[LLVMdev] llvm and parallel [Re: gcc in c++]

[Eli Friedman]

On Thu, Jul 3, 2008 at 11:00 AM, Devang Patel <dpatel at apple.com> wrote:
>
> On Jul 3, 2008, at 10:28 AM, Eli Friedman wrote:
>
>> On Thu, Jul 3, 2008 at 9:09 AM, Devang Patel <dpatel at apple.com> wrote:
>>>> On Wed, Jul 2, 2008 at 7:00 PM, Mike Stump <mrs at apple.com> wrote:
>>>>> If we apply their logic to llvm, we can dominate, if we just
>>>>> target 8
>>>>> cores.  :-)
>>>
>>> Does this mean llvm can not dominate if llvm target 1 core machine
>>> also ?
>>
>> Making an optimizer/code generator parallel is fundamentally a lot
>> easier than making a browser parallel because the problems parallelize
>> a lot more naturally.
>
> It is hard for me to agree or disagree because I do not know typical
> web browser architecture.

Okay... in case you're interested, I can give a quick description of
some of the issues: first is the pipeline of actually rendering a
page, parsing->DOM construction->style resolution->layout->painting,
which is essentially serial; within the parsing, DOM construction, and
layout stages, it's very difficult to do parallelization because later
pieces of the document depend on earlier pieces.  (This is fast enough
single-threaded to not be a serious issue on a desktop computer these
days, but it's still an issue on lower-power devices, and for heavy
DOM manipulation.) Another is that Javascript is fundamentally
single-threaded, so it's extremely difficult to parallelize.  And
multiple pages can interact with each other's state, which complicates
things even further.

>>  There are essentially two chunks of code in the
>> llvm pipeline that take up large amounts of time: the optimization
>> passes and the code generator.  I think parallelizing both of these is
>> feasible with conventional parallelism techniques, with very little
>> penalty for the single-core case.
>>
>>
>> That said, that wasn't the point I was trying to make by linking to
>> that blog post; I was really trying to point in the direction of
>> building new programming tools with LLVM.  Parallelizing a web-browser
>> to many cores involves much more difficult issues.
>
> host vs. target
>
> If LLVM itself is parallelized than it can spit out optimized code
> faster if the host has multiple cores. This is a win for developers
> who use LLVM based compiler, e.g. web browser developers.
>
> If LLVM targets machines with multiple cores then the generated code
> runs faster on machines with 8 cores. This is a big one for end users
> whose application is compiled using LLVM based tools., e.g. web
> browser users. Mike is well versed in compiler host/target terminology
> so I asked him the question.

Oh, okay... sorry to drag your question off-topic.

-Eli