www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - LDC with Profile-Guided Optimization (PGO)

reply Johan Engelen <j j.nl> writes:
Hi all,
   I have been working on adding profile-guided optimization (PGO) 
to LDC [1][2][3].
At this point, I'd like to hear your input and hope you can help 
with testing!

Unfortunately, to try it out, you will need to build LDC with 
LLVM3.7 yourself. PGO should work on OS X, Linux, and Windows.

A first implementation is mostly complete now: it can generate an 
executable that will output profile data, and it can use profile 
data during a second compilation pass (and it will tell LLVM 
about branch frequencies). LDC does not do any PGO optimizations 
(yet): LLVM should do that.

It works like PGO with Clang, with the fprofile-instr-generate 
and fprofile-instr-use cmdline options [4]:

 ldc2 -fprofile-instr-generate=test.profraw -run test.d
 llvm-profdata merge test.profraw -output test.profdata
 ldc2 -profile-instr-use=test.profdata test.d -of=test


You should now have the executable "test" with an amazing 
performance boost ;-)

You can inspect the generated code using LDC's -output-ll switch. 
Functions should be annotated with call frequencies, and most 
branches should be annotated with branch_weights metadata. For 
example:


 ...
 !12 = !{!"function_entry_count", i64 234}


for "void for_loop()" that is called 234 times, and

 br i1 %3, label %if, label %else, !prof !17
 ...
 !17 = !{!"branch_weights", i32 5, i32 3}


for "if (condition) {...} else {...}"
The branch_weights have an offset of 1, so the above means that 
the condition was true 4 times, and false 2 times. If a certain 
piece of code is never executed, no metadata is added (i.e. you 
won't see {!"branch_weights", i32 1, i32 1}). Some branches are 
intentionally not instrumented/annotated if they lead to 
terminating code (e.g. array boundschecks and auto-generated 
nullptr checks on this at class method entry).

I hope you will be able to test and comment on the work. I am 
very interested in hearing about performance 
gains(/losses/no-change) for your programs. I am curious to learn 
for what kinds of code it makes a difference in practice.

Thanks!
   Johan

(future work will probably include coverage analysis (llvm-cov) 
and support for sampling-based profiles, which should fit 
naturally with the current implementation)

[1] http://wiki.dlang.org/LDC_LLVM_profiling_instrumentation
[2] https://github.com/JohanEngelen/ldc/tree/pgo  (warning: I 
will rebase soon)
[3] https://github.com/ldc-developers/ldc/pull/1219
[4] 
http://clang.llvm.org/docs/UsersManual.html#profiling-with-instrumentation
Dec 15 2015
parent reply Johan Engelen <j j.nl> writes:
On Tuesday, 15 December 2015 at 23:05:38 UTC, Johan Engelen wrote:

 Hi all,
   I have been working on adding profile-guided optimization 
 (PGO) to LDC [1][2][3].
 At this point, I'd like to hear your input and hope you can 
 help with testing!

 Unfortunately, to try it out, you will need to build LDC with 
 LLVM3.7 yourself. PGO should work on OS X, Linux, and Windows.


Would it help if binaries are available?
Or is general interest low?

-Johan
Dec 22 2015
next sibling parent reply CraigDillabaugh <craig.dillabaugh gmail.com> writes:
On Tuesday, 22 December 2015 at 14:49:51 UTC, Johan Engelen wrote:

 On Tuesday, 15 December 2015 at 23:05:38 UTC, Johan Engelen 
 wrote:

 Hi all,
   I have been working on adding profile-guided optimization 
 (PGO) to LDC [1][2][3].
 At this point, I'd like to hear your input and hope you can 
 help with testing!

 Unfortunately, to try it out, you will need to build LDC with 
 LLVM3.7 yourself. PGO should work on OS X, Linux, and Windows.


 Would it help if binaries are available?
 Or is general interest low?

 -Johan


Maybe many folks reading your post don't know what PGO is? 
Perhaps you need to do a bit of a sales job to convince folks 
that it is pretty cool, and worth trying out.

Craig
Dec 22 2015
parent Mattcoder <stop spam.com> writes:
On Tuesday, 22 December 2015 at 17:09:21 UTC, CraigDillabaugh 
wrote:

 Maybe many folks reading your post don't know what PGO is?


https://en.m.wikipedia.org/wiki/Profile-guided_optimization

Matt.
Dec 22 2015
prev sibling next sibling parent reply Mattcoder <stop spam.com> writes:
On Tuesday, 22 December 2015 at 14:49:51 UTC, Johan Engelen wrote:

 On Tuesday, 15 December 2015 at 23:05:38 UTC, Johan Engelen 
 Would it help if binaries are available?


Definitely!

Matt.
Dec 22 2015
parent Johan Engelen <j j.nl> writes:
On Tuesday, 22 December 2015 at 17:36:30 UTC, Mattcoder wrote:

 On Tuesday, 22 December 2015 at 14:49:51 UTC, Johan Engelen 
 wrote:

 On Tuesday, 15 December 2015 at 23:05:38 UTC, Johan Engelen 
 Would it help if binaries are available?


 Definitely!


Sorry for false hope :(. I wouldn't even know where to upload 
them...

I hope to have PGO in LDC master soon. It's working, just need 
some code cleanup. Once merged into master, you can play with it 
in nightly builds. For windows, successfull builds of master with 
LLVM3.7 (needed for PGO) are uploaded to github: 
http://wiki.dlang.org/Latest_pre-built_LDC_for_Win64
Jan 05 2016
prev sibling next sibling parent Etienne Cimon <etcimon gmail.com> writes:
On Tuesday, 22 December 2015 at 14:49:51 UTC, Johan Engelen wrote:

 On Tuesday, 15 December 2015 at 23:05:38 UTC, Johan Engelen 
 wrote:

 Hi all,
   I have been working on adding profile-guided optimization 
 (PGO) to LDC [1][2][3].
 At this point, I'd like to hear your input and hope you can 
 help with testing!

 Unfortunately, to try it out, you will need to build LDC with 
 LLVM3.7 yourself. PGO should work on OS X, Linux, and Windows.


 Would it help if binaries are available?
 Or is general interest low?

 -Johan


Sorry I don't read the forums often. This is definitely going to 
be a game changer for me, I need PGO to help with Botan 
performance issues and I'm going to be developing an embedded 
server on Intel Edison with vibe.d/botan/http2 soon, this 
compiler could make quite the difference. I'll be testing it one 
I get my prototype!
Dec 31 2015
prev sibling parent welkam <wwwelkam gmail.com> writes:
On Tuesday, 22 December 2015 at 14:49:51 UTC, Johan Engelen wrote:

 Would it help if binaries are available?
 Or is general interest low?

 -Johan


Reducing steps will always help. I am interested, but not to the 
point of figuring out how to compile newest LDC myself.
Jan 05 2016