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

[Mehdi AMINI via llvm-dev]

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.

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.
What about any bitcode shipped after r317488 but before the version bump?
Not worth taking into account because of the short period of time?

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
>
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