build: Add Windows port of LLVM LTO config.

build: Add Windows port of LLVM LTO config; start using thin archives with lld.

This uses "-Xclang -emit-llvm-bc" because the clang-cl driver does not
support LTO.

BUG=507755
R=thakis@chromium.org

Committed: https://crrev.com/438f52c765ac1e4d170e7feed3a9024f04e170ef
Cr-Commit-Position: refs/heads/master@{#359228}

[Nico, 2015-11-11 05:00:00]

lgtm

should clang-cl grow a driver-mode flag for this? -Xclang is pretty ugly…

https://codereview.chromium.org/1432133002/diff/1/build/common.gypi
File build/common.gypi (right):

https://codereview.chromium.org/1432133002/diff/1/build/common.gypi#newcode6009
build/common.gypi:6009: # llvm-lib is a replacement for lib.exe that understands
bitcode.
we don't bundle this, do we? how big is it?

[pcc1, 2015-11-11 20:22:10]

Please take another look.

> should clang-cl grow a driver-mode flag for this? -Xclang is pretty ugly…

It should. I've added a TODO.

https://codereview.chromium.org/1432133002/diff/1/build/common.gypi
File build/common.gypi (right):

https://codereview.chromium.org/1432133002/diff/1/build/common.gypi#newcode6009
build/common.gypi:6009: # llvm-lib is a replacement for lib.exe that understands
bitcode.
On 2015/11/11 05:00:00, Nico wrote:
> we don't bundle this, do we? how big is it?

Hm, since we do apparently bundle lld-link and lld-link supports the /lib flag
(which causes it to act like llvm-lib) we can just do that instead.

And since it turns out we can pass flags in $AR we might as well also pass
/llvmlibthin which causes lld-link to produce thin archives on Windows.

And since that's useful even when not doing LTO, it seems like a good idea to
use it all the time when use_lld==1.

I've done all that in the new patch set.

[pcc1, 2015-11-11 20:23:34]

Description was changed from

==========
build: Add Windows port of LLVM LTO config.

This uses "-Xclang -emit-llvm-bc" because the clang-cl driver does not
support LTO.

BUG=507755
R=thakis@chromium.org
==========

to

==========
build: Add Windows port of LLVM LTO config; start using thin archives with lld.

This uses "-Xclang -emit-llvm-bc" because the clang-cl driver does not
support LTO.

BUG=507755
R=thakis@chromium.org
==========

[Nico, 2015-11-11 21:03:23]

this looks much nicer, thanks! lgtm++

[pcc1, 2015-11-11 23:31:11]

The CQ bit was checked by pcc@chromium.org

[commit-bot: I haz the power, 2015-11-11 23:34:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1432133002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1432133002/20001

[commit-bot: I haz the power, 2015-11-12 03:17:04]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-11-12 20:03:45]

Patchset 2 (id:??) landed as
https://crrev.com/438f52c765ac1e4d170e7feed3a9024f04e170ef
Cr-Commit-Position: refs/heads/master@{#359228}