Re: [PATCH v5] pgo: add clang's Profile Guided Optimization infrastructure

On Mon, Jan 18, 2021 at 10:56 PM Bill Wendling [off-list ref] wrote:

On Mon, Jan 18, 2021 at 9:26 AM Sedat Dilek [off-list ref] wrote:

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On Mon, Jan 18, 2021 at 1:39 PM Sedat Dilek [off-list ref] wrote:

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On Mon, Jan 18, 2021 at 3:32 AM Bill Wendling [off-list ref] wrote:

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On Sun, Jan 17, 2021 at 4:27 PM Sedat Dilek [off-list ref] wrote:

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[ big snip ]

[More snippage.]

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[ CC Fangrui ]

With the attached...

   [PATCH v3] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for
undefined symbols

...I was finally able to boot into a rebuild PGO-optimized Linux-kernel.
For details see ClangBuiltLinux issue #1250 "Unknown symbol
_GLOBAL_OFFSET_TABLE_ loading kernel modules".

Thanks for confirming that this works with the above patch.

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@ Bill Nick Sami Nathan

1, Can you say something of the impact passing "LLVM_IAS=1" to make?

The integrated assembler and this option are more-or-less orthogonal
to each other. One can still use the GNU assembler with PGO. If you're
having an issue, it may be related to ClangBuiltLinux issue #1250.

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2. Can you please try Nick's DWARF v5 support patchset v5 and
CONFIG_DEBUG_INFO_DWARF5=y (see attachments)?

I know Nick did several tests with PGO. He may have looked into it
already, but we can check.

Reproducible.

LLVM_IAS=1 + DWARF5 = Not bootable

I will try:

LLVM_IAS=1 + DWARF4

I was not able to boot into such a built Linux-kernel.

PGO will have no effect on debugging data. If this is an issue with
DWARF, then it's likely orthogonal to the PGO patch.

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For me worked: DWARF2 and LLVM_IAS=1 *not* set.

Of course, this could be an issue with my system's LLVM/Clang.

Debian clang version
12.0.0-++20210115111113+45ef053bd709-1~exp1~20210115101809.3724

Please use the official clang 11.0.1 release
(https://releases.llvm.org/download.html), modifying the
kernel/pgo/Kconfig as I suggested above. The reason we specify clang
12 for the minimal version is because of an issue that was recently
fixed.

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Can you give me a LLVM commit-id where you had success with LLVM_IAS=1
and especially CONFIG_DEBUG_INFO_DWARF5=y?
Success means I was able to boot in QEMU and/or bare metal.

The DWARF5 patch isn't in yet, so I don't want to rely upon it too much.

That means to build the first PGO-enabled kernel with clang-11 and
rebuild in a second step again with the same clang-11.

Just FYI:
I was able to boot into a Linux-kernel  rebuild with *no  LLVM_IAS=1*
(means use "GNU AS 2.35.1") set and DWARF5 using LLVM=1 from
LLVM/Clang-12.

- Sedat -