[LLVMdev] Bitcode parsing performance

[Manman Ren]

I briefly looked at the bit code files and some types are not uniqued, here
is one example:
!3903 = metadata !{i32 786454, metadata !3904, null, metadata !"int64_t",
i32 198, i64 0, i64 0, i64 0, i32 0, metadata !2258} ; [ DW_TAG_typedef ]
[int64_t] [line 198, size 0, align 0, offset 0] [from long int]

!4019 = metadata !{i32 786454, metadata !4020, null, metadata !"int64_t",
i32 198, i64 0, i64 0, i64 0, i32 0, metadata !2258} ; [ DW_TAG_typedef ]
[int64_t] [line 198, size 0, align 0, offset 0] [from long int]

!3904 = metadata !{metadata !"runtime/int.cpp", metadata
!"/home/kmod/icbd/jit"}
!4020 = metadata !{metadata !"runtime/list.cpp", metadata
!"/home/kmod/icbd/jit"}

The file names are different for the two typedefs.

Manman


On Fri, Jan 10, 2014 at 12:14 AM, Eric Christopher <echristo at gmail.com>wrote:

> That was likely type information and should mostly be fixed up. It's still
> not lazily loaded, but is going to be ridiculously smaller now.
>
> -eric
>
> On Fri Jan 10 2014 at 12:11:52 AM, Sean Silva <chisophugis at gmail.com>
> wrote:
>
>> This Summer I was working on LTO and Rafael mentioned to me that debug
>> info is not lazy loaded, which was the cause for the insane resource usage
>> I was seeing when doing LTO with debug info. This is likely the reason that
>> the lazy loading was so ineffective for your debug build.
>>
>> Rafael, am I remembering this right/can you give more information? I
>> expect that this will have to get fixed before pitching LLD as a turnkey
>> LTO solution (not sure where in the priority list it is).
>>
>> -- Sean Silva
>> On Thu, Jan 9, 2014 at 5:37 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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