[llvm] r269638 - ThinLTO: fix non-determinism in bitcode writing

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

> On 2016-May-16, at 02:04, Mehdi Amini via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> Author: mehdi_amini
> Date: Mon May 16 04:04:55 2016
> New Revision: 269638
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=269638&view=rev
> Log:
> ThinLTO: fix non-determinism in bitcode writing
> 
> Calls are initialized from a DenseMap. We can sort them using the
> value id to recover some determinism during serialization.
> 
> From: mehdi_amini <mehdi_amini at 91177308-0d34-0410-b5e6-96231b3b80d8>
> 
> 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=269638&r1=269637&r2=269638&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Bitcode/Writer/BitcodeWriter.cpp (original)
> +++ llvm/trunk/lib/Bitcode/Writer/BitcodeWriter.cpp Mon May 16 04:04:55 2016
> @@ -3177,8 +3177,15 @@ void ModuleBitcodeWriter::writePerModule
> 
>   NameVals.insert(NameVals.end(), Refs.begin(), Refs.end());
> 
> +  std::vector<FunctionSummary::EdgeTy> Calls = FS->calls();

Is there a case for a SmallVector here?  Is there some number that would avoid an allocation in most cases (without blowing out the stack)?

Regardless, you can move this auxiliary vector outside the loop and clear() it between uses.  This will avoid allocating unnecessarily on every iteration.

> +  std::sort(Calls.begin(), Calls.end(),
> +            [this](const FunctionSummary::EdgeTy &L,
> +                   const FunctionSummary::EdgeTy &R) {
> +              return VE.getValueID(L.first.getValue()) <
> +                     VE.getValueID(R.first.getValue());
> +            });
>   bool HasProfileData = F.getEntryCount().hasValue();
> -  for (auto &ECI : FS->calls()) {
> +  for (auto &ECI : Calls) {
>     NameVals.push_back(VE.getValueID(ECI.first.getValue()));
>     assert(ECI.second.CallsiteCount > 0 && "Expected at least one callsite");
>     NameVals.push_back(ECI.second.CallsiteCount);
> 
> 
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