[LLVMdev] Bitcode parsing performance

[Chandler Carruth]

Any chance you can share either your bitcode file or some other bitcode
file that seems about the same size and generally representative of the
performance problems you're having?


On Thu, Jan 9, 2014 at 4:37 PM, Kevin Modzelewski <kmod at dropbox.com> wrote:

> Hi all, I'm trying to reduce the startup time for my JIT, but I'm running
> into the problem that the majority of the time is spent loading the bitcode
> for my standard library, and I suspect it's due to debug info.  My stdlib
> is currently about 2kloc in a number of C++ files; I compile them with
> clang -g -emit-llvm, then link them together with llvm-link, call opt -O3
> on it, and arrive at a 1MB bitcode file.  I then embed this as a binary
> blob into my executable, and call ParseBitcodeFile on it at startup.
>
> Unfortunately, this parsing takes about 60ms right now, which is the main
> component of my ~100ms time to run on an empty source file (another ~20ms
> is loading the pre-jit'd image through an ObjectCache).  I thought I'd save
> some time by using getLazyBitcodeModule, since the IR isn't actually needed
> right away, but this only reduced the parsing time (ie the time of the
> actual getLazyBitcodeModule() call) to 45ms, which I thought was
> surprising.  I also tested computing the bytewise-xor of the bitcode file
> to make sure that it was fully read into memory, which took about 5ms, so
> the majority of the time does seem to be spent parsing.
>
> Then I switched back to ParseBitcodeFile, but now I added the
> "-strip-debug" flag to my opt invocation, which reduced the bitcode file
> down to about 100KB, and reduced the parsing time to 20ms.  What surprised
> me the most was that if I then switched to getLazyBitcodeModule, the
> parsing time was cut down to 3ms, which is what I was originally expecting.
>  So when lazy loading, stripping out the debug info cuts down the
> initialization time from 45ms to 3ms, which is why I suspect that
> getLazyBitcodeModule is still parsing all of the debug info.
>
>
> To work around it, I can generate separate builds, one with debug info and
> one without, but I'd like to avoid doing that. I did some simple profiling
> of what getLazyBitcodeModule was doing, and it wasn't terribly informative
> (spends most of its time in parsing-related functions); does anyone have
> any ideas if this is something that could be fixable or if I should just
> move on?
>
> Thanks,
> Kevin
>
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