please provide a little more info here. this is too vague.

The discriminator encoding in create_llvm_prof is guarded by `defined(HAVE_LLVM)` in source_info.h. However, it seems that source_info.h doesn’t include config.h, which has the definition of HAVE_LLVM. 

So the discriminator encoding is disabled in current AutoFDO tools.

The pull request is merged [1]. Quote from the fix, which provides a good context:

"""
Now LLVM debug metadata uses discriminator to express not only
multiple expressions in a single line, but also other optimization
information such as unrolling factor. Therefore, create_llvm_prof needs
to be in accordance with this to provide precise discriminator info.

In source_info.h file, which is updated by this diff, checks if
HAVE_LLVM is defined to decide if discriminator encoding needs to be
supported or not. However, the file that define HAVE_LLVM (config.h) has
not been included by source_info.h. This made create_llvm_prof
to generate wrong profile data that are ignorant about discriminator
encoding.

...
"""

[1] https://github.com/google/autofdo/pull/59

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/1d38bafe4f1316a06e260fad36c6a36b5e339a0a

commit 1d38bafe4f1316a06e260fad36c6a36b5e339a0a
Author: Yunlian Jiang <yunlian@google.com>
Date: Tue Dec 12 19:09:13 2017

autofdo: fix discriminator encoding.

This fixes the discriminator encoding problem in the autofdo profiling
creation tool.

BUG= chromium:792153 
TEST=sudo emerge autofdo works.

Change-Id: Ia6e34685cac8fbf44621d53b4b9cc2cd2d6a0b90
Reviewed-on: https://chromium-review.googlesource.com/809913
Commit-Ready: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[modify] https://crrev.com/1d38bafe4f1316a06e260fad36c6a36b5e339a0a/sys-devel/autofdo/Manifest
[delete] https://crrev.com/555554226f1050b0bd83836f7957e8f19bb98a3c/sys-devel/autofdo/autofdo-0.17-r1.ebuild
[rename] https://crrev.com/1d38bafe4f1316a06e260fad36c6a36b5e339a0a/sys-devel/autofdo/autofdo-0.18.ebuild

This bug requires manual review: M64 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), kbleicher@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Added M-64 label; recommended for searching.

Can I have a bit of context as well?  Is this an infrastructure build change, or ?

Thanks

This is a performance improvement CL. It generates more accurate profiles of Chrome so that compiler can produce better code.

Understood, but it's not a M64 regression or introduced prior to branch.  Rejecting request.

kbleicher@: This is a fix to a performance regression IMHO. We lost ~6% speedometer scores due to some prior changes of toolchain and only recovered ~4% so far. This brings the remaining 2% back.

Shall we reconsider this in R64?

This bug requires manual review: M64 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), kbleicher@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Approving merge to M64 Chrome OS.

[It appears that a bunch of old cros issues bulk-added the "Infra" component recently, but they should probably be "Infra>Client>ChromeOS".]

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/555eba2b98d01a802a5dcfcb2a95c3f80f6a2461

commit 555eba2b98d01a802a5dcfcb2a95c3f80f6a2461
Author: Yunlian Jiang <yunlian@google.com>
Date: Tue Jan 02 19:25:13 2018

autofdo: fix discriminator encoding.

This fixes the discriminator encoding problem in the autofdo profiling
creation tool.

BUG= chromium:792153 
TEST=sudo emerge autofdo works.

Change-Id: Ia6e34685cac8fbf44621d53b4b9cc2cd2d6a0b90
Reviewed-on: https://chromium-review.googlesource.com/809913
Commit-Ready: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
(cherry picked from commit 1d38bafe4f1316a06e260fad36c6a36b5e339a0a)
Reviewed-on: https://chromium-review.googlesource.com/846204
Commit-Queue: Ting-Yuan Huang <laszio@chromium.org>
Tested-by: Ting-Yuan Huang <laszio@chromium.org>
Trybot-Ready: Ting-Yuan Huang <laszio@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>

[modify] https://crrev.com/555eba2b98d01a802a5dcfcb2a95c3f80f6a2461/sys-devel/autofdo/Manifest
[delete] https://crrev.com/2821d29d12622caa12e5a8033402718d41ab621a/sys-devel/autofdo/autofdo-0.17-r1.ebuild
[rename] https://crrev.com/555eba2b98d01a802a5dcfcb2a95c3f80f6a2461/sys-devel/autofdo/autofdo-0.18.ebuild

This issue has been approved for a merge. Please merge the fix to any appropriate branches as soon as possible!

If all merges have been completed, please remove any remaining Merge-Approved labels from this issue.

Thanks for your time! To disable nags, add the Disable-Nags label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

This issue has been approved for a merge. Please merge the fix to any appropriate branches as soon as possible!

If all merges have been completed, please remove any remaining Merge-Approved labels from this issue.

Thanks for your time! To disable nags, add the Disable-Nags label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot