[LLVMdev] Bitcode parsing performance

[Eric Christopher]

Adrian may have handled this recently?
On Jan 13, 2014 3:34 PM, "Manman Ren" <manman.ren at gmail.com> wrote:

> 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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>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
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