[llvm] r269629 - ThinLTO: fix non-determinism in bitcode writing

[Duncan P. N. Exon Smith via llvm-commits]

> On 2016-May-15, at 21:50, Mehdi Amini via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> Author: mehdi_amini
> Date: Sun May 15 23:50:47 2016
> New Revision: 269629
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=269629&view=rev
> Log:
> ThinLTO: fix non-determinism in bitcode writing

You committed a bunch of these.  It would be great if the subjects somehow described what was different about them.  E.g., "ThinLTO: Fix non-determinism when refs to bitcode".

> Refs are initialized from a DenseSet. We can sort them using the
> value id to recover some determinism during serialization.
> 
> From: Mehdi Amini <mehdi.amini at apple.com>
> 
> Modified:
>    llvm/trunk/lib/Bitcode/Writer/BitcodeWriter.cpp
> 
> Modified: llvm/trunk/lib/Bitcode/Writer/BitcodeWriter.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Bitcode/Writer/BitcodeWriter.cpp?rev=269629&r1=269628&r2=269629&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Bitcode/Writer/BitcodeWriter.cpp (original)
> +++ llvm/trunk/lib/Bitcode/Writer/BitcodeWriter.cpp Sun May 15 23:50:47 2016
> @@ -3168,8 +3168,14 @@ void ModuleBitcodeWriter::writePerModule
>   NameVals.push_back(FS->instCount());
>   NameVals.push_back(FS->refs().size());
> 
> +  // Compute refs in a separate vector to be able to sort them for determinism.
> +  std::vector<uint64_t> Refs;

I would probably add a scope around this variable to make it clear that it's dead.

> +  Refs.reserve(FS->refs().size());
>   for (auto &RI : FS->refs())
> -    NameVals.push_back(VE.getValueID(RI.getValue()));
> +    Refs.push_back(VE.getValueID(RI.getValue()));
> +  std::sort(Refs.begin(), Refs.end());
> +
> +  NameVals.insert(NameVals.end(), Refs.begin(), Refs.end());

You don't need a separate vector/allocation.  Instead:
--
{
   unsigned SizeBeforeRefs = NameVals.size();
   for (...)
     NameVals.push_back(...);
   std::sort(NameVals.begin() + SizeBeforeRefs, NameVals.end());
}
--

> 
>   bool HasProfileData = F.getEntryCount().hasValue();
>   for (auto &ECI : FS->calls()) {
> 
> 
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