llvm dev - Jul 2013 - [LLVMdev] [LLVM Dev] [Discussion] Function-based parallel LLVM backend code generation

On Tue, Jul 16, 2013 at 1:37 PM, Xinliang David Li <xinliangli at
gmail.com>wrote:
> On Tue, Jul 16, 2013 at 1:33 PM, Chandler Carruth <chandlerc at
google.com>
> wrote:
> > On Tue, Jul 16, 2013 at 1:18 PM, Xinliang David Li <xinliangli at
gmail.com
> >
> > wrote:
> >>
> >> Ignoring FE time which can be fully parallelized and assuming 10%
> >> compile time is spent in serial module passes, 25% time is spent
in
> >> CGSCC pass, the maximum speed up that can be gained by using
function
> >> level parallelism is less than 3x.  Even adding support for
parallel
> >> compilation for leaves of CG in CGSCC pass won't help too much
-- the
> >> percentage of leaf functions is < 30% in large apps I have
seen.
> >
> >
> > Can you clarify what you're basing these assumption on or how you
derived
> > your data?
> >
>
> Those numbers are purely speculative -- does Clang has an option to
> dump the time breakout of each passes such as -ftime-report in GCC?
>
We have the functionality... I thought we wired -ftime-report up to it? If
that doesn't work I'll have to go digging.
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On Tue, Jul 16, 2013 at 1:40 PM, Chandler Carruth <chandlerc at
google.com> wrote:
> On Tue, Jul 16, 2013 at 1:37 PM, Xinliang David Li <xinliangli at
gmail.com>
> wrote:
>>
>> On Tue, Jul 16, 2013 at 1:33 PM, Chandler Carruth <chandlerc at
google.com>
>> wrote:
>> > On Tue, Jul 16, 2013 at 1:18 PM, Xinliang David Li
>> > <xinliangli at gmail.com>
>> > wrote:
>> >>
>> >> Ignoring FE time which can be fully parallelized and assuming
10%
>> >> compile time is spent in serial module passes, 25% time is
spent in
>> >> CGSCC pass, the maximum speed up that can be gained by using
function
>> >> level parallelism is less than 3x.  Even adding support for
parallel
>> >> compilation for leaves of CG in CGSCC pass won't help too
much -- the
>> >> percentage of leaf functions is < 30% in large apps I have
seen.
>> >
>> >
>> > Can you clarify what you're basing these assumption on or how
you
>> > derived
>> > your data?
>> >
>>
>> Those numbers are purely speculative -- does Clang has an option to
>> dump the time breakout of each passes such as -ftime-report in GCC?
>
>
> We have the functionality... I thought we wired -ftime-report up to it? If
> that doesn't work I'll have to go digging.
I just checked -- it produces a flat report -- it would be nice to
produce something similar to -debug-pass=Structure outputs.

David

On Tue, Jul 16, 2013 at 1:46 PM, Xinliang David Li <xinliangli at
gmail.com>wrote:
> On Tue, Jul 16, 2013 at 1:40 PM, Chandler Carruth <chandlerc at
google.com>
> wrote:
> > On Tue, Jul 16, 2013 at 1:37 PM, Xinliang David Li <xinliangli at
gmail.com
> >
> > wrote:
> >>
> >> On Tue, Jul 16, 2013 at 1:33 PM, Chandler Carruth <chandlerc at
google.com
> >
> >> wrote:
> >> > On Tue, Jul 16, 2013 at 1:18 PM, Xinliang David Li
> >> > <xinliangli at gmail.com>
> >> > wrote:
> >> >>
> >> >> Ignoring FE time which can be fully parallelized and
assuming 10%
> >> >> compile time is spent in serial module passes, 25% time
is spent in
> >> >> CGSCC pass, the maximum speed up that can be gained by
using function
> >> >> level parallelism is less than 3x.  Even adding support
for parallel
> >> >> compilation for leaves of CG in CGSCC pass won't help
too much -- the
> >> >> percentage of leaf functions is < 30% in large apps I
have seen.
> >> >
> >> >
> >> > Can you clarify what you're basing these assumption on or
how you
> >> > derived
> >> > your data?
> >> >
> >>
> >> Those numbers are purely speculative -- does Clang has an option
to
> >> dump the time breakout of each passes such as -ftime-report in
GCC?
> >
> >
> > We have the functionality... I thought we wired -ftime-report up to
it?
> If
> > that doesn't work I'll have to go digging.
>
> I just checked -- it produces a flat report -- it would be nice to
> produce something similar to -debug-pass=Structure outputs.

Yea, improving the nested timing information and other improvements to
reflect the pass management structure are on my list for the significant
changes to the pass manager infrastructure I mentioned in my original
comments.
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