llvm dev - Apr 2017 - How to test that GlobalsAA is non unintentionally invalidated

Hello,

During the last month we've faced two cases where some performance
degradations were observed due to GlobalsAA not being preserved till the LICM:
  1) New pass pgo-memop-opt (fixed in
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20170403/443360.html)
  2) GVNHoist (when it was moved to the simplification part of the pipeline)
So it would be good to guard against such issues. However, this highly depends
on the particular structure of the optimization pipeline.
LIT test like this seems to work:

; RUN: opt < %s -O3 -debug-pass=Details -disable-output 2>&1 |
FileCheck %s
; CHECK-NOT: is not preserving 'Globals Alias Analysis'
; CHECK: 'Loop Invariant Code Motion' is not preserving 'Rotate
Loops'
<some IR with simple loop>

but looks too hard-coded. Are there ways to do it in a cleaner manner?

Thanks,
Andrei

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On Fri, Apr 7, 2017 at 3:09 AM, Elovikov, Andrei via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello,
>
> During the last month we've faced two cases where some performance
> degradations were observed due to GlobalsAA not being preserved till the
> LICM:
>
Do you mean "compile time" or "generated code performance"?

>   1) New pass pgo-memop-opt (fixed in http://lists.llvm.org/
> pipermail/llvm-commits/Week-of-Mon-20170403/443360.html)
>   2) GVNHoist (when it was moved to the simplification part of the
> pipeline)
> So it would be good to guard against such issues. However, this highly
> depends on the particular structure of the optimization pipeline.
> LIT test like this seems to work:
>
> ; RUN: opt < %s -O3 -debug-pass=Details -disable-output 2>&1 |
FileCheck %s
> ; CHECK-NOT: is not preserving 'Globals Alias Analysis'
> ; CHECK: 'Loop Invariant Code Motion' is not preserving 'Rotate
Loops'
> <some IR with simple loop>
>
> but looks too hard-coded. Are there ways to do it in a cleaner manner?
>
> Yes. Add a flag explicitly to output the state of preserved analysis, for
the specific goal of being used in tests, and use that.

But note: I'm not sure it's entirely reasonable to expect everything in
between here and there to preserve something you want preserved.
These are tradeoffs, and adding a test so you can see when it's not true
seems reasonable, but so would xfailing that test if we add something in
between :)
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On 7 Apr 2017, at 12:09, Elovikov, Andrei via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hello,

During the last month we've faced two cases where some performance
degradations were observed due to GlobalsAA not being preserved till the LICM:
 1) New pass pgo-memop-opt (fixed in
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20170403/443360.html)
 2) GVNHoist (when it was moved to the simplification part of the pipeline)
So it would be good to guard against such issues. However, this highly depends
on the particular structure of the optimization pipeline.
LIT test like this seems to work:

; RUN: opt < %s -O3 -debug-pass=Details -disable-output 2>&1 |
FileCheck %s
; CHECK-NOT: is not preserving 'Globals Alias Analysis'
; CHECK: 'Loop Invariant Code Motion' is not preserving 'Rotate
Loops'
<some IR with simple loop>

but looks too hard-coded. Are there ways to do it in a cleaner manner?

Thanks,
Andrei

I added a somewhat similar test, with similar intentions as part of
https://reviews.llvm.org/D19806, committed in
http://llvm.org/viewvc/llvm-project?view=revision&revision=268370. That test
didn't get touched in the 10 months it has been in tree, so it seems to me
that testing it like this has a reasonable chance of highlighting accidentally
dropping GlobalsAA. Maybe just adding a simple loop that should be optimized by
LICM to the same test file (test/Transforms/PhaseOrdering/globalaa-retained.ll)
might be all what's needed?

Thanks,

Kristof


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