[llvm-dev] Tablegen ridiculously slow when compiling for Debug

[Joan Lluch via llvm-dev]

Hi Praveen,

Thanks for the tip, but Xcode seems to spend all the time running tablegen "custom shell scripts", one by one at a time, not linking. Linking is actually very fast, possibly less than a second. The “scripts” that take longer are  “AArch64CommonTableGen" and “AMDGPUCommonTableGen”.  As said this is on LLVM 9.0. 

However, on LLVM 7.0.1, the same process takes just 5-6 seconds in total, with individual “scripts” taking significantly less than 1 second each. There must be some difference between LLVM 9.0 and LLVM 7.0 that might cause this (?)

John


> On 30 Jun 2019, at 11:17, Praveen Velliengiri <praveenvelliengiri at gmail.com> wrote:
> 
> 
> cmake BUILD_SHARED_LIBS option, it builds llvm as .so not as .a. It will use less memory during linking so you can increase the link threads and your build time will be lesser. 
> Check this in : https://llvm.org/docs/CMake.html <https://llvm.org/docs/CMake.html> 
> 
> 
> On Sun, 30 Jun 2019 at 14:42, Joan Lluch <joan.lluch at icloud.com <mailto:joan.lluch at icloud.com>> wrote:
> Hi Praveen,
> 
> Please, can you elaborate on this?. What do do mean by “building as shared objects”.
> 
> Thanks,
> 
> John
> 
> 
> 
>> On 30 Jun 2019, at 07:32, Praveen Velliengiri <praveenvelliengiri at gmail.com <mailto:praveenvelliengiri at gmail.com>> wrote:
>> 
>> Maybe try building llvm as a shared objects.. 
>> 
>> On Jun 30, 2019 1:30 AM, "Joan Lluch via llvm-dev" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> Hi Florian,
>> 
>> Ok, I ran this:
>> 
>> cmake -S LLVM -DCMAKE_INSTALL_PREFIX=INSTALL -DLLVM_OPTIMIZED_TABLEGEN=On -G Xcode
>> 
>> Compiled it again from clean, and the situation is worse than before. Incremental builds take an incredible amount of time stuck in running Tablegen scripts for all targets. Now this happens both in Release and Debug configurations. Just before this, at least Release compiled fine, but that’s no longer the case. 
>> 
>> Any other suggestions?  What could actually cause this? 
>> 
>> Thanks
>> John
>>  
>> 
>> 
>>> On 29 Jun 2019, at 19:37, Florian Hahn <florian_hahn at apple.com <mailto:florian_hahn at apple.com>> wrote:
>>> 
>>> Hi,
>>> 
>>>> On Jun 29, 2019, at 18:26, Joan Lluch via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>> 
>>>> Hi all,
>>>> 
>>>> On LLVM version 7.0.1, incremental builds are very fast for both Release and Debug. I’m compiling with Xcode
>>>> 
>>>> I recently downloaded LLVM 9.0 from the LLVM-mirror Github repository and found that Incremental "Debug” builds take a ridiculously long time due to Tablegen taking ages (literally more than 10 minutes) to generate files. This makes it totally unusable for debug purposes. However, incremental ‘Release’ builds only take a few seconds.
>>>> 
>>>> Why is that?. Any suggestions?.
>>> 
>>> 
>>> 
>>> You could give setting LLVM_OPTIMIZED_TABLEGEN a try (https://llvm.org/docs/CMake.html <https://llvm.org/docs/CMake.html>).
>>> 
>>> Cheers,
>>> Florian
>> 
>> 
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