Mention ThinLTO with PGO in ReleaseNotes 4.0

[Prazek]

Summary

It would be good to also mention
other improvements in ThinLTO for 4.0 release

[Prazek]

Maybe I should change ThinLTO for '-flto=thin' and also mention this improvement in LLVM release notes?

BTW '-flto=thin' is not documented in UsersManual

[mehdi_amini]

docs/ReleaseNotes.rst

69

We need to mention ThinLTO in the Release Notes, so thanks for initiating this.
We likely should go with a paragraph summarizing the improvements, I don't know though if there is anything else than 1) PGO and 2) Significant build-time and binary-size improvements with debug-info.

[tejohnson]

In D28746#646793, @Prazek wrote:

Maybe I should change ThinLTO for '-flto=thin' and also mention this improvement in LLVM release notes?

BTW '-flto=thin' is not documented in UsersManual

It's documented here:

http://clang.llvm.org/docs/index.html

and here:

http://clang.llvm.org/docs/CommandGuide/clang.html

but not (yet) here:

http://clang.llvm.org/docs/UsersManual.html

I can add that.

Thanks!

docs/ReleaseNotes.rst

69

Yes this is a good start. One of the big improvements from PGO is the integration with indirect call promotion. I would suggest something like:

Improvements to ThinLTO (-flto=thin):

• Integration with profile data. When available, profile data enables more accurate function importing decisions, as well as cross-module indirect call promotion.
• Significant build-time and binary-size improvements when compiling with debug info (-g).

[Prazek]

update

[mehdi_amini]

LGTM, thanks.

docs/ReleaseNotes.rst

75

I'd add a parenthesis: Integration with profile data (PGO).

[Prazek]

Cool, This still might require some small fixes - I haven't generate docs for it, so I will check it before release.

[Prazek]

(PGO)

[Prazek]

Shoud I add the same note to LLVM Release notes?

[mehdi_amini]

Oh I didn't notice that we were in the clang Release Notes, actually I've only updated the LLVM ones in the past.

So yes, I think this should be in LLVM as well, I don't know if it should differ in some way?

[Prazek]

In D28746#646875, @mehdi_amini wrote:

Oh I didn't notice that we were in the clang Release Notes, actually I've only updated the LLVM ones in the past.

So yes, I think this should be in LLVM as well, I don't know if it should differ in some way?

It probably should not differ in any way. I think it is very important to have note in clang's release notes, because this is where clang users look :)

[Prazek]

Pushed on branch.
Please check if everything looks good when RC will be released.

docs/ReleaseNotes.rst

.. code-block:: c++
            for (int i = 0 ; i < n; i++)
                a.foo();
        }
        void test(int n) {
            A a;
            indirect(a, n);
        }
 
+- Significantly improved generated code for ThinLTO with profile data. It is
+  achieved by importing hot-chains of functions and by not importing functions
+  called from cold callsite.
+
 
 -  ...
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 -  ...
 
 New Compiler Flags
 ------------------
 
 The option -Og has been added to optimize the debugging experience.
 For now, this option is exactly the same as -O1. However, in the future,
 some other optimizations might be enabled or disabled.