[PATCH] D113073: [lld-macho] Cache library paths from findLibrary

Tue Nov 9 09:05:45 PST 2021

keith added a comment.

In D113073#3118477 <https://reviews.llvm.org/D113073#3118477>, @oontvoo wrote:

> In D113073#3106528 <https://reviews.llvm.org/D113073#3106528>, @keith wrote:
>
>> In D113073#3106474 <https://reviews.llvm.org/D113073#3106474>, @oontvoo wrote:
>>
>>> In D113073#3105039 <https://reviews.llvm.org/D113073#3105039>, @keith wrote:
>>>
>>>> In D113073#3104904 <https://reviews.llvm.org/D113073#3104904>, @int3 wrote:
>>>>
>>>>> By the way, thanks for contributing all these optimizations! I was quite surprised to hear that ld64 was faster, given that LLD is typically much faster for our own workloads, but I guess you have rather different inputs. Hopefully we can make LLD the fastest Mach-O linker for all builds :)
>>>>
>>>> Thanks for all the reviews! For context our project is a huge iOS application with on the order of thousands of static libraries, and many iOS system framework + system library dependencies. Is this a case you've benchmarked? If so I'd be interested to dig a bit deeper into the differences to try and understand why it has been slower for us.
>>>
>>> For one of our largest ios apps that I've measured(different from int3's ):
>>>
>>> - ~7100 archives
>>> - 56 frameworks  (including system ones)
>>> - 12 weak frameworks
>>
>> Thanks for the info! What's your bottleneck at this point? After all my changes here our biggest area is we now spend 20+ seconds in `lld::macho::readFile`
>
> *With* all of the patches improving load input applied, linking the app above is only slightly faster than our optimised LD64:
> (cross-linking, on Linux - Broadwell (yeah, kind of old) RAM 32GB)
>
>   x ./LLD_with_cache_patches.txt
>   + ./LD64_local_imprv.txt
>   +-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
>   |x     *           x  *x  x  * *          +      +   +                         +                                                                +                      +|
>   |        |_|________A__M______|__________________M____________A____________________________________________________|                                                    |
>   +-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
>       N           Min           Max        Median           Avg        Stddev
>   x  10         23.64         23.89         23.82        23.799   0.077667382
>   +  10         23.69         25.01         24.04        24.146     0.4379041
>   Difference at 95.0% confidence
>   	0.347 +/- 0.295482
>   	1.45804% +/- 1.24157%
>   	(Student's t, pooled s = 0.314478)
>
> Looking at the trace, the bottleneck is still in loading input and writing output. 
> It seems it could be improved further ...
>
> F20184452: Screen Shot 2021-11-09 at 10.20.13 AM.png <https://reviews.llvm.org/F20184452>

Thanks for the data point! You might have mentioned before but does this mean these changes _were_ an improvement for you as well? Do you have a time for this benchmark before these changes?

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