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

[Wan, Xiaofei]

-----Original Message-----
From: Shuxin Yang [mailto:shuxin.llvm at gmail.com] 
Sent: Wednesday, July 17, 2013 1:50 AM
To: Wan, Xiaofei
Cc: Evan Cheng; Shuxin Yang; LLVM Developers Mailing List (llvmdev at cs.uiuc.edu)
Subject: Re: [LLVMdev] [LLVM Dev] [Discussion] Function-based parallel LLVM backend code generation


On 7/16/13 7:23 AM, Wan, Xiaofei wrote:
> 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 
> https://docs.google.com/document/d/1QSkP6AumMCAVpgzwympD5pI3btPJt4SRgj
> Y-vhyfySg/edit?usp=sharing
>
> 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
Why difficult?

> 2. How to make sure the generated binary is same each time
It depends on what is the same.  In the merged version, constant may keep one copy, while in the partition version,  constant may be duplicated as the post-IPO passes may generated some constant, and they cannot share with the same constant generated in other partitions.

All these issues don't sound to be a problem in practice.

> 3. if 2 can't be achieved, it is difficult to validate the correctness 
> of parallelism

It is nothing about the correctness.

[Xiaofei] why? I don't understand it very well here, you mean it can generate totally identical binaries as the original llc, including the function order (function order may not affect code quality, but we should make sure the output is same in each run)?