Add use_thin_lto switch to start evaluating ThinLTO.

Add use_thin_lto switch to start evaluating ThinLTO.

Local experiments show that it's possible to compile
Chrome and tests with ThinLTO, and the linking time is 3.5x smaller
compared to the full LTO. With additional profiling it might reach 10x,
as there're hints that Chrome is hitting some corner cases, which can be
optimized.

The plan is to setup ThinLTO / Clang ToT buildbot, then port
whole program devirtualization, then CFI, then get rid of
full LTO completely.

BUG=645295
Committed: https://crrev.com/8e6253caa689d2c20f5173406f1e7cb64d7f9b0f
Cr-Commit-Position: refs/heads/master@{#418098}

[krasin1, 2016-09-12 20:36:11]

krasin@chromium.org changed reviewers:
+ dpranke@chromium.org

[krasin1, 2016-09-12 20:36:11]

Hi Dirk,

this is the first step in getting LTO fast. It seems that ThinLTO has just got
ready (at least, in Clang ToT) to build Chrome, and we would like to start
porting CFI and whole-program devirtualization to get rid of the slow FullLTO.
See blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html for more
details on ThinLTO and why is it going to be awesome, when works for us.

[krasin1, 2016-09-12 20:39:11]

The CQ bit was checked by krasin@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-09-12 20:39:35]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Dirk Pranke, 2016-09-12 21:15:38]

lgtm w/ nit.

https://codereview.chromium.org/2333843002/diff/40001/build/config/compiler/B...
File build/config/compiler/BUILD.gn (right):

https://codereview.chromium.org/2333843002/diff/40001/build/config/compiler/B...
build/config/compiler/BUILD.gn:467: ldflags += [ "-fwhole-program-vtables" ]
Can you move these four lines up next to lines 450-451 (or even merge them)? I
think that'd read better.

[krasin1, 2016-09-12 21:35:06]

https://codereview.chromium.org/2333843002/diff/40001/build/config/compiler/B...
File build/config/compiler/BUILD.gn (right):

https://codereview.chromium.org/2333843002/diff/40001/build/config/compiler/B...
build/config/compiler/BUILD.gn:467: ldflags += [ "-fwhole-program-vtables" ]
On 2016/09/12 21:15:38, Dirk Pranke wrote:
> Can you move these four lines up next to lines 450-451 (or even merge them)? I
> think that'd read better.

Done. Good idea.

[krasin1, 2016-09-12 22:01:31]

The CQ bit was checked by krasin@chromium.org

[krasin1, 2016-09-12 22:01:31]

The patchset sent to the CQ was uploaded after l-g-t-m from dpranke@chromium.org
Link to the patchset: https://codereview.chromium.org/2333843002/#ps60001
(title: "merge -flto and -whole-program-vtables flags")

[commit-bot: I haz the power, 2016-09-12 22:02:27]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-12 23:17:10]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2016-09-12 23:20:37]

Description was changed from

==========
Add use_thin_lto switch to start evaluating ThinLTO.

Local experiments show that it's possible to compile
Chrome and tests with ThinLTO, and the linking time is 3.5x smaller
compared to the full LTO. With additional profiling it might reach 10x,
as there're hints that Chrome is hitting some corner cases, which can be
optimized.

The plan is to setup ThinLTO / Clang ToT buildbot, then port
whole program devirtualization, then CFI, then get rid of
full LTO completely.

BUG=645295
==========

to

==========
Add use_thin_lto switch to start evaluating ThinLTO.

Local experiments show that it's possible to compile
Chrome and tests with ThinLTO, and the linking time is 3.5x smaller
compared to the full LTO. With additional profiling it might reach 10x,
as there're hints that Chrome is hitting some corner cases, which can be
optimized.

The plan is to setup ThinLTO / Clang ToT buildbot, then port
whole program devirtualization, then CFI, then get rid of
full LTO completely.

BUG=645295

Committed: https://crrev.com/8e6253caa689d2c20f5173406f1e7cb64d7f9b0f
Cr-Commit-Position: refs/heads/master@{#418098}
==========

[commit-bot: I haz the power, 2016-09-12 23:20:38]

Patchset 4 (id:??) landed as
https://crrev.com/8e6253caa689d2c20f5173406f1e7cb64d7f9b0f
Cr-Commit-Position: refs/heads/master@{#418098}