Disable ThinLTO optimizations on Linux with LLD.

Disable ThinLTO optimizations on Linux with LLD.

Unfortunately:
- Binaries built with ThinLTO at -O2/--lto-O2 (which is what you
  currently get with use_thin_lto=true) are too large.
- Binaries built with -Os introduce significant performance regressions.
  This issue most likely affects ThinLTO as well, at least according to
  local benchmarking.

So that leaves -O2/--lto-O0, which is pretty much what we have now
with full LTO. (With full LTO, --lto-O1 basically just means apply
CFI, whole-program devirtualization, dead code elimination and CFG
simplification.)

Efforts have been made on the LLVM side to make ThinLTO at --lto-O0
generate binaries with as good quality as full LTO with --lto-O1
(simplifying the code generated for CFI checks (r304921) and allowing
for more dead code elimination (r305482)). Now that those are complete,
we are ready to start using --lto-O0 with ThinLTO.

BUG=660216
R=thakis@chromium.org

Review-Url: https://codereview.chromium.org/2939923004
Cr-Commit-Position: refs/heads/master@{#479875}
Committed: https://chromium.googlesource.com/chromium/src/+/da072d13b2a1cc8459c29aa4e52584a4491a4ad6

[pcc1, 2017-06-15 21:06:07]

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

[commit-bot: I haz the power, 2017-06-15 21:06:43]

Dry run: CQ is trying da patch.

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

[Nico, 2017-06-15 21:11:01]

lgtm

[pcc1, 2017-06-15 21:12:09]

The CQ bit was unchecked by pcc@chromium.org

[pcc1, 2017-06-15 21:12:10]

The CQ bit was checked by pcc@chromium.org

[commit-bot: I haz the power, 2017-06-15 21:12:25]

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, 2017-06-15 22:38:42]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1497561130424350, "parent_rev":
"bf4adbb1a2b7066a30297d5df51707fd468d391d", "commit_rev":
"da072d13b2a1cc8459c29aa4e52584a4491a4ad6"}

[commit-bot: I haz the power, 2017-06-15 22:38:54]

Description was changed from

==========
Disable ThinLTO optimizations on Linux with LLD.

Unfortunately:
- Binaries built with ThinLTO at -O2/--lto-O2 (which is what you
  currently get with use_thin_lto=true) are too large.
- Binaries built with -Os introduce significant performance regressions.
  This issue most likely affects ThinLTO as well, at least according to
  local benchmarking.

So that leaves -O2/--lto-O0, which is pretty much what we have now
with full LTO. (With full LTO, --lto-O1 basically just means apply
CFI, whole-program devirtualization, dead code elimination and CFG
simplification.)

Efforts have been made on the LLVM side to make ThinLTO at --lto-O0
generate binaries with as good quality as full LTO with --lto-O1
(simplifying the code generated for CFI checks (r304921) and allowing
for more dead code elimination (r305482)). Now that those are complete,
we are ready to start using --lto-O0 with ThinLTO.

BUG=660216
R=thakis@chromium.org
==========

to

==========
Disable ThinLTO optimizations on Linux with LLD.

Unfortunately:
- Binaries built with ThinLTO at -O2/--lto-O2 (which is what you
  currently get with use_thin_lto=true) are too large.
- Binaries built with -Os introduce significant performance regressions.
  This issue most likely affects ThinLTO as well, at least according to
  local benchmarking.

So that leaves -O2/--lto-O0, which is pretty much what we have now
with full LTO. (With full LTO, --lto-O1 basically just means apply
CFI, whole-program devirtualization, dead code elimination and CFG
simplification.)

Efforts have been made on the LLVM side to make ThinLTO at --lto-O0
generate binaries with as good quality as full LTO with --lto-O1
(simplifying the code generated for CFI checks (r304921) and allowing
for more dead code elimination (r305482)). Now that those are complete,
we are ready to start using --lto-O0 with ThinLTO.

BUG=660216
R=thakis@chromium.org

Review-Url: https://codereview.chromium.org/2939923004
Cr-Commit-Position: refs/heads/master@{#479875}
Committed:
https://chromium.googlesource.com/chromium/src/+/da072d13b2a1cc8459c29aa4e525...
==========

[commit-bot: I haz the power, 2017-06-15 22:38:55]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/da072d13b2a1cc8459c29aa4e525...