915), though this is no fixpoint search, but an optimization problem to choose the least cost alternative. You can not construct all variations anyway. For a simple example consider loop unrolling an infinite loop.
Hence, unlike Regehr I would not devalue machine learning. I would not bet on it either, though. ;)