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

[Gerolf Hoflehner via llvm-dev]

> 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.

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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