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

[Quentin Colombet via llvm-dev]

Thanks to all a better fix landed on both trunk and LLVM 6.0!

> On Feb 16, 2018, at 11:37 AM, Teresa Johnson <tejohnson at google.com> wrote:
> 
> 
> 
> On Fri, Feb 16, 2018 at 8:26 AM, Mehdi AMINI <joker.eph at gmail.com <mailto:joker.eph at gmail.com>> wrote:
> 
> 
> 2018-02-13 17:46 GMT-08:00 Mehdi AMINI <joker.eph at gmail.com <mailto:joker.eph at gmail.com>>:
> 
> 
> 2018-02-13 13:29 GMT-08:00 Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>>:
> Hi Mehdi,
> 
> 
>> On Feb 13, 2018, at 12:34 PM, Mehdi AMINI <joker.eph at gmail.com <mailto: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 :).
> 
> 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 <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?
> 
> For our usage of ThinLTO, we don't reuse bitcode across different revisions, so it wouldn't cause an issue. The LTO caching in lib/LTO/LTO.cpp also encodes the llvm revision, so it shouldn't be an issue there either.
> 
> Teresa
> 
> 
> Best,
> 
> -- 
> Mehdi
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> -- 
> Teresa Johnson |	 Software Engineer |	 tejohnson at google.com <mailto:tejohnson at google.com> |	 408-460-2413

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