[llvm-dev] Can LLVM emit machine code faster with no optimization passes?

Jonas Maebe via llvm-dev
Wed, 12 Oct 2016 19:58:01 -0000

On 12/10/16 20:32, Matthias Braun via llvm-dev wrote:
> But just as food for though: What if msvc did some minimal
> optimisations, found out that half the sourcecode is unreachable and
> removes it, while llvm with no optimisations just compiles everything?

llvm is actually extremely slow when it has to remove lots of dead code.
I experienced that in the beginning when working on our llvm backend. I
had some bugs in our code generator that caused about half of the llvm
IR code to be dead, and compiling that code with -O1 made llvm extremely
slow.

Another thing that makes llvm incredibly slow is loading/storing large
aggregates directly (I know, now, that you're not supposed to do that).
I guess it's the generation of the resulting spilling code that takes
forever. See e.g. http://pastebin.com/krXhuEzF

All that said: we will also keep our original code generators in our
compiler, and keep llvm as an option to optimise extra. In terms of
speed, our code generators are much less complex and hence much faster
than llvm's. We don't have instruction selection, but directly generate
assembler via virtual methods of our parse tree node classes. That would
be very hard to beat, even if things have gotten slower lately due to
the addition of extra abstraction layers to support generating JVM
bytecode and, yes, LLVM IR :)

There are also a few other reasons, but they're not relevant to this
thread. (*)


Jonas

(*) We support several platforms that LLVM no longer supports and/or
will probably never support (OS/2, 16 and 32 bit MS-DOS, Gameboy
Advance, Amiga, Darwin/PowerPC), and the preference of some code
generator/optimisation developers to write Pascal rather than C++ (our
compiler is a self-hosted Pascal compiler)