D34393: Adding code padding for performance stability - infrastructure

[Easwaran Raman via llvm-commits]

I applies the two patches to llvm (synced at r311358). The attached
perf_nops.mir test fails with the message:
LLVM ERROR: symbol '.L0$pb' can not be undefined in a subtraction expression

Indeed if I generate the assembly, I don't see L0$pb defined. Am I missing
something?

On Sun, Aug 20, 2017 at 7:35 AM, Paparo Bivas, Omer <
omer.paparo.bivas at intel.com> wrote:

> Hi,
>
> Attached is the said patch (CodePadding_x86.patch). I'm also attaching the
> infrastructure patch it relies on (CodePadding_inf.patch), which is
> identical to the one I've sent a few days ago.
>
> Thanks,
> Omer
>
> -----Original Message-----
> From: Omer Paparo Bivas via Phabricator [mailto:reviews at reviews.llvm.org]
> Sent: Sunday, August 20, 2017 17:30
> To: Paparo Bivas, Omer <omer.paparo.bivas at intel.com>; Aboud, Amjad <
> amjad.aboud at intel.com>; Rackover, Zvi <zvi.rackover at intel.com>; Ansari,
> Zia <zia.ansari at intel.com>; craig.topper at gmail.com;
> rafael.espindola at gmail.com
> Cc: Elovikov, Andrei <andrei.elovikov at intel.com>; gberry at codeaurora.org;
> wmi at google.com; eraman at google.com; kyle+llvm at iteratee.net;
> chandlerc at gmail.com; davidxl at google.com; llvm-dev at redking.me.uk;
> simon.dardis at imgtec.com; mgorny at gentoo.org; javed.absar at arm.com;
> llvm-commits at lists.llvm.org; Ben Simhon, Oren <oren.ben.simhon at intel.com>;
> Saba, Lama <lama.saba at intel.com>
> Subject: [PATCH] D34393: Adding code padding for performance stability -
> infrastructure
>
> opaparo added a comment.
>
> In https://reviews.llvm.org/D34393#844569, @davidxl wrote:
>
> > Tried the two patches with our internal benchmark -- the alignment
> related performance issue is still there.  The problem disappears with
> -mllvm -x86-experimental-pref-loop-alignment=5 is used.
>
>
> I can think of two reasons that might have caused the patch to fail to
> help you:
>
> 1. This optimization is not enabled for all architectures. Use -target-cpu
> to specify the target (if you haven't already) to make sure you are
> compiling for the desired architecture, which will enable the optimization
> (if relevant). See the constructor of X86MCCodePadder for the list of
> architectures for which the optimization is enabled.
> 2. You might have encountered a different alignment issue than the one
> handled by this patch. I will send you soon another patch (which was not
> yet uploaded for review) that addresses another alignment issue (in
> addition to the first one), which will hopefully handle your case.
> Regardless of the results you'll get, could you send a small reproducer for
> this problem? I would like to investigate it and see the kind of alignment
> issue we are facing here.
>
>
>
>
> https://reviews.llvm.org/D34393
>
>
>
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