Disable arm-gepopt by default

James Molloy james at jamesmolloy.co.uk
Mon Apr 20 13:54:24 PDT 2015

Hi Michael,

>From a bleary-eyed look at those dumps, it looks to me as if fewer
addressing modes are being matched. For example I'm seeing stuff like this:

-00000001000314b8 ubfm x1, x2, #1, #8
-00000001000314bc ldr w1, [x12, x1, lsl #2]
+0000000100031be4 add x15, x11, w15, uxtb #2
+0000000100031be8 ldr w15, [x15, #2120]

So, is this just pattern permutation - addressing modes aren't being
matched as well as they can because the DAG has been permuted? (should be
easy to fix if so). Or is it due to information loss somewhere along the
line? (I seem to remember that the gep matching pass will produce some
inttoptr/ptrtoint pairs sometimes).

My main concern with disabling it is the likelihood that it'll be left
unfixed, and bitrot. Do you intend to look at it after disabling it? I'd be
happier if I knew it was on someone's backlog.

Adding Hao, as he wrote this and hopefully can have more insight!



On Mon, 20 Apr 2015 at 21:36 Michael Zolotukhin <mzolotukhin at apple.com>

> Hi James,
> I’m attaching two disasm files for the TwoFish hot function, the first one
> (slow.s) is obtained by compiler with default options, the second one
> (fast.s) - with aarch64-gep-opt=false.
> It’s a pretty big function, and for me it’s not obvious what piece of code
> is responsible for the total performance. However, slow.s is 100 lines
> longer, and that’s corresponds to the performance I observe (for slow.s
> geekbench score of the Twofish is 733, for fast.s - 951, higher is better).
> The tests arm64-cse.ll and arm64-addr-mode-folding.ll also might give some
> ideas what’s wrong - and we need to fix them, or properly explain why it’s
> ok to disable them (please note that they were written exactly to detect
> issues that had been fixed once).
> I totally support that we need to understand what’s wrong, and what I
> suggest is a temporal disabling until it’s done.
> Thanks,
> Michael
> On Apr 18, 2015, at 9:23 AM, James Molloy <james at jamesmolloy.co.uk> wrote:
> Hi Michael,
> I'm not too keen on disabling this without understanding what it's doing
> wrong. It seems like a large hammer.
> If this is the pass I'm thinking of, it was introduced by Hao to help
> remove many redundant multiplies formed by complex GEPs, and did manifest
> in spec, gobmk was the main culprit iirc.
> It will also affect in order cores more than out of order ones- would it
> be ok to understand whether the regression is a deficiency in the pass
> easily fixed or if it is something more fundamental?
> James
> On Fri, 17 Apr 2015 at 19:48, Michael Zolotukhin <mzolotukhin at apple.com>
> wrote:
>> Hi,
>> Recently I’ve found that LLVM regressed on a TwoFish test (from
>> geekbench) when comparing relatively new compiler and a year-old one. The
>> performance regression is pretty big - about 20%, so I decided to
>> investigate it further and understand what change exposed that. Triaging
>> led me to r222328 and r222331, which introduced a new ARM64-specific pass
>> for hoisting constant offsets from addresses. It claimed some gains on
>> SPECS, but when I turned it off, I saw no regressions there (I checked
>> SPEC2006/ref-input set, built with PGO and LTO).
>> Moreover, when the patch was committed, it effectively disabled two
>> existing regression tests, which is not good I think.
>> Is it ok to disable this pass by default and enable the tests back? We
>> can turn it back on when we are confident that all tests pass, and there
>> are no new regressions. Any objections?
>> Thanks,
>> Michael
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