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

[mats petersson via llvm-dev]

On 15 October 2016 at 02:36, Gerolf Hoflehner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> > On Oct 12, 2016, at 12:58 PM, Jonas Maebe via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > 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.
>
> When you encounter such a problem i encourage you to file a bug and give
> people the opportunity to analyze and  - in case - perhaps fix the
> underlying issue. Here I don’t know what you mean by “extremely slow” or
> “incredibly slow”  - nor your basis of comparison.
>

If I remember correctly, the reply on the LLVM chat when I first mentioned
this was "but don't do that, clang has code go avoid that, so you should do
the same" (in other words, LLVM is not supposed to do well in the example
given, because you are supposed to use memcpy or similar to copy larger
data structures) - I think this is a but "unfriendly" to compiler writers,
but I see the point in some way.

I'm not sure there are other cases, where this symptom is obvious.

--
Mats

>
> Thanks
> Gerolf
>
> >
> > 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)
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