[llvm-dev] [RFC] Profile guided section layout

[Rafael Avila de Espindola via llvm-dev]

Tobias Edler von Koch <tobias at codeaurora.org> writes:

> Hi Rafael,
>
> On 07/31/2017 04:20 PM, Rafael Avila de Espindola via llvm-dev wrote:
>> However, do we need to start with instrumentation? The original paper
>> uses sampling with good results and current intel cpus can record every
>> branch in a program.
>>
>> I would propose starting with just an lld patch that reads the call
>> graph from a file. The format would be very similar to what you propose,
>> just weight,caller,callee.
>
> The advantage of the proposed approach (weighted callgraph section) is 
> that it's completely transparent: it works regardless of the particular 
> profiling methodology (as long as there's !perf metadata when the pass 
> runs). For this reason, it fits neatly into an *existing* PGO-based 
> build flow. I only need to add 1 compiler flag to enable it. That's a 
> big plus.
>
> On the other hand, I could see how your idea (callgraph input file for 
> linker) would be useful in situations where I just want to do section 
> layout but no PGO in the compiler... and of course for testing of the 
> linker's sorting algorithm.
>
> So there's merits in both, but for my use cases Michael's original 
> approach is the most practical.

Yes, I must stress that I am not proposing having the option of reading
the callgraph from another file *instead* of reading it from .o
files. Just that doing it first decouples most of the lld patch form the
llvm changes and can be useful in cases where only samples are available.

Cheers,
Rafael