[LLVMdev] [LLVM Dev] [Discussion] Function-based parallel LLVM backend code generation

Wan, Xiaofei xiaofei.wan at intel.com
Tue Jul 16 07:23:03 PDT 2013

Yes, the purpose is similar, we started this job from last year;
But it Shuxin's solution is module based (correct me if I am wrong), we tried this solution and failed for many reasons, it is described in my design document

we need discuss two solution and compare them, then adopt one solution

The biggest difference of module based parallelism and function based parallelism are
1. how to partition module into different pieces which consume similar time, it is a difficult question
2. How to make sure the generated binary is same each time
3. if 2 can't be achieved, it is difficult to validate the correctness of parallelism

Wan Xiaofei

-----Original Message-----
From: Evan Cheng [mailto:evan.cheng at apple.com] 
Sent: Tuesday, July 16, 2013 8:28 PM
To: Wan, Xiaofei
Cc: LLVM Developers Mailing List (llvmdev at cs.uiuc.edu); Shuxin Yang
Subject: Re: [LLVMdev] [LLVM Dev] [Discussion] Function-based parallel LLVM backend code generation

Please see Shuxin's proposal on "parallelizing post-IPO stage". It seems the two projects are related.


On Jul 16, 2013, at 3:33 AM, "Wan, Xiaofei" <xiaofei.wan at intel.com> wrote:

> Hi, community:
> For the sake of our business need, I want to enable "Function-based parallel code generation" to boost up the compilation of single module, please see the details of the design and provide your feedbacks on below aspects, thanks!
> 1. Is this idea the proper solution for my requirement 2. This new 
> feature will be enabled by llc -thd=N and has no impact on original 
> llc when -thd=1 3. Can this new feature of llc be accepted by 
> community and merged into LLVM code tree
> Patches
> The patch is divided into four separated parts, the all-in-one patch could be found here:
> http://llvm-reviews.chandlerc.com/D1152
> Design
> https://docs.google.com/document/d/1QSkP6AumMCAVpgzwympD5pI3btPJt4SRgj
> Y-vhyfySg/edit?usp=sharing
> Background
> 1. Our business need to compile C/C++ source files into LLVM IR and link them into a big BC file; the big BC file is then compiled into binary code on different arch/target devices.
> 2. Backend code generation is a time-consuming activity happened on target device which makes it an important user experience.
> 3. Make -j or file based parallelism can't help here since there is only one big BC file; function-based parallel LLVM backend code generation is a good solution to improve compilation time which will fully utilize multi-cores.
> Overall design strategy and goal
> 1. Generate totally same binary as what single thread output 2. No 
> impacts on single thread performance & conformance 3. Little impacts 
> on LLVM code infrastructure
> Current status and test result
> 1. Parallel llc can generate same code as single thread by "objdump 
> -d", it could pass 10 hours stress test for all performance benchmark 
> 2. Parallel llc can introduce ~2.9X performance gain on XEON sever for 
> 4 threads
> Thanks
> Wan Xiaofei
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