llvm dev - Feb 2018 - [RFC] Should we bump the bitcode version in LLVM 6.0?

Hi Mehdi,

> On Feb 13, 2018, at 12:34 PM, Mehdi AMINI <joker.eph at gmail.com>
wrote:
> 
> 
> 
> 2018-02-08 17:34 GMT-08:00 Quentin Colombet via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>:
> Hi,
> 
> TL;DR
> r317488 changed the way fast math flags are laid out in the bitcode and
anyone compiling a pre-llvm-6.0 bitcode with llvm-6.0 will lose all the
optimizations guarded by isFast and a pre-llvm-6.0 compiler compiling a llvm-6.0
bitcode will potentially generate incorrect code w.r.t. fast math expectations.
> 
> Should we bump the bitcode version because of that and have the
autoupgrader properly rewrite the fast-math to preserve that semantic?
> (I believe we should!)
> 
> 
> * Context *
> 
> With https://reviews.llvm.org/D39304
<https://reviews.llvm.org/D39304> / r317488 we got rid of the umbrella
UnsafeMath flag and introduced 3 more flags that better represent the different
things that happen under fast-math.
> 
> From a bitcode perspective, this change looks like this:
> Before r317488 we had 6 bits that respectively represented:
> 
> UnsafeMath
> nnan
> ninf
> nsz
> arcp
> contract
> *unset*
> 
> (The order may not match what is exactly in the bitcode.)
> 
> After r317488 we had 7 bits that respectively represented:
> reassoc (-UnsafeMath- is gone)
> nnan
> ninf
> nsz
> arcp
> contract
> *afn* (new bit)
> 
> Before r317488, fast-math was true if UnsafeMath was true (this should also
imply all the other flags are sets). After r317488, fast-math is true if all the
bits are set, in particular the afn, new one, too.
> 
> 
> * Problem *
> 
> Given we currently have no way to check if a bitcode file has been
generated pre-r317488 or post-r317488 that means that:
> 1. a post-r317488 compiler is going to skip any optimization guarded by
isFast for all pre-r317488 bitcode file (remember the afn bit is not set here)
> 2. a pre-r317488 compiler is going to run any optimization guarded by
unsafeAlgebra for any post-r317488 bitcode file that has the reassoc bit
(remember we repurposed UnsafeMath)
> 
> Scenario #2 might be unlikely but we’re potentially breaking the semantic
of the program. It is particularly dangerous because there is nothing that is
going to tell us that we are in this situation “downgrade" situation.
> #1 means that any code that uses unsafeMath is going to get a performance
hit.
> 
> In other words, one scenario implies generating wrong code and the other,
runtime performance regressions.
> 
> 
> 
> Scenario #1 is unsupported AFAIK, unless I missed something the bitcode is
not forward compatible: loading newer bitcode with an older LLVM has never been
supported as far I can remember.
I agree, but we cannot detect that this is the situation we are in and debugging
would prove hard. That said, if we say we don’t care, so be it :).
> 
> Scenario #2 is very much like other performance regression when we drop old
metadata (i.e. bitcode upgrade isn't performance proof in general but only
"best effort", there have been multiple instance of this in the past).
> 
> Usually IIRC we try not to version the bitcode at all this way (i.e.
bitcode does not have a linear versioning that is regularly bumped) but instead
make sure the encoding itself allows an easy upgrade. I.e. the encoding of the
FMF should have been such that the reader can detect and upgrade to the new IR
representation.
> Now this is too late here I guess, so bumping may be a possible trade-off.
Do you think we should do it, or live with the performance drop?
I know this performance drop is not acceptable for our use cases, but I don’t
want to impose our ruling on this.

> What about any bitcode shipped after r317488 but before the version bump?
Not worth taking into account because of the short period of time?
I was hoping we can ignore that period of time because it was not in any
official LLVM release.

Best,
-Quentin
> 
> Best,
> 
> -- 
> Mehdi
>  
> 
> 
> * Feedback Needed *
> 
> I believe this change is big enough that it would be worth bumping the
bitcode version so that the upgrader can do the right thing *before* we release
it to the public with LLVM-6.0.
> 
> That being said, I don’t know what are the implications of such bump and if
people really don’t care about the performance problem that might be okay. The
silent downgrade path is however concerning.
> 
> Should we bump the bitcode version because of that change and have the
autoupgrader properly rewrite the fast-math flags to
> preserve the semantic and make sure there are no silent downgrade?
> 
> 
> Thanks,
> -Quentin
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
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2018-02-13 13:29 GMT-08:00 Quentin Colombet <qcolombet at apple.com>:
> Hi Mehdi,
>
>
> On Feb 13, 2018, at 12:34 PM, Mehdi AMINI <joker.eph at gmail.com>
wrote:
>
>
>
> 2018-02-08 17:34 GMT-08:00 Quentin Colombet via llvm-dev <
> llvm-dev at lists.llvm.org>:
>
>> Hi,
>>
>> TL;DR
>> r317488 changed the way fast math flags are laid out in the bitcode and
>> anyone compiling a pre-llvm-6.0 bitcode with llvm-6.0 will lose all the
>> optimizations guarded by isFast and a pre-llvm-6.0 compiler compiling a
>> llvm-6.0 bitcode will potentially generate incorrect code w.r.t. fast
math
>> expectations.
>>
>> Should we bump the bitcode version because of that and have the
>> autoupgrader properly rewrite the fast-math to preserve that semantic?
>> (I believe we should!)
>>
>>
>> * Context *
>>
>> With https://reviews.llvm.org/D39304 / r317488 we got rid of the
>> umbrella UnsafeMath flag and introduced 3 more flags that better
represent
>> the different things that happen under fast-math.
>>
>> From a bitcode perspective, this change looks like this:
>> Before r317488 we had 6 bits that respectively represented:
>>
>> UnsafeMath
>> nnan
>> ninf
>> nsz
>> arcp
>> contract
>> *unset*
>>
>> (The order may not match what is exactly in the bitcode.)
>>
>> After r317488 we had 7 bits that respectively represented:
>> reassoc (-UnsafeMath- is gone)
>> nnan
>> ninf
>> nsz
>> arcp
>> contract
>> *afn* (new bit)
>>
>> Before r317488, fast-math was true if UnsafeMath was true (this should
>> also imply all the other flags are sets). After r317488, fast-math is
true
>> if all the bits are set, in particular the afn, new one, too.
>>
>>
>> * Problem *
>>
>> Given we currently have no way to check if a bitcode file has been
>> generated pre-r317488 or post-r317488 that means that:
>> 1. a post-r317488 compiler is going to skip any optimization guarded by
>> isFast for all pre-r317488 bitcode file (remember the afn bit is not
set
>> here)
>> 2. a pre-r317488 compiler is going to run any optimization guarded by
>> unsafeAlgebra for any post-r317488 bitcode file that has the reassoc
bit
>> (remember we repurposed UnsafeMath)
>>
>> Scenario #2 might be unlikely but we’re potentially breaking the
semantic
>> of the program. It is particularly dangerous because there is nothing
that
>> is going to tell us that we are in this situation “downgrade"
situation.
>> #1 means that any code that uses unsafeMath is going to get a
performance
>> hit.
>>
>> In other words, one scenario implies generating wrong code and the
other,
>> runtime performance regressions.
>>
>
>
>
> Scenario #1 is unsupported AFAIK, unless I missed something the bitcode is
> not forward compatible: loading newer bitcode with an older LLVM has never
> been supported as far I can remember.
>
>
> I agree, but we cannot detect that this is the situation we are in and
> debugging would prove hard. That said, if we say we don’t care, so be it
:).
>
I just mean that in general many things can blow up when the bitcode
evolves, and we don't expect or support (as far as I know) llvm-3.9 to be
used to read/process bitcode from llvm-4.0 for instance.
(I remember implementing in the Apple LLVM a hard check and failure for
this: we don't even try)


>
>
> Scenario #2 is very much like other performance regression when we drop
> old metadata (i.e. bitcode upgrade isn't performance proof in general
but
> only "best effort", there have been multiple instance of this in
the past).
>
>
> Usually IIRC we try not to version the bitcode at all this way (i.e.
> bitcode does not have a linear versioning that is regularly bumped) but
> instead make sure the encoding itself allows an easy upgrade. I.e. the
> encoding of the FMF should have been such that the reader can detect and
> upgrade to the new IR representation.
> Now this is too late here I guess, so bumping may be a possible trade-off.
>
>
> Do you think we should do it, or live with the performance drop?
>
I'm not saying this shouldn't be done, I don't have a dog in this
fight: I
was just providing another data point "as a matter of general policy"
in
the past there have been other cases where we haven't been able (or rather
chose to not put effort) into preserving all the performance when upgrading
(I think TBAA comes to my mind right now, but other kind of metadata have
also been lost). Now every case is different, and the FMF can be considered
too important to be left on the table.
For example it would impact LTO when loading libraries built with Xcode9
and before whenever the next released post-r317488 come up.


> I know this performance drop is not acceptable for our use cases, but I
> don’t want to impose our ruling on this.
>
>
> What about any bitcode shipped after r317488 but before the version bump?
> Not worth taking into account because of the short period of time?
>
>
> I was hoping we can ignore that period of time because it was not in any
> official LLVM release.
>
Very possible, I just wanted to make sure this is a conscientious choice :)

Cheers,

-- 
Mehdi


>
> Best,
> -Quentin
>
>
> Best,
>
> --
> Mehdi
>
>
>>
>>
>> * Feedback Needed *
>>
>> I believe this change is big enough that it would be worth bumping the
>> bitcode version so that the upgrader can do the right thing *before* we
>> release it to the public with LLVM-6.0.
>>
>> That being said, I don’t know what are the implications of such bump
and
>> if people really don’t care about the performance problem that might be
>> okay. The silent downgrade path is however concerning.
>>
>> Should we bump the bitcode version because of that change and have the
>> autoupgrader properly rewrite the fast-math flags to
>> preserve the semantic and make sure there are no silent downgrade?
>>
>>
>> Thanks,
>> -Quentin
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
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2018-02-13 17:46 GMT-08:00 Mehdi AMINI <joker.eph at gmail.com>:
>
>
> 2018-02-13 13:29 GMT-08:00 Quentin Colombet <qcolombet at apple.com>:
>
>> Hi Mehdi,
>>
>>
>> On Feb 13, 2018, at 12:34 PM, Mehdi AMINI <joker.eph at
gmail.com> wrote:
>>
>>
>>
>> 2018-02-08 17:34 GMT-08:00 Quentin Colombet via llvm-dev <
>> llvm-dev at lists.llvm.org>:
>>
>>> Hi,
>>>
>>> TL;DR
>>> r317488 changed the way fast math flags are laid out in the bitcode
and
>>> anyone compiling a pre-llvm-6.0 bitcode with llvm-6.0 will lose all
the
>>> optimizations guarded by isFast and a pre-llvm-6.0 compiler
compiling a
>>> llvm-6.0 bitcode will potentially generate incorrect code w.r.t.
fast math
>>> expectations.
>>>
>>> Should we bump the bitcode version because of that and have the
>>> autoupgrader properly rewrite the fast-math to preserve that
semantic?
>>> (I believe we should!)
>>>
>>>
>>> * Context *
>>>
>>> With https://reviews.llvm.org/D39304 / r317488 we got rid of the
>>> umbrella UnsafeMath flag and introduced 3 more flags that better
represent
>>> the different things that happen under fast-math.
>>>
>>> From a bitcode perspective, this change looks like this:
>>> Before r317488 we had 6 bits that respectively represented:
>>>
>>> UnsafeMath
>>> nnan
>>> ninf
>>> nsz
>>> arcp
>>> contract
>>> *unset*
>>>
>>> (The order may not match what is exactly in the bitcode.)
>>>
>>> After r317488 we had 7 bits that respectively represented:
>>> reassoc (-UnsafeMath- is gone)
>>> nnan
>>> ninf
>>> nsz
>>> arcp
>>> contract
>>> *afn* (new bit)
>>>
>>> Before r317488, fast-math was true if UnsafeMath was true (this
should
>>> also imply all the other flags are sets). After r317488, fast-math
is true
>>> if all the bits are set, in particular the afn, new one, too.
>>>
>>>
>>> * Problem *
>>>
>>> Given we currently have no way to check if a bitcode file has been
>>> generated pre-r317488 or post-r317488 that means that:
>>> 1. a post-r317488 compiler is going to skip any optimization
guarded by
>>> isFast for all pre-r317488 bitcode file (remember the afn bit is
not set
>>> here)
>>> 2. a pre-r317488 compiler is going to run any optimization guarded
by
>>> unsafeAlgebra for any post-r317488 bitcode file that has the
reassoc bit
>>> (remember we repurposed UnsafeMath)
>>>
>>> Scenario #2 might be unlikely but we’re potentially breaking the
>>> semantic of the program. It is particularly dangerous because there
is
>>> nothing that is going to tell us that we are in this situation
“downgrade"
>>> situation.
>>> #1 means that any code that uses unsafeMath is going to get a
>>> performance hit.
>>>
>>> In other words, one scenario implies generating wrong code and the
>>> other, runtime performance regressions.
>>>
>>
>>
>>
>> Scenario #1 is unsupported AFAIK, unless I missed something the bitcode
>> is not forward compatible: loading newer bitcode with an older LLVM has
>> never been supported as far I can remember.
>>
>>
>> I agree, but we cannot detect that this is the situation we are in and
>> debugging would prove hard. That said, if we say we don’t care, so be
it :).
>>
>
> I just mean that in general many things can blow up when the bitcode
> evolves, and we don't expect or support (as far as I know) llvm-3.9 to
be
> used to read/process bitcode from llvm-4.0 for instance.
> (I remember implementing in the Apple LLVM a hard check and failure for
> this: we don't even try)
>
>
>
>>
>>
>> Scenario #2 is very much like other performance regression when we drop
>> old metadata (i.e. bitcode upgrade isn't performance proof in
general but
>> only "best effort", there have been multiple instance of this
in the past).
>>
>>
>> Usually IIRC we try not to version the bitcode at all this way (i.e.
>> bitcode does not have a linear versioning that is regularly bumped) but
>> instead make sure the encoding itself allows an easy upgrade. I.e. the
>> encoding of the FMF should have been such that the reader can detect
and
>> upgrade to the new IR representation.
>> Now this is too late here I guess, so bumping may be a possible
trade-off.
>>
>>
>> Do you think we should do it, or live with the performance drop?
>>
>
> I'm not saying this shouldn't be done, I don't have a dog in
this fight: I
> was just providing another data point "as a matter of general
policy" in
> the past there have been other cases where we haven't been able (or
rather
> chose to not put effort) into preserving all the performance when upgrading
> (I think TBAA comes to my mind right now, but other kind of metadata have
> also been lost). Now every case is different, and the FMF can be considered
> too important to be left on the table.
> For example it would impact LTO when loading libraries built with Xcode9
> and before whenever the next released post-r317488 come up.
>
>
>
>> I know this performance drop is not acceptable for our use cases, but I
>> don’t want to impose our ruling on this.
>>
>>
>> What about any bitcode shipped after r317488 but before the version
bump?
>> Not worth taking into account because of the short period of time?
>>
>>
>> I was hoping we can ignore that period of time because it was not in
any
>> official LLVM release.
>>
>
Need to at least make sure everyone knows what to expect, so the
cherry-pick for the LLVM 6.0 release should include an entry in the Release
Notes.

Hans: this can only happen if you're still OK to cherry-pick in LLVM 6.0
branch https://reviews.llvm.org/D43253

Duncan, Steven, Peter, Teresa: this change can break LTO if we read back
bitcode emitted from trunk since r317488 (last November), any concerns?

Best,

-- 
Mehdi
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